Service Assessment

Tropical Storm Allison 

Heavy Rains and Floods 

Texas and Louisiana

June 2001 

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Weather Service

Silver Spring, Maryland
Cover: The photograph shows severe flooding in downtown Houston, Buffalo Bayou, Texas,

on June 9, 2001. (Courtesy of Harris County Flood Control District/Steve Fitzgerald)
Service Assessment

Tropical Storm Allison 

Heavy Rains and Floods 

Texas and Louisiana

June 2001

September 2001

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

National Oceanic and Atmospheric Administration

Scott B. Gudes, Acting Administrator

National Weather Service

John J. Kelly, Jr., Assistant Administrator
ii

Preface

Tropical Storm Allison was the most costly tropical storm in U.S. history. While the

storm impacted a large part of the country, worst hit was southeast Texas and southern Louisiana. 

In these two areas alone, there were 24 fatalities and more than $5 billion in damage.

Due to the magnitude of this flooding event, a service assessment team was dispatched to

examine the warning and forecast services provided by the National Weather Service (NWS) to

emergency managers, flood control/public works agencies, and the public in southeast Texas and

southern Louisiana. Service assessments provide a valuable contribution to our ongoing efforts

to improve the quality and timeliness of our products and services for the protection of life and

property. Findings and recommendations from this assessment will help to improve techniques,

products, and services.

John J. Kelly, Jr.

Assistant Administrator

 for Weather Services

September 2001
iii

Table of Contents

Page

Preface ..................................................................... ii

Service Assessment Team ...................................................... iv

Acronyms ................................................................... vi

Service Assessment Report ...................................................... 1

Facts ....................................................................... 25

Findings and Recommendations ................................................. 29

Best Practices ................................................................ 31

Appendix A Tropical Storm Allison Rainfall Storm Total ......................... A-1

Appendix B Texas and Louisiana Fatalities Directly Attributed to 

Tropical Storm Allison ........................................... B-1

Appendix C HPC Forecast Operations Branch Monthly QPF Threat Scores

24-Hour Day 1 Forecast, June 2001 .................................. C-1
iv

Service Assessment Team

This service assessment team was activated on June 15, 2001. The Weather Forecast

Office (WFO) Houston/Galveston, Texas, briefed the team on June 18. Team members visited

Southern Region Headquarters (SRH), River Forecast Centers (RFCs), WFOs, Hydrometeorological Prediction Center (HPC), and various emergency management, flood control,

and broadcast media outlets. In addition, telephone interviews were conducted with a number of

internal and external organizations. The team comprised the following individuals.

Larry Mooney Team Leader, Meteorologist in Charge (MIC), WFO Denver/Boulder,

Colorado

Julie Adolphson Science and Operations Officer, WFO Northern Indiana, Syracuse, Indiana

John Feldt Hydrologist in Charge (HIC), Southeast River Forecast Center, Peachtree

City, Georgia 

Peter Gabrielsen Deputy Chief, Hydrologic Services Division, NWS Eastern Region

Headquarters, Bohemia, New York

Ken Graham Meteorologist in Charge, WFO Corpus Christi, Texas

Jeff Orrock Warning Coordination Meteorologist, WFO Newport/Morehead City,

North Carolina

Kevin Stewart Professional Engineer (Chair, National Hydrologic Warning Council),

Urban Drainage and Flood Control District, Denver, Colorado

Ron Trumbla National Oceanic and Atmospheric Administration (NOAA) Public

Affairs Specialist, NWS SRH, Fort Worth, Texas

Other valuable contributors include:

William Lerner NWS Headquarters, Office of Climate, Water, and Weather Services,

Silver Spring, Maryland

Linda Kremkau NWS Headquarters, Office of Climate, Water, and Weather Services,

Silver Spring, Maryland
v

The team would like to thank the individuals from the following organizations who took

time to talk with team members.

NWS Offices

WFO Houston/Galveston, TX

WFO Lake Charles, LA

WFO New Orleans/Baton Rouge, LA

Center Weather Service Unit, Houston, TX

Spaceflight Meteorology Group, Houston, TX

Lower Mississippi River Forecast Center (LMRFC), Slidell, LA

West Gulf River Forecast Center (WGRFC), Fort Worth, TX

NWS Southern Region Headquarters (SRH), Fort Worth, TX

Hydrometeorological Prediction Center (HPC), Camp Springs, MD

Tropical Prediction Center (TPC), Miami, FL

State Emergency Managers

Texas Division of Emergency Management (TDEM)

Louisiana Office of Emergency Preparedness

County/Parish Emergency Managers

Galveston County, TX Harris County, TX

Houston County, TX Jefferson, County, TX

Montgomery County, TX Polk County, TX

East Baton Rouge Parish, LA

City Emergency Managers

Friendswood, TX Houston, TX

Pasadena, TX City of Baton Rouge, LA

Other Officials

Harris County Flood Control District

Jefferson County Drainage District No. 6

Louisiana State University, Southern Regional Climate Center

Louisiana Office of State Climatology

Media

KHOU-TV, Houston, TX KTRK-TV, Houston, TX

KPRC-TV, Houston, TX KXLN-TV, Houston, TX

KRIV-TV, Houston, TX WBRZ-TV, Baton Rouge, LA

KTRH-Radio, Houston, TX
vi

Acronyms

AFD Area Forecast Discussion

ALERT Automated Local Evaluation in Real-Time

AWIPS Advanced Weather Interactive Processing System

CWA County Warning Area

DEM Division of Emergency Management

EAS Emergency Alert System

EMWIN Emergency Managers Weather Information Network

HCM Hydrometeorological Coordination Message

HIC Hydrologist in Charge

HMD Hydrometeorological Discussion

HPC Hydrometeorological Prediction Center

LMRFC Lower Mississippi River Forecast Center

MANS Media Alert Notification System

MAP Mean Areal Precipitation

MIC Meteorologist in Charge

MSL Mean Sea Level

NCEP National Centers for Environmental Prediction

NOAA National Oceanic and Atmospheric Administration

NWR NOAA Weather Radio

NWS National Weather Service

OEM Office of Emergency Management

QPF Quantitative Precipitation Forecast

RFC River Forecast Center

ROC Regional Operations Center

SRH Southern Region Headquarters

TDEM Texas Division of Emergency Management

TPC Tropical Prediction Center

WFO Weather Forecast Office

WGRFC West Gulf River Forecast Center

WSR-88D Weather Surveillance Radar-1988 Doppler
vii

Neighborhood flooding near Interstate 10, Houston, Texas. (Courtesy of Harris County Flood

Control District/Steve Fitzgerald)
viii

Cars and trucks stranded on Interstate 10, west of downtown Houston, Texas. (Courtesy of Paige

Morrison)
1

Service Assessment Report

Introduction

Tropical Storm Allison produced severe storms, torrential rainfall, and associated

flooding across the southern and eastern sections of the United States from June 5 to June 18,

2001. After making landfall near Galveston, Texas, on June 5, the storm moved inland to near

Lufkin, Texas. Allison drifted back into the Gulf of Mexico on June 9, turned to the northeast,

and made landfall again on June 10 near Morgan City, Louisiana. After causing 24 deaths in

Texas and Louisiana, Allison moved across southern Mississippi, southern Alabama, southwest

Georgia, and northern Florida, causing 9 more deaths. By mid-week, Allison stalled over North

Carolina and produced more heavy rainfall and flooding before tracking northeast along the

DelMarVa Peninsula and moving off the New England coast on June 18. Seven additional

deaths occurred in Pennsylvania and one in Virginia. Figure 1 shows the path of Tropical Storm

Allison, and Figure 2 shows the associated rainfall.

Tropical Storm Allison caused more damage than any tropical storm in U.S. history, with

estimates in excess of $5 billion. Most of the damage and fatalities (22) occurred in Houston,

Texas. Storm rainfall totals peaked at 36.99 inches (Port of Houston) in Texas and 29.86 inches

(Thibodaux) in Louisiana. Since this was the area of extreme rainfall and greatest impact in

terms of damage and fatalities, the report focuses on NWS performance in southeast Texas and

southern Louisiana.

Overview

Tropical Storm Allison began as a disorganized area of thunderstorms in the southwest

Gulf of Mexico. On June 5, the NWS’s Tropical Prediction Center (TPC) declared the system a

tropical storm. Of particular interest is that Allison made landfall twice. The storm first struck

Galveston Island, Texas, on June 5, drifted back over the Gulf of Mexico, and the remnants of

Allison made landfall again over southern Louisiana on June 10. During this period, torrential

rainfall caused tremendous flooding along the Texas and Louisiana Gulf coasts. Rainfall storm

totals from Allison are listed in Appendix A.
2

20

25

30

35

40

45

-100 -95 -90 -85 -80 -75 -70 -65 -60

Allison

5-17 June 2001

Hurricane

Tropical Storm

Tropical Dep.

Extratropical

Subtr. Storm

Subtr. Dep.

00 UTC Pos/Date

12 UTC Position

10

9

8

7

6

19

18

15

14

13

12

11

16

17

1000 mb

 Figure 1. Track of Tropical Storm Allison, June 5-17, 2001. (NOAA/NCEP/TPC)
3

Figure 2. Tropical Storm Allison rainfall totals. (Courtesy of the Southeast Regional

Climate Center)
4

Texas

A high pressure system east of the Florida peninsula acted to steer the cyclone northward,

prompting the evacuation of the west end of Galveston Island, an area not protected by the

seawall which was constructed after the Galveston hurricane of 1900. Allison made its initial

landfall on Galveston Island during the evening of June 5. Allison drifted north to Lufkin, about

120 miles north of Houston. During this time, the Houston area received 4 to 6 inches of rain

with isolated totals of 8 to 10 inches.

During the night of June 5-6, heavy rains deluged northwest Jefferson and Orange

Counties near the Louisiana border. Six to 10 inches of rain fell in less than 5 hours, flooding

homes, closing streets, and stalling hundreds of cars in China, Beaumont, Nome, and Vidor. The

remnants of Tropical Storm Allison’s circulation center remained well defined between two high

pressure areas, allowing the center to drift south and west to near Huntsville (about 60 miles

north of Houston) during the day on June 7. Additional heavy rain, 8 to 12 inches, occurred over

the Sugarland-Stafford area of Fort Bend and Harris Counties, west and southwest of Houston,

flooding streets and homes. Without a strong steering current, Allison drifted farther southwest

toward the Gulf of Mexico, and continued to draw in additional moisture. During the early

morning hours of June 8, a band of heavy rain moved over Beaumont. Although up to 10 inches

of rain fell, it was in areas previously spared, resulting in only minor flooding. The center of

circulation was now between College Station and Huntsville drifting slowly southwest.

The combination of daytime heating and copious moisture set the stage for heavy and

persistent rain over the Houston area on the night of June 8-9. Thunderstorms formed over the

western side of the Houston area and rapidly intensified as they moved east. Flash flooding

began rapidly during the afternoon rush hour and became widespread by dark. Between

June 8-9, five of the six major bayou systems experienced severe flooding, some to record levels. 

All major interstates were closed due to flooding. Two-thirds of Harris County (Houston)

received over 10 inches of rain. Amounts ranged from 2 inches in the extreme west to more than

20 inches in northeast Houston. Greens Bayou received more than 26 inches of rain in about

10 hours. Figure 3 is a map of the rainfall totals during the heaviest period of rain. 

During this portion of the event, 22 people were killed and more than 45,000 homes and

businesses flooded. More than 70,000 vehicles were flooded. Ten of the fatalities occurred

either in a vehicle or while fleeing a vehicle. Six deaths resulted while walking in flood waters,

and three from electrocution. The 45-building Texas Medical Center complex suffered damages

of $2 billion and included the loss of decades of medical research records and experiments. 

Large-scale patient evacuations occurred and medical services were disrupted for several days

due to power outages. The underground portions of downtown Houston were flooded. In

addition to the 22 deaths in Houston, a man was killed while swimming in a drainage ditch at
5

Figure 3. Harris County ALERT precipitation totals from 6 p.m., June 8, through 4 a.m.,

June 9. (Courtesy of Harris County OEM)
6

Mauriceville (Orange County), Texas, on June 9. (See Appendix B for the list of Texas and

Louisiana fatalities directly attributed to Tropical Storm Allison.)

Louisiana

The remnants of Allison drifted off the middle Texas coast during the evening of June 9. 

By midday on June 10, the center was still offshore about 70 miles south-southwest of Cameron,

Louisiana, and moving to the east-northeast. Allison moved inland near Morgan City on the

early evening of June 10. Tracking north of New Orleans across Lake Pontchartrain, the center

of Allison exited Louisiana around dawn on June 11.

In terms of storm rainfall totals and area impacted, Allison ranks among the worst to hit

Louisiana in the past 100 years. The greatest storm total was 29.86 inches at Thibodaux

(30 miles northeast of Morgan City). Salt Point in St. Mary Parish recorded 27.55 inches of rain,

and Baton Rouge had 19.15 inches.

The most significant flooding occurred on June 6-7. Much of the city of Thibodaux was

flooded when 15.16 inches of rain fell in 24 hours. Dozens of homes were evacuated in

Lafayette, Iberia, St. Mary, and Vermilion Parishes. Deep water submerged Highways 90 and

182 in this region for more than 12 hours. In East Baton Rouge Parish, flooding resulted in the

evacuation of 1,800 residents and flooded 1,000 homes. Over 2,000 homes were flooded in

Livingston, Lafourche, St. Tammany, and Ascension Parishes. This was the most significant

flooding of the Amite and Comite Basins since 1983. Fifty homes were flooded in Jefferson

Parish. About 75 people were evacuated from their homes in south Slidell during the early

morning of June 11. Despite numerous reports of people stranded in their vehicles, no flood

deaths occurred in Louisiana. The only fatality occurred near Zachary when a small tornado

caused a tree to fall onto a vehicle, killing its occupant (see Appendix B). Damage in excess of

$50 million has been attributed to Allison in Louisiana.

Warning and Forecast Services

National Centers

The National Centers for Environmental Prediction (NCEP) provides forecast product

guidance to RFCs, WFOs, and other NWS field offices. Of the nine centers comprising NCEP,

the Tropical Prediction Center and the Hydrometeorological Prediction Center were most

involved in providing services during Tropical Storm Allison in Louisiana and Texas.
1

 All times listed in this service assessment are CDT.

7

Tropical Prediction Center

The TPC monitors the movement and strength of tropical weather systems, provides

official NWS forecasts, and issues associated coastal watches and warnings for the United States.

On May 21, the TPC observed a tropical wave off the west coast of Africa and tracked it

across the Atlantic and the Yucatan Peninsula. It moved into the Bay of Campeche on the

evening of June 4. The 4:30 a.m., Central Daylight Time (CDT),1 June 5, Tropical Weather

Outlook issued by TPC indicated no tropical storm development was expected. By 7 a.m., the

TPC analysis of satellite imagery and surface observations suggested the presence of a surface

low about 120 nautical miles south of Galveston, Texas. Data Buoy 42002 (240 nautical miles

southeast of Sabine, Texas) was out of service during the passage of the weather system that was

to later become Tropical Storm Allison. A replacement buoy became operational on August 21.

Had this buoy been in service, the development of Tropical Storm Allison might have been

detected earlier. A post storm review by TPC states, “Determination of a closed surface low

pressure system was delayed by at least 6 hours as a result of Buoy 42002 not reporting due to its

previous collision with a ship.” The service assessment team agrees with this opinion.

The possibility of tropical development was first mentioned by TPC in a Special Tropical

Disturbance Statement issued at 7:45 a.m., June 5. This statement cautioned heavy rains and

tropical storm force winds would be spreading onto the mid and upper Texas coast and coastal

Louisiana later in the day if development continued. Around 1 p.m., weather information from

another buoy 60 nautical miles south of Freeport, Texas, and observations from an Air Force

Reserve reconnaissance flight confirmed a circulation 80 nautical miles south-southwest of

Galveston.

The initial Tropical Storm Warning for Allison was issued in a special advisory at 2 p.m.

for the area from Sargent, Texas, to Morgan City, Louisiana, where the broad circulation was

expected to make landfall that evening, June 5. Tides were predicted to be 2 to 4 feet above

normal with maximum winds around 60 miles per hour (mph). Locally heavy rainfall up to

5 inches was predicted.

At 4 p.m., June 5, when the storm was centered 35 miles south of the Houston/Galveston

area, the TPC forecast included the possibility of isolated rainfall amounts up to 8 inches. 

Heaviest rainfall and strongest winds in a tropical storm are normally near the center. In the

7 p.m. advisory, TPC warned the weather associated with Allison was not near the center but

rather north, east, and southeast. Because heavy rain was already spreading into southeast Texas

and southern Louisiana, and little movement of the storm had been detected, the forecast called

for isolated rainfall totals to 10 inches.
8

At 10 p.m., June 5, Allison’s center was 20 miles southwest of Galveston. TPC

emphasized the strong winds (now weakened from 60 mph to 45 mph) and the heavy rain were

well to the north and east. Ten inches of rain had fallen at a few locations in southeast Texas,

and additional amounts of 4 to 5 inches with isolated amounts to 10 inches were still possible.

At 5 a.m., June 6, TPC issued its last public advisory on Allison when coastal warnings

were discontinued. The threat of heavy rain was highlighted in this advisory even though Allison

had weakened to a tropical depression and all tropical storm warnings were discontinued. 

Consistent with operational protocol, responsibility for preparing storm summaries on Allison

was transferred from TPC to HPC.

HPC continued to issue storm summaries for the next 3 days. On the evening of June 9,

the remains of Allison moved back across the Texas coast into the Gulf of Mexico and a new

low-level circulation developed. As part of its marine warning and forecast responsibilities, TPC

issued a Gale Warning for the northeast Gulf of Mexico about 3 hours prior to Allison reaching

subtropical storm status. (A subtropical storm is one occurring in lower latitudes but has

characteristics of both tropical cyclones and storms occurring farther north.) Normally, TPC

would have again assumed responsibility for advisories on Allison. However, since HPC had

been issuing these storm summaries for the past 3 days, the gale force winds were expected to be

of short duration, and most of the storm’s circulation remained over land, a coordinated decision

was made to have HPC continue issuing storm summaries.

The onset of tropical storm force winds along the upper Texas coast occurred within

3 hours of the issuance of the Tropical Storm Warning. Rainfall forecasts in TPC products were

effectively coordinated with HPC, the WFOs, and the RFCs. Rainfall amounts of 4 to 5 inches

with isolated amounts of 8 to 10 inches were forecast. Observed totals were generally in the 4- to

6-inch range in the Houston area with isolated amounts of 8 to 10 inches in eastern Harris

County. Tides observed in Texas were within the 2- to 4-foot above normal range predicted

when the Tropical Storm Warning was issued.

Hydrometeorological Prediction Center

HPC’s mission is to provide forecast, guidance, and analysis products and services to

support the public forecast activities of the NWS and its customers.

During Tropical Storm Allison, HPC forecasters provided accurate guidance to WFOs

and RFCs. Updated Quantitative Precipitation Forecasts (QPFs) were made outside of the

regularly scheduled issuance times. The morning HPC QPF on June 5 called for rainfall to

exceed 5 inches in southeast Texas. A WFO Houston/Galveston forecaster indicated this was

important input into his decision to issue the initial Flash Flood Watch at 10:30 a.m. This watch

was issued more than 3 hours before Allison reached tropical storm strength and 6 hours in

advance of the onset of flash flooding. Guidance from HPC to WFO Houston/Galveston on

June 9 enabled the WFO to determine the threat was over and terminate the Flash Flood Watch
9

sooner than expected. The WFO Houston/Galveston MIC stated, “kudos are in order” for HPC

assistance.

Another forecast product widely used by field personnel is the Excessive Rainfall

Potential Outlook, which identifies areas where rainfall is expected to exceed amounts that will

produce flash flooding. Areas expected to receive 5 inches or more of rain are depicted on this

graphical product as a hatched area. Figure 4 is the Excessive Rainfall Potential Outlook issued

by HPC at 12:25 p.m. on Friday, June 8, which correctly highlighted the overnight flash flood

event that claimed 22 lives in Houston.

Frequent coordination calls were made from HPC to affected offices. A West Gulf River

Forecast Center (WGRFC) contingency forecast for major flooding on Buffalo Bayou (24 hours

before the event) was a direct result of coordination between the HPC and the WGRFC. WFO

forecasters also noted excellent coordination calls with HPC.

Figure 5 compares the HPC QPF forecasts versus actual rainfall. The map on the top

(Figure 5a) is the total rainfall from HPC’s forecasts for the period June 5 to June 12. The map

on the bottom (Figure 5b) is the actual precipitation from radar estimates and rainfall

measurements for the same period. The comparison of HPC’s cumulative QPF forecasts to the

rainfall totals for June 5 to June 12 documents a high level of accuracy and strong performance

by HPC forecasters. In fact, during June 2001, HPC set several new records for accuracy in its

QPF program (see Appendix C).

After assuming responsibility for Allison from the TPC on June 6, the HPC issued

48 storm summaries (one every 6 hours) which provided information on storm position, watch/

warning status, rainfall, forecasts, and historical information.

River Forecast Centers

The RFCs are tasked with providing timely river stage and flood forecasts, radar and

rainfall analyses, and hydrological expertise to WFOs, water management agencies, and state

emergency managers. The RFCs responsible for river and flood forecast services with respect to

Allison’s heavy rain and resultant flooding were the West Gulf RFC (WGRFC) in Fort Worth,

Texas, and the Lower Mississippi RFC (LMRFC) in Slidell, Louisiana. RFCs are not normally

staffed 24 hours a day.
10

Figure 4. HPC Excessive Rainfall Potential Outlook issued at 12:25 p.m.,

June 8, 2001. (NOAA/NCEP/HPC)
11

Figure 5a. HPC Cumulative Quantitative Precipitation

Forecasts. (NOAA/NCEP/HPC)

Figure 5b. Mean Areal Precipitation Estimates. (NOAA) 

Figure 5. Comparison of HPC QPF forecasts versus actual rainfall.
12

West Gulf River Forecast Center

Recognizing the seriousness of the hydrologic event, the WGRFC began 24-hour

operations on June 5. This extended coverage was provided to ensure partners and customers

had continuous access to the RFC during a critical situation and provided a continuous flow of

flood forecasts and updates.

There was ongoing telephone coordination with WFOs Houston/Galveston and Lake

Charles, HPC, San Jacinto River Authority, Trinity River Authority, Texas Division of

Emergency Management (TDEM), SRH Regional Operations Center (ROC), and the Federal

Emergency Management Agency. Hydrometeorological Coordination Messages (HCMs) and

telephone calls were the primary methods used to communicate with WFOs. HCMs contain

specific information regarding adjustments to radar rainfall estimates, hydrometeorological

analysis, and potential for both short- and long-term river impacts. A total of 23 HCMs were

issued and several provided “heads-up” notice for the serious flooding potential.

The WGRFC produces an experimental River Flood Watch that is disseminated to the

public by the WFOs. This experimental product provided information that highlighted the

likelihood of a major flood. Thirteen WGRFC river flood watches were produced with an

average lead time to crest of 36 hours. In the case of the San Jacinto River, the experimental

River Flood Watch resulted in a lead time of almost 48 hours.

Another example of WGRFC’s response was issuance of several non-routine graphical

products on the offices’ Internet page, including 24-hour rainfall totals derived from radar rainfall

estimates and rain gauge measurements.

Radar estimated precipitation data was adequate throughout most of the event. When a

minor underestimation was observed, WGRFC forecasters coordinated with affected WFOs,

adjusted the data, and thereby improved accuracy of precipitation estimates to customers and

partners. Early on June 9, a communication connection between a private vendor and the

Internet failed, severing access to Southern Region field offices. As a result, Harris County

ALERT (Automated Local Evaluation in Real-Time) rainfall data was not received at the

WGRFC. An ALERT system is a local network of rainfall and stream gauges that provides realtime precipitation and water level measurements in support of local flood warning decisions. 

These data would have more precisely calibrated WGRFC precipitation estimates, resulting in a

more accurate knowledge of where and how much rainfall had occurred. Working through SRH,

the vendor restored Internet service by late morning on June 9.

Forecasting proved to be difficult for some small rivers/bayous that rose rapidly to record

levels. Forecasts for Buffalo Bayou at Shepherd Drive in Houston underforecast the river stage

by 6 to 12 feet with little or no lead time. Also, the forecasts for the West Fork of the San Jacinto

River near Conroe were not accurate. Both software and procedural problems have been

identified by the WGRFC and the NWS Office of Hydrologic Development as contributing to
13

these inaccuracies. RFCs employ forecast procedures that use data from past floods. When

extreme rainfall occurs resulting in record floods, how a river responds becomes less predictable. 

How rainfall is distributed over a basin also affects how a river responds. With Allison, rainfall

was concentrated at the lower end of the basin, and the RFC did not have a precedent for

adjusting its procedure. For Buffalo Bayou, the time interval used for the forecast model was set

at 6-hour time steps. A 3-hour interval is needed for basins that respond rapidly during heavy

rainfall. A software problem existed in the model used to generate the forecasts for the West

Fork of the San Jacinto River. (Finding 1)

During Allison, contingency forecasts showed potential for compensating for weaknesses

in forecast procedures during extreme or unprecedented rainfall and flood events. Contingency

forecasts are made based on potential “worst case” situations. On June 7, a contingency forecast

called for a major flood at Buffalo Bayou 24 hours before the event. Another such forecast for

Sour Lake on Pine Island Bayou called for a crest that verified within a half foot. For 16 forecast

points, the average 24-hour crest forecast error was less than a foot.

Lower Mississippi River Forecast Center

The LMRFC provided outstanding service to its customers and partners. This included

specific guidance for river and flash flooding as well as operational support to emergency

managers and the U.S. Geological Survey.

The LMRFC began 24-hour operations on June 5. Normally, the LMRFC uses 12-hour

QPF to derive its river forecasts. During Allison, the RFC did expand usage of the QPF to

24 hours when conditions warranted. For longer range contingency forecasts, a special 48-hour

QPF was used. These contingency forecasts were the basis for river flood watches issued by the

WFOs at the suggestion of the LMRFC. During the event, the LMRFC issued Hydrometeorological Discussions (HMDs) on a more frequent basis. River forecasts were updated every

6 hours and QPF every 12 hours. Coordination of QPF forecasts was accomplished through

conference calls with HPC and the WFOs.

The LMRFC directly coordinated with East Baton Rouge Emergency Management,

Louisiana State Climatologist, and state emergency management officials in Texas and

Louisiana. Extensive coordination also occurred with the mayor of Baton Rouge during times of

critical decision making. There was extensive interaction with WFOs New Orleans/Baton Rouge

and Lake Charles. The LMRFC also coordinated with interests along the flooded Amite River.

Rainfall in excess of 20 inches produced moderate to major flooding at seven LMRFC 

river forecast points. The LMRFC provided excellent lead times for these locations. For rivers

in the Lake Pontchartrain drainage, lead times to flood stage ranged from 18 hours at two of the

forecast points to 31 hours at three other sites. For the two forecast points in the Calcasieu Basin,

the lead times to flood stage were 18.5 hours; and 2 days, 11.5 hours; respectively.
14

The LMRFC Web site was used extensively by State Climatologist Jay Grymes who

stated, “when I am asked about the most useful sites available for monitoring Louisiana weather,

the LMRFC is among the very first I mention.” A graphical version of the HMD on the RFC

Web site was well received by customers. Normally, the LMRFC posts all river forecasts in both

graphical and text format on its Web site one hour after the forecast is sent to the WFOs. In

response to customer feedback, this delay was reduced to 30 minutes during this event to provide

information faster to customers.

Weather Forecast Offices

WFOs are responsible for issuing timely meteorological and hydrological warnings,

watches, forecasts, and statements for their areas of responsibility. They are the primary NWS

contact point for state and local officials, the news media, and the general public. Southeast

Texas and southern Louisiana are served by WFOs Houston/Galveston, Lake Charles, and New

Orleans/Baton Rouge.

WFO Houston/Galveston, Texas

WFO Houston/Galveston provided outstanding service to its customers and maintained

an exceptional level of cooperation with its partners. The threat of heavy rain was initially

mentioned in the morning forecast issued at 4:30 a.m. on June 5. This was 12 hours before the

heavy rains began. Recognizing that the storm had the potential to produce a significant weather

event, the WFO issued a Hazardous Weather Outlook at 5:19 a.m. The Outlook mentioned the

threat of rainfall up to 3 inches. The initial Flash Flood Watch for Houston/Harris County was

issued at 10:30 a.m., calling for rainfall totals along and east of Highway 59 to exceed 5 inches

with isolated amounts to 8 inches. At 1:38 p.m., a Coastal Flood Warning was issued for tides of

4 to 5 feet from High Island to Freeport. This included coastal sections of Galveston Bay.

 A High Wind Warning was issued for the area east of a Conroe-Livingston line at

3:27 p.m., June 5. By late afternoon, the WFO began issuing a series of flash flood and river

flood warnings, extending well into June 6. The WFO issued a Hurricane Local Statement at

7:25 p.m., calling for overnight rainfall totals of 7 to 14 inches in northern Galveston and Harris

Counties.

Overnight rainfall totals ending on the morning of June 6 were in the 4- to 6-inch range

with isolated amounts of 8 to 10 inches in eastern Harris County. Observed tides were within the

forecasted range.

The Flash Flood Watch was extended on Wednesday, June 6, and remained in effect for

all or portions of southeast Texas until late Sunday afternoon, June 10. A River Flood Watch

was issued at 10:39 a.m. on June 6. Flash flood and river flood warnings continued to be issued

throughout this period as well.
15

As early as Wednesday, June 6, WFO Houston/Galveston forecasters increased the

likelihood of rain in the forecast for Friday and Saturday, June 8-9, in anticipation of another

period of excessive rainfall. In the Thursday afternoon, June 7, Area Forecast Discussion (AFD),

the forecaster cautioned “CANNOT RULE OUT SEVERAL MORE ROUNDS OF 5 TO

10 INCH RAINFALL EVENTS...OVER THE WEEKEND.” In the AFD issued at 3:37 a.m.,

Friday, June 8, the forecaster cautioned the storm could produce more heavy rain through

Saturday night, June 9, and some of the thunderstorms could be severe. This was stated again in

the 5:05 a.m. extension of the Flash Flood Watch, forecasting rainfall in excess of 10 inches and

emphasizing this was a potentially dangerous situation. A number of flash flood warnings were

issued in the early morning of June 8 for counties surrounding Harris County. Meanwhile, a

break in the precipitation and cloud cover over metropolitan Houston was just a precursor for the

most significant round of Allison’s rains. In the mid-morning forecast discussion, the forecaster

warned, “THIS HEAVY RAIN EVENT IS NOWHERE NEAR FROM BEING OVER.” The

combination of afternoon heating and a strong surge of moisture still feeding into the area

enhanced the risk of thunderstorm development. An early afternoon update characterized the

situation as a “DANGEROUS FLASH FLOOD THREAT” and advised the forecast of greater

than 10 inches might have to be increased before the night was over.

By late Friday afternoon, June 8, rain began to fall again in Houston. At 4:25 p.m., an

Urban Small Stream Flood Advisory was issued for Harris County as rainfall was expected to

cause flooding of streets and feeder roads near the Texas Medical Center, Bush Intercontinental

Airport, and other locations throughout the county. At 5:10 p.m., a Flash Flood Warning was

issued for Harris County and specifically mentioned the areas that were later flooded extensively

along I-45, Highway 59, and Highway 288. The Flash Flood Warning remained in effect until

midnight, Saturday, June 9.

Warnings and statements for Harris County stated clearly this was a very dangerous flood

situation and urged people not to travel in the county until the flood threat had passed. 

Subsequent statements and warnings provided specific information on expected or existing

locations of flooding. An example of the detail and seriousness of the flood threat can be seen in

the 7:12 p.m., June 8, statement issued by the WFO. It noted, “THE HARRIS COUNTY

EMERGENCY MANAGER REPORTS FLOODING AT I-10 AND JOHN RALSTON...

SOUTHWEST FREEWAY AT WESTPARK...NORMANDY AT I-10...AND WIDESPREAD

STREET FLOODING OVER THE EAST SIDE OF HOUSTON. THIS IS A VERY

DANGEROUS SITUATION! TRAVEL IS NOT RECOMMENDED.” The 7:55 p.m. extension

of the Flash Flood Warning stated, “MUCH OF HOUSTON INSIDE LOOP 610 HAS

RECEIVED 3 TO 5 INCHES OF RAINFALL...MORE IS ON THE WAY. DO NOT TRAVEL

IN HARRIS COUNTY UNTIL THIS FLOOD THREAT HAS PASSED.”

The most intense rainfall in Houston occurred between 11 p.m., Friday, June 8, and

4 a.m., Saturday, June 9. At times, the hourly rainfall rates exceeded 5 inches an hour. Twentyfour-hour rainfall totals in the Houston area averaged just under 10 inches. An ALERT rain
16

gauge in the Greens Bayou area reported more than 28 inches in the 24 hours ending at 7 a.m.,

Saturday, June 9. Figure 2 shows the rainfall distribution across the Houston metropolitan area. 

Flash and river flood warnings and statements were issued throughout Saturday, June 9,

in the Houston county warning area (CWA) as forecasters continued to stress the flood danger. 

By late Sunday afternoon, June 10, the flash flood threat finally ended in the Houston service

area. Figure 6 shows Allison’s rainfall storm totals for June 4-10 for the Houston area.

Between June 5 and June 10, WFO Houston/Galveston experienced 39 flash flood events

in its service area and issued 99 flash flood warnings. All of the events were included in a

warning for an accuracy of 100 percent. Nine warnings had no subsequent flash flood event

associated with them for a false alarm rate of 9 percent. This performance was outstanding when

compared to national performance during 2000 (accuracy was 86 percent and false alarm rate was

38 percent). The average lead time was 40 minutes.

The Weather Surveillance Radar-1988 Doppler (WSR-88D) experienced an outage on

Friday, June 1, and the WFO electronics staff worked over the weekend to restore operations. 

Media partners commented that this effort was helpful and appreciated. Partners and customers

were also impressed by the quality of the rainfall data. The radar was switched into tropical

rainfall mode during the late afternoon on June 5. Minor biases were noted and adjustments

made in coordination with the WGRFC.

The WFO Houston/Galveston staff is capable of activating the pagers of key emergency

managers and government officials in their area. With Allison’s rapid formation, the WFO pager

system was a vital notification method for the emergency management community. This

capability was used throughout the event to provide rapid dissemination of new information.

The WFO relied on its Internet connection to monitor data from the Harris County Office

of Emergency Management (OEM) ALERT system. WFO access to this system was lost early

on the morning of June 9 when the Internet service provider experienced an outage. No

significant impact resulted as the WFO was already in frequent telephone contact with the OEM

and the Harris County Flood Control District. This interaction was critical during June 8-9 when

the main threats were from urban flash flooding and the rapid flooding of the bayous.

Because of the small size of the bayous in the Houston area, most are not included in the

WGRFC forecast system. WFO Houston/Galveston derived forecasts for the bayous and small

basins by tapping the expertise of the engineers at the Harris County Flood Control District,

using ALERT data, and interpreting their local radar information. Harris County has initiated a

project to develop new forecast tools for these small basins and has invited NWS participation in

this effort. The NWS Advanced Weather Interactive Processing System (AWIPS) Release 5.1.2

includes a site-specific hydrologic model which can be used for small-gauged basins. 

(Finding 2)
17

Figure 6. Harris County OEM rainfall storm totals, June 4-10, 2001. (Courtesy of Harris

County OEM)
18

The WGRFC generates Mean Areal Precipitation (MAP) averages from radar rainfall

estimates and observed rainfall measurements. Currently, coverage of this product does not

include the Houston metropolitan area. MAP estimates would have been useful for assessing the

flood risk and supplementing the analysis where gauges exist. (Finding 3)

The WFO was well staffed during this extended event. The administrative support

assistant came in on June 9 to augment the operations staff and provide critical communications

support. An amateur radio operator collected valuable reports from the flooded areas and was

prepared to provide backup communications with key partners if needed.

The assessment team learned that excellent working relationships exist between WFO

Houston/Galveston and its partners and customers. During Allison, these relationships resulted

in close cooperation and effective two-way exchange of information. A strong performance by

the WFO produced a high level of customer satisfaction and praise. Montgomery County

Emergency Manager Jim Strong stated, “This was the best prediction of flooding that we have

received in 10 years...Bill Read and his staff nailed it.” David Lamb, Houston County

Emergency Manager, had “nothing but praise” for the WFO. Jim White, Emergency

Management Coordinator for Harris County, described his organization’s relationship with the

WFO as “extremely good.” A similar response was provided by Craig McDowell, Emergency

Manager for the City of Houston. Bob White of the City of Houston OEM stated there was

“good information flow throughout the event...no surprises.” Mr. White also noted, “NWS uses

urban and small stream flood advisories real effectively.” He also expressed appreciation for the

WFO’s willingness to include information from the OEM in its messages.

Media partners responded with favorable comments as well. Ed Brandon of KTRK-TV

stated, “I thought their performance was superb. They were quick to respond and pretty explicit

about the severity of the event.” This was echoed by Frank Billingsly at KPRC who stated,

“WFO Houston never let its guard down. I thought the office did an excellent job of conveying

the severity and urgency of the event.” Hector Villarreal of KXLN simply commented, “The

NWS did the job for us.” 

WFO Lake Charles, Louisiana

WFO Lake Charles served its customers in an excellent manner during this historic flood

event. A Special Weather Statement issued by WFO Lake Charles early on Monday, June 4,

cautioned heavy rains were possible on Tuesday, June 5. This information was included again in

an early morning statement on June 5, followed around noon by a very detailed statement on

local impact. It noted, “HEAVY RAIN BANDS WILL MOVE NORTHWARD FROM THE

COAST. RAINFALL AMOUNTS OF 3 TO 5 INCHES WILL BE POSSIBLE BY

WEDNESDAY MORNING. AT THIS TIME...RIVER LEVELS WILL RISE TO NEAR

FLOOD STAGE.” These actions occurred well before the formation of Tropical Storm Allison

and the onset of heavy rain. On the afternoon of June 5, the WFO issued a Flash Flood Watch

for all of the counties and parishes it serves. The lead time was 18 hours.
19

Between June 6 and June 10, the WFO Lake Charles CWA experienced 16 flash flood

events. Only one of the events was not preceded by a warning for an accuracy of 94 percent. 

The WFO issued 47 flash flood warnings. Eleven of these had no subsequent flooding event

associated with them, for a false alarm rate of 24 percent. Lead time averaged 24 minutes. Flash

flood warnings contained detailed information for partners and customers. To increase feedback,

WFO Lake Charles included a phone number in its warning messages to report flooding. A staff

member was assigned to handle these calls.

The Jefferson County Drainage District No. 6 operates an ALERT system in southeast

Texas. While some ALERT data is periodically faxed to WFO Lake Charles by the District, no

Web site or other real-time connection exists for the WFO to access this data. The WFO is

working with the District to obtain this data on a continuous basis. 

The NOAA Weather Radio (NWR) transmitter site at Lafayette was flooded on

Wednesday, June 6. The system remained off the air until flood waters had receded and a

repairman could get to the site. Operations resumed on the evening of Sunday, June 10. During

this period, critical warning information was disseminated via normal media outlets, the

Emergency Alert System (EAS), etc. Some overlap coverage was provided by the NWR

transmitter in Morgan City, Louisiana. The MIC has taken action to minimize the recurrence of

this outage.

The WFO coordinated well with the WGRFC and LMRFC and provided timely river

flood warnings for the Calcasieu River. During the event, the WFO contacted Jefferson County

Emergency Management on a routine basis and also exchanged information with the Drainage

District. Both of these agencies relied heavily on the WFO Web site for radar and other

products. The overall coordination effort with partners was extensive and effective. Users were

very complimentary. Cecil Peltier, Assistant District Manager of the Jefferson County Drainage

District No. 6, stated, the NWS did an “excellent job.” Darlene Koch, Jefferson County

Emergency Manager, was very complimentary about overall services and coordination provided

by WFO Lake Charles. She stated she can “always rely on NWS.”

In May, more than 300 elected officials, emergency service, and media representatives

attended a Hurricane Workshop. The key topic was response. Additional workshops were held

at other locations in the WFO service area.

WFO staff focal points are used to manage the office hydrology program with assistance

provided by the Service Hydrologist at WFO New Orleans/Baton Rouge.

WFO New Orleans/Baton Rouge, Louisiana

WFO New Orleans/Baton Rouge issued an initial Flash Flood Watch for most of

southeast Louisiana at 7:30 p.m. on Tuesday, June 5. This was about 16 hours before the first

flooding in Terrebonne Parish and up to 21 hours prior to flooding in other southeast Louisiana
20

parishes. At 10:52 p.m., June 5, a River Flood Watch was issued for the Amite and Comite

Rivers. The threat to the Amite, Comite, and smaller rivers draining into Lake Pontchartrain was

again highlighted in a Flash Flood/Flood Watch issued at 10:54 a.m. on June 6. All of these

rivers flooded within 72 hours.

 Between June 5 and June 11, WFO New Orleans/Baton Rouge issued 87 flash flood

warnings for the Louisiana portion of its service area. Thirty were not followed by a flash flood

event, for a false alarm rate of 34 percent. Thirty flash flood events occurred in the WFO service

area in Louisiana. Five were not preceded by a Flash Flood Warning, for an accuracy of

83 percent. The average lead time was 39 minutes.

WFO New Orleans/Baton Rouge issued over 140 river flood warnings and statements

during a 7-day period. These flood warnings and statements included forecasts for up to

15 locations along the river. At times, multiple warnings were issued for as many as 13 forecast

points on three different basins. On a few occasions, the interval was excessive between the

LMRFC’s issuance of river forecasts and WFO New Orleans/Baton Rouge’s issuance of river

flood warnings and statements. At an emergency management meeting sponsored by the Amite

River Basin Commission in July, there was strong customer support for more rapid dissemination

of river forecasts and warnings. In response, the WFO New Orleans/Baton Rouge MIC and the

LMRFC HIC have identified procedural changes and training requirements that will address this

need and reduce lag time between the LMRFC and WFO issuances. (Finding 4)

The WFO coordinated extensively with state and local partners during the event, logging

over 60 coordination calls. Following Allison, the mayor of Baton Rouge noted in a letter to the

WFO, “The abundance of information your office provides continues to aid our response

community in decision making as we work to ensure our common goal—public safety.” 

In addition, the WFO coordinated closely with HPC on rainfall forecasts. A Flash Flood

Decision Tree developed by WFO New Orleans/Baton Rouge proved valuable during the event

and was used to make adjustments to the QPF forecasts.

Southern Region Headquarters

SRH provides supervisory, administrative, operational, and technical oversight and

support to the WFOs and RFCs in its area of responsibility. The Regional Operations Center

(ROC) is a component of SRH and serves as a coordination and support unit for field offices. 

During significant weather events, the ROC is staffed 24 hours a day and provides a variety of

services, including communication system support, field office coordination services, media, and

interagency support.
21

During the period of heaviest rainfall associated with Allison, the ROC helped provide a

larger picture of the event. Services included:

C Twice-a-day hydrometeorological briefings to the Texas DEM

C Coordination and support to field offices as requested

C A wide variety of media interviews.

Local media interviews were handled by the respective field office, while national and

international media interviews were conducted by SRH/ROC staff and the NOAA Public Affairs

Specialist assigned to SRH. Interviews were provided to CNN, CNN.com, MSNBC, CNBC,

ABC World News Tonight, and GHTV-London. The WFOs indicated this assistance was

valuable since it allowed them to focus their support to local media and other partners.

Jack Colley, Assistant State Coordinator for the Texas DEM, stated, “Southern Region

Headquarters and River Forecast Center participation in our conference calls was essential to

deploying resources and ensuring the safety of recovery workers.” The Texas DEM State

Coordinator, Tom Millwee, was also very appreciative of the assistance provided by SRH and the

WFOs in Texas.

Partner and Customer Coordination and Response

The effectiveness of the weather and flood warning system is dependent on close

coordination, cooperation, and a clear consistent message among the various agencies

responsible for public safety. This includes the NWS, emergency management community, flood

control/public works agencies, the media, and state and local governments.

Emergency Management and Flood Control/Public Works Agencies

Coordination between NWS offices and partners in emergency management and flood

control/public works agencies was excellent. This cooperative effort extended beyond the

normal WFO-local emergency manager relationship to include RFCs, SRH, the Louisiana Office

of Emergency Preparedness, and TDEM. It was evident to the assessment team that over an

extended period of time, a high level of respect and trust had been built up between the NWS and

these partners.

Most of the emergency managers interviewed participated in annual NWS workshops,

had prior interactions with the WFOs, and were aware of the full suite of products available to

them. As a result, they were proactive in monitoring weather conditions and the WFOs’ AFDs

and statements, and were not caught off guard by the heavy rainfall. These preparedness

activities paid off once a Tropical Storm Warning for Allison was issued. Within an hour of

issuance, the mayor of Galveston called for a voluntary evacuation of the west end of Galveston

Island since that area is not protected by the seawall. The issuance of the warning, subsequent
22

conference calls, and follow-up statements resulted in activation of many Emergency Operation

Centers.

Emergency management partners were aggressive in making NWS information available

to local decision makers and the public. In Houston, the Harris County Appraisal District

operates an Emergency Managers Weather Information Network (EMWIN) with the cooperation

of Harris County OEM, the Texas National Guard, and the NWS. The NWS provides a data

stream via satellite of national and local NWS text and graphic products. The Houston EMWIN

system captures this signal and re-broadcasts the information over VHF radio frequencies. With

a simple radio receiver and personal computer, the information can be displayed and

subsequently provided to other users or the public via the Internet. The system also has pager

activation capabilities.

The Internet site operated by the Harris County OEM was a major contributor to the

success of the warning effort in southeast Texas. This site was a “one-stop” point for

information and coordination. It provided immediate access to real-time and historic rainfall and

river stage information; links to EMWIN, local radar, and news media outlets; rapid exchange of

information among local governments; and awareness and safety information. Harris County

also supports an e-mail list server, the Media Alert Notification System (MANS), which provides

the latest information to more than 140 media outlets in the Houston area.

With the cooperation of a Lake Charles Mutual Aid Association and the local emergency

manager, WFO Lake Charles can immediately broadcast NWS warning information over the

Mutual Aid Association’s radio system to 100 major business and government organizations. 

Mutual aid organizations in Baton Rouge and Beaumont also disseminate NWS information and

collect reports of hazardous weather. Other examples of assistance include Friendswood, Texas,

where the emergency management office re-broadcasts NWR programming on a city-owned AM

radio station. In Polk County, Texas, about 200 NWRs were distributed to key locations such as

hospital and schools. In Pasadena, Texas, the emergency manager used an e-mail notification

system and personal briefings to provide weather information as well as its expected impact

information to all city officials.

Both the Harris County OEM and the Harris County Flood Control District expressed a

strong need for river forecasts to be issued in reference to Mean Sea Level (MSL). (Finding 5)

Broadcast Media

Upon assuming responsibility for Allison from TPC, the HPC had to support the

significant requirements of the national news media. While this transition placed heavy demands

upon HPC resources, it was accomplished in an effective manner that impressed the news media.

WFO Houston/Galveston held a Hurricane Workshop in May 2001 for local media,

providing details on NWS products and dissemination. After the Allison floods, television
23

stations were complimentary, stating the workshop before the hurricane season gave them a

much clearer understanding and appreciation of the WFO’s operations and products. Workshops

held by WFO Lake Charles received similar praise and results.

The Houston broadcast media indicated WFO Houston/Galveston was very accessible

and user friendly prior to and during the flood event. Frank Billingsly of KPRC in Houston

noted there was an “open line of communication to the WFO at all times.” Products were

described as timely and informative, and addressed the appropriate level of concern for the

growing threat to life and property. Hector Villarreal of KXLN in Houston stated, “All the

watches and warnings were concise and precise. I could read them right off the copy.”

All of the media interviewed used the AFD on a daily basis and found it a valuable tool,

providing insight into the severity of the event. They were also impressed with the reliability and

accuracy of WSR-88D rainfall estimates. During the height of the event in Houston, the intensity

of the rain disrupted the microwave signal from the vendor that provided data and warning

products to one of the television stations. The station immediately went to the NWS Internet site

to obtain WSR-88D data and NWS products. The television meteorologist at Baton Rouge

indicated the Internet was a critical part of his operations.

Some weathercasters expressed a desire for more information on road closings. Others

noted the information came so quickly there was little time to explain what was happening and

provide safety information. Two interviewees expressed a preference for the use of the word

“Warning” rather than “Advisory” for urban street flooding. One television station noted the

written text format of river flood warnings and statements was too cumbersome for rapid

television dissemination.

Several emergency managers noted the media was very proactive at covering the event

and relaying NWS information to the public on the seriousness of the situation. Jennifer Shields

Hawes, City of Pasadena Emergency Manager, stated, “the media had continuous coverage of

the event and provided a lot of safety information to the public.”

The EAS is a voluntary effort on the part of the broadcast media to ensure rapid relay and

dissemination of critical life-saving information. There was widespread cooperation from the

media, and the EAS was activated in both Louisiana and Texas, ensuring wide dissemination of

warning messages.

Public

There were no flood related deaths from Allison during the first 3 days of the event. With

the onset of torrential rainfall on the evening of Friday, June 8, the first fatality in Texas

occurred. Most of the 22 deaths in Houston occurred from around midnight to the early morning

hours of Saturday, June 9.
24

In Houston, three people died in their homes by electrocution. Seven drowned in their

vehicles. Two of these deaths occurred after the rainfall ended on Sunday afternoon, June 10. 

A couple’s vehicle was swept off a bridge while they were trying to reach their daughter who was

isolated by flood waters but in no danger. Nine people died while on foot; three of these when

flood waters forced them to flee their vehicles. Two residents at an assisted living facility

drowned after getting separated from a group of patients and staff who were walking between the

nursing home and a nearby hospital. A woman died when she went to move her car from a

flooded parking garage four floors below ground in a high-rise building. In addition to the

22 deaths in Houston, a man was killed while swimming in a drainage ditch at Mauriceville

(Orange County), Texas, on June 9 (see Appendix B).

It is difficult to understand why there were 22 fatalities on June 8-9. Sunshine returned to

Houston on the morning and early afternoon of June 8, and WFO Houston/Galveston was clearly

concerned about a possible drop in the public’s perception of the flood risk as several of its

messages stressed the threat had not ended. The ground was saturated from the previous days’

rains and had limited ability to absorb additional rainfall. With the bayous and creeks already

full of water, the rainfall that night produced almost instantaneous flooding. While heavy rains

are not unusual in Houston, many individuals have not experienced intense rainfall rates of 4 to

5 inches an hour. In this case, the water spread outside the traditional flood plain into areas that

may never have flooded before. The flooding of areas outside of the historical flood plains and

inexperience with sustained excessive rainfall rates of 4 to 5 inches an hour are factors that were

in place on June 8-9 when most of the fatalities occurred. With the exception of the two deaths

on Sunday afternoon, June 10, all of the Houston fatalities perished while a Flash Flood Warning

was in effect. Other than rapidly flowing water in a ditch, there was no flash flood threat in

Orange County when the man died.

The sole fatality in Louisiana occurred at 6:30 a.m., June 7, about 3 miles north-northwest

of Zachary, Louisiana. A small tornado blew a tree onto a vehicle occupied by a 70-year-old man

(see Appendix B). No warning was in effect at that time.
25

Facts

Introduction

FACT: Tropical Storm Allison caused more damage than any tropical storm in

U.S. history, with estimates in excess of $5 billion.

FACT: Storm rainfall totals peaked at 36.99 inches (Port of Houston) in Texas and

29.86 inches (Thibodaux) in Louisiana. 

Overview

FACT: In Houston, Texas, 22 people were killed and more than 45,000 homes and

businesses flooded. More than 70,000 vehicles were flooded. In addition, a man

was killed while swimming in a drainage ditch at Mauriceville (Orange County),

Texas, on June 9.

FACT: In terms of storm rainfall totals and area impacted, Allison ranks among the worst

to hit Louisiana in the past 100 years.

FACT: In Louisiana, the only fatality occurred near Zachary when a small tornado caused

a tree to fall onto a vehicle, killing its occupant.

Warning and Forecast Services

FACT: Rainfall forecasts in TPC products were effectively coordinated with HPC, the

WFOs, and the RFCs. Rainfall amounts of 4 to 5 inches with isolated amounts of

8 to 10 inches were forecast. Observed totals were generally in the 4- to 6-inch

range in the Houston area with isolated amounts of 8 to 10 inches in eastern

Harris County.

FACT: The Excessive Rainfall Potential Outlook issued by HPC at 12:25 p.m. on Friday,

June 8, correctly highlighted the overnight flash flood event that claimed 22 lives

in Houston.

FACT: The comparison of HPC’s cumulative QPF forecasts to the rainfall totals for

June 5 to June 12 documents a high level of accuracy and strong performance by

HPC forecasters.
26

FACT: Rainfall in excess of 20 inches produced moderate to major flooding at seven

LMRFC river forecast points. The LMRFC provided excellent lead times for

these locations. For rivers in the Lake Pontchartrain drainage, lead times to flood

stage ranged from 18 hours at two of the forecast points to 31 hours at three other

sites. For the two forecast points in the Calcasieu Basin, the lead times to flood

stage were 18.5 hours; and 2 days, 11.5 hours; respectively. 

Weather Forecast Offices

FACT: As early as Wednesday, June 6, WFO Houston/Galveston forecasters increased

the likelihood of rain in the forecast for Friday and Saturday, June 8-9, in

anticipation of another period of excessive rainfall. In the Thursday afternoon,

June 7, Area Forecast Discussion (AFD), the forecaster cautioned “CANNOT

RULE OUT SEVERAL MORE ROUNDS OF 5 TO 10 INCH RAINFALL

EVENTS...OVER THE WEEKEND.”

FACT: Between June 5 and June 10, WFO Houston/Galveston experienced 39 flash flood

events in its service area and issued 99 flash flood warnings. All of the events

were included in a warning for an accuracy of 100 percent. Nine warnings had no

subsequent flash flood event associated with them for a false alarm rate of

9 percent. This performance was outstanding when compared to national

performance during 2000 (accuracy was 86 percent and false alarm rate was

38 percent). The average lead time was 40 minutes.

FACT: Excellent working relationships exist between WFO Houston/Galveston and its

partners and customers. During Allison, these relationships resulted in close

cooperation and effective two-way exchange of information. A strong

performance by the WFO produced a high level of customer satisfaction and

praise.

FACT: A Special Weather Statement issued by WFO Lake Charles early on Monday,

June 4, cautioned heavy rains were possible on Tuesday, June 5. On the afternoon

of June 5, the WFO issued a Flash Flood Watch for all of the counties and

parishes it serves. The lead time was 18 hours.

FACT: Between June 6 and June 10, the WFO Lake Charles CWA experienced 16 flash

flood events. Only one of the events was not preceded by a warning for an

accuracy of 94 percent. The WFO issued 47 flash flood warnings. Eleven of

these had no subsequent flooding event associated with them, for a false alarm

rate of 24 percent. Lead time averaged 24 minutes.
27

FACT: Between June 5 and June 11, WFO New Orleans/Baton Rouge issued 87 flash

flood warnings for the Louisiana portion of its service area. Thirty were not

followed by a flash flood event, for a false alarm rate of 34 percent. Thirty flash

flood events occurred in the WFO service area in Louisiana. Five were not

preceded by a Flash Flood Warning, for an accuracy of 83 percent. The average

lead time was 39 minutes.

FACT: Jack Colley, Assistant State Coordinator for the Texas DEM, stated, “Southern

Region Headquarters and River Forecast Center participation in our conference

calls was essential to deploying resources and ensuring the safety of recovery

workers.”

Partner and Customer Coordination and Response

FACT: Coordination between NWS offices and partners in emergency management and

flood control/public works agencies was excellent. This cooperative effort

extended beyond the normal WFO-local emergency manager relationship to

include RFCs, SRH, the Louisiana Office of Emergency Preparedness, and

TDEM. 

FACT: The Internet site operated by the Harris County OEM was a major contributor to

the success of the warning effort in southeast Texas. This site was a “one-stop”

point for information and coordination. It provided immediate access to real-time

and historic rainfall and river stage information; links to EMWIN, local radar, and

news media outlets; rapid exchange of information among local governments; and

awareness and safety information. Harris County also supports an e-mail list

server, the Media Alert Notification System (MANS), which provides the latest

information to more than 140 media outlets in the Houston area.

FACT: The Houston broadcast media indicated WFO Houston/Galveston was very

accessible and user friendly prior to and during the flood event.

FACT: The flooding of areas outside of the historical flood plains and inexperience with

sustained excessive rainfall rates of 4 to 5 inches an hour are factors that were in

place on June 8-9 when most of the fatalities occurred. With the exception of the

two deaths on Sunday afternoon, June 10, all of the Houston fatalities perished

while a Flash Flood Warning was in effect.
28

Interstate 10 North Loop, Houston, Texas. (Courtesy of Harris County Flood Control District/

Steve Fitzgerald)
29

Findings and Recommendations

Warning and Forecast Services

Finding 1: Forecasting proved to be difficult for some small rivers/bayous that

rose rapidly to record levels. Forecasts for Buffalo Bayou at Shepherd

Drive in Houston underforecast the river stage by 6 to 12 feet with

little or no lead time. Also, the forecasts for the West Fork of the San

Jacinto River near Conroe were not accurate. Both software and

procedural problems have been identified by the WGRFC and the

NWS Office of Hydrologic Development as contributing to these

inaccuracies.

Recommendation 1a: The WGRFC, with the assistance of the NWS Office of Hydrologic

Development, should make the necessary software corrections and

implement procedural changes needed for small basins that respond

rapidly during heavy rainfall. These changes should be made by

April 1, 2002.

Recommendation 1b: Regions should evaluate RFC procedures for small basins that respond

rapidly during heavy rainfall. RFCs should be tasked with changing

procedures where inadequate.

Weather Forecast Offices

Finding 2: Because of the small size of the bayous in the Houston area, most are

not included in the WGRFC forecast system. Harris County has

initiated a project to develop new forecast tools for these small basins

and has invited NWS participation in this effort. The NWS Advanced

Weather Interactive Processing System (AWIPS) Release 5.1.2

includes a site-specific hydrologic model which can be used for smallgauged basins.

Recommendation 2a: The WFO Houston/Galveston MIC and the WGRFC HIC should meet

with the appropriate Harris County officials to determine the NWS role

in the project. Once the NWS role is determined, progress will be

tracked through the follow-up service assessment reporting process.
30

Recommendation 2b: Once the AWIPS site-specific hydrologic model is deployed in

Build 5.1.2, WFO Houston/Galveston should determine what

contribution this model would make to local flood warning operations.

Finding 3: The WGRFC generates Mean Areal Precipitation (MAP) averages

from radar rainfall estimates and observed rainfall measurements. 

Currently, coverage of this product does not include the Houston

metropolitan area. MAP estimates would have been useful for

assessing the flood risk and supplementing the analysis where gauges

exist.

Recommendation 3a: The WGRFC should extend the MAP analysis across the Houston

metropolitan area to the Texas coast.

Recommendation 3b: Regions should evaluate MAP coverage in their regions. RFCs should

be tasked with extending MAP coverage where inadequate.

Finding 4: On a few occasions, the interval was excessive between the LMRFC’s

issuance of river forecasts and WFO New Orleans/Baton Rouge’s

issuance of river flood warnings and statements. At an emergency

management meeting sponsored by the Amite River Basin

Commission in July, there was strong customer support for more rapid

dissemination of river forecasts and warnings. In response, the WFO

New Orleans/Baton Rouge MIC and the LMRFC HIC have identified

procedural changes and training requirements that will address this

need and reduce lag time between the LMRFC and WFO issuances.

Recommendation 4: The WFO New Orleans/Baton Rouge MIC, with the assistance of the

LMRFC HIC, should develop and implement a plan to change WFO

procedures and provide training to staff to ensure rapid dissemination

of river forecasts and river flood warnings.

Partner and Customer Coordination and Response

Finding 5: Both the Harris County OEM and the Harris County Flood Control

District expressed a strong need for river forecasts to be issued in

reference to Mean Sea Level (MSL). 

Recommendation 5: WFO Houston/Galveston should provide river stages in MSL in river

flood warnings and statements.
31

Best Practices

1. Contingency River Forecasts

Both the LMRFC and WGRFC issued contingency river forecasts based on worst case

precipitation forecasts. In several cases, these forecast the onset of major flooding well in

advance of the event. The contingency forecasts were also the basis for several river

flood watches.

2. Internal River Flood Watch

The WGRFC produces an experimental River Flood Watch that is disseminated to the

public by the WFOs. This experimental product provided information that highlighted

the likelihood of a major flood. Thirteen WGRFC river flood watches were produced

with an average lead time to crest of 36 hours. In the case of the San Jacinto River, an

experimental River Flood Watch resulted in a lead time of almost 48 hours.

3. Phone Number in Warning Messages

To increase feedback, WFO Lake Charles included a phone number in its warning

messages to report flooding. A staff member was assigned to handle these calls.

4. Flash Flood Decision Tree

A Flash Flood Decision Tree developed by WFO New Orleans/Baton Rouge proved

valuable during the event and was used to make adjustments to the QPF forecasts.

5. Southern Region Headquarters/Public Affairs Assistance with

Interviews

Local media interviews were handled by the respective field office, while national and

international media interviews were conducted by SRH/ROC staff and the NOAA Public

Affairs Specialist assigned to SRH. This assistance allowed the WFOs to focus their

support to their local media and other partners where life-saving dissemination was

critical.

6. WFO Pager System

The WFO Houston/Galveston staff is capable of activating the pagers of key emergency

managers and government officials in their area. With Allison’s rapid formation, the

WFO pager system was a vital notification method for the emergency management

community. This capability was used throughout the event to provide rapid

dissemination of new information.
32

National Guardsmen leading a truck through a flooded Houston, Texas, neighborhood. 

(Courtesy of Jason Hinkel, Galveston County Office of Emergency Management, and Wayne

Brown, Risk Manager for Galveston County)
A-1

Appendix A

Tropical Storm Allison 

Rainfall Storm Totals

(Inches)

Southeast Texas Reports from WFOs Houston/Galveston and Lake Charles

Port of Houston (Harris County) 36.99 Pennington (Houston County) 15.60

Beaumont Research (Jefferson County) 27.24 Tomball Hooks Airport (Harris County) 15.02

Pearland Clover Fld. (Brazoria County) 21.41 Hunstville (Walker County) 13.01

Houston Hobby Airport (Harris County) 20.84 Segno (Polk County) 12.99

Deer Park (Harris County) 20.50 Sugarland Airport (Fort Bend County) 12.17

Westbury (Harris County) 19.53 Newton (Newton County) 12.08

League City, NWS (Galveston County) 19.41 Alvin (Brazoria County) 11.23

Conroe (Montgomery County) 17.48 Baytown (Harris County) 9.83

Bush Intercontinental (Harris County) 16.48 Galveston (Galveston County) 9.77

Harris County Office of Emergency Management ALERT System

Greens Bayou at Mt. Houston Pkwy 38.78 Garners Bayou at Beltway 8 24.72

Hunting Bayou at I-10 35.83 Buffalo Bayou at Turning Basin 22.91

Greens Bayou at Ley Road 33.66 Clear Creek at Telephone Road 22.56

Cowart Creek at Baker (Friendswood) 28.31 Greens Bayou at Bammel N Houston 20.39

Vince Bayou at W. Ellaine 26.85 Brays Bayou at Stella Link 19.76

Hunting Bayou at Lockwood 25.12 Buffalo Creek at Milam 16.02
A-2

Southern Louisiana Reports from WFOs New Orleans/Baton Rouge and Lake Charles

Thibodaux (Lafourche Parish) 29.86 Folsom (Saint Tammany Parish) 19.66

Salt Point (Saint Mary Parish) 27.55 Convent (Saint James Parish) 19.59

Slidell, NWS (Saint Tammany Parish) 21.35 Baton Rouge (East Baton Rouge Parish) 19.15

Gretna/Terrytown (Jefferson Parish) 21.30 St. Francisville (West Feliciana Parish) 19.11

Morgan City (Saint Mary Parish) 20.96 Paradis (Lafourche Parish) 18.45

Reserve (Saint John The Baptist Parish) 20.62 Bogalusa (Washington Parish) 17.66

Breaux Bridge (Saint Martin Parish) 20.13 Jackson (East Feliciana Parish) 17.11

Pontchatoula (Tangipahoa Parish) 20.03 New Orleans Int’l (Jefferson Parish) 14.33
B-1

Appendix B

Texas and Louisiana Fatalities

Directly Attributed to

Tropical Storm Allison

Houston, Texas

Date Cause Age Gender Circumstances

6/8/01 Drowning 70 Male In automobile

6/8/01 Drowning 38 Male Fleeing flooded automobile

6/8/01 Drowning 34 Female Fleeing flooded automobile

6/8/01 Drowning 21 Male Unknown

6/9/01 Drowning 22 Male Pickup washed into underpass

6/9/01 Electrocution 53 Female Grabbed an antenna

6/9/01 Electrocution 30 Male Grabbed person holding antenna

6/9/01 Drowning 57 Male Van overturned in flood waters

6/9/01 Drowning 23 Male Walking to friend’s house

6/9/01 Drowning 65 Male Homeless man found under bridge

6/9/01 Drowning 37 Male Vehicle swept off bridge

6/9/01 Drowning 32 Male Walking

6/9/01 Drowning 62 Male Evacuation of assisted care facility

6/9/01 Drowning 55 Female Evacuation of assisted care facility

6/9/01 Drowning 21 Male Fleeing vehicle; rescue attempt failed

6/9/01 Drowning 35 Female Walking

6/9/01 Drowning 44 Male Walking

6/9/01 Drowning 52 Male Last seen in car

6/9/01 Electrocution 45 Female Holding iron cord

6/9/01 Drowning 42 Female Wall of water slammed into elevator 

6/10/01 Drowning 71 Male Truck washed into ditch

6/10/01 Drowning 69 Female Truck washed into ditch
B-2

Mauriceville, Texas

06/9/01 Drowning 30s Male Swimming in a ditch

3 Miles North of Zachary, Louisiana

6/7/01 Trauma 70 Male Small tornado knocked down tree on vehicle
C-1

Appendix C

HPC Forecast Operations Branch

Monthly QPF Threat Scores

24-Hour Day 1 Forecast

June 2001

Precipitation Threshold HPC Threat Score Best Model Threat Score

0.5 inch .333 .296 (ETA)

1.0 inch .235 .198 (ETA)

2.0 inch .201 .128 (ETA)

3.0 inch .205 .015 (Aviation)

For the four increments of rainfall on the left, the table above shows a comparison of the HPC

threat scores with the best threat score of the NCEP numerical forecast models. The threat score

is based on the overlap between the forecast and observed areas of precipitation greater than the

threshold precipitation value. If there is no overlap between the forecast and observed areas, the

threat score has a value of 0, and if there is complete overlap (a perfect forecast), the value is 1. 

The scores are for the first 24 hours of the QPF forecast.

The HPC scores for each of the four thresholds are record high values for the month of June. 

The 3-inch score in June was mainly associated with Allison’s rains.