REPORT TO CONGRESS

GAPS IN NEXRAD RADAR COVERAGE 

Developed pursuant to: Section 414 of the Weather Research and Forecasting Act of 2017, 

Public Law 115-25, and the Consolidated Appropriations Act, 2017, Public Law 115-31, and the 

accompanying House Report 114-605 
Louis W. Uccellini, Director

National Weather Service

National Oceanic and Atmospheric Administration

Neil A. Jacobs, Ph.D. 

Assistant Secretary of Commerce for Environmental Observation and Prediction, 

Performing the Duties of Under Secretary of Commerce for Oceans and Atmosphere
PUBLIC LAW 115-25, THE WEATHER RESEARCH AND FORECASTING ACT OF 2017, 

SECTION 414, INCLUDED THE FOLLOWING LANGUAGE

(a) STUDY ON GAPS IN NEXRAD COVERAGE.— 

(1) IN GENERAL.—Not later than 180 days after the date of the enactment of this 

Act, the Secretary of Commerce shall complete a study on gaps in the coverage of 

the Next Generation Weather Radar of the National Weather Service 

(‘‘NEXRAD’’). 

(2) ELEMENTS.—In conducting the study required under paragraph (1), the 

Secretary shall— (

A) identify areas in the United States where limited or no NEXRAD 

coverage has resulted in— 

(i) instances in which no or insufficient warnings were given for 

hazardous weather events, including tornadoes; or 

(ii) degraded forecasts for hazardous weather events that resulted 

in fatalities, significant injuries, or substantial property damage; 

and 

(B) for the areas identified under subparagraph (A)— 

(i) identify the key weather effects for which prediction would 

improve with improved radar detection; 

(ii) identify additional sources of observations for high impact 

weather that were available and operational for such areas on the 

day before the date of the enactment of this Act, including dense 

networks of x-band radars, Terminal Doppler Weather Radar 

(commonly known as ‘‘TDWR’’), air surveillance radars of the 

Federal Aviation Administration, and cooperative network 

observers; 

(iii) assess the feasibility and advisability of efforts to integrate 

and upgrade Federal radar capabilities that are not owned or 

controlled by the National Oceanic and Atmospheric 

Administration, including radar capabilities of the Federal 

Aviation Administration and the Department of Defense;

(iv) assess the feasibility and advisability of incorporating Stateoperated and other non-Federal radars into the operations of the 

National Weather Service; 

(v) identify options to improve hazardous weather detection and 

forecasting coverage; and 

(vi) provide the estimated cost of, and timeline for, each of the 

options identified under clause (v). 

(3) REPORT.—Upon the completion of the study required under paragraph (1), 

the Secretary shall submit to the Committee on Commerce, Science, and 

Transportation of the Senate and the Committee on Science, Space, and 

Technology of the House of Representatives a report that includes the findings of 

the Secretary with respect to the study. 
PUBLIC LAW 115-31, CONSOLIDATED APPROPRIATIONS ACT, 2017, HOUSE REPORT

114-605, INCLUDED THE FOLLOWING LANGUAGE

 

NEXRAD Coverage Report.—NOAA shall complete a study on gaps in NEXRAD 

coverage. Within this study, NOAA shall identify areas in the United States with limited 

or no NEXRAD coverage below 6,000 feet above ground level of the surrounding terrain. 

NOAA should identify the effects on prediction of improved radar detection, and identify 

additional sources of observations for high impact weather that are currently available 

and operational for such areas. NOAA shall assess the feasibility and advisability of 

efforts to integrate and upgrade Federal radar capabilities and incorporate other nonNOAA radars into NWS operations in such areas, and the cost and timeline for carrying 

out such radar improvements. NOAA shall submit the study findings to the Committee 

within 180 days of enactment of this Act. Not later than 30 days after the completion of 

the study, NOAA shall develop a plan to improve radar coverage in the identified areas.

THIS REPORT RESPONDS TO REQUESTS FROM THE COMMITTEE ON COMMERCE, 

SCIENCE, AND TRANSPORTATION OF THE SENATE; THE COMMITTEE OF SCIENCE, 

SPACE, AND TECHNOLOGY OF THE HOUSE OF REPRESENTATIVES REQUEST; AND 

THE SENATE AND HOUSE APPROPRIATIONS COMMITTEES.
TABLE OF CONTENTS

I. Executive Summary 6 

II. Background 7 

III. Study Methodology 11 

IV. Tornado Warning Performance 11 

V. Flash Flood Warning Performance 15 

VI. Other Observational Sources Available 20 

VII. Options to Improve Hazardous Weather Detection and 

Forecasting Coverage 24 

VIII. Cost and Timeline for Hazardous Weather Detection and 

Forecasting Coverage 26 

IX. Summary 26 

List of Figures

Figure 1. NEXRAD coverage below 10,000 ft. AGL. 7

Figure 2. Tornado event fraction (from all events) as a function of 

tornado events per state for 2008-2016. 12

Figure 3. Warned tornado event fraction (from all events) as a 

function of tornado events per state for 2008-2016. 14

Figure 4. Tornado false alarm ratio (FAR) for 2008-2016, and 

percentage of WFO-area covered by NEXRAD. Fitted line is 

essentially insensitive to radar coverage.

15 

Figure 5. Ratio of warned (red) to unwarned (cyan) flash flood 

events from 2008-2016 normalized by the total number of events 

inside or outside the 6 kft. beam bottom altitude according to the 

radar coverage type (within radar coverage, outside of radar 

coverage, or crossing limit of radar coverage). 

17 

Figure 6. Flash flood (6a) warned-event ratio (probability of 

detection), (6b) false alarm ratio, (6c) critical success index, (a 

performance measure including warned and unwarned events), and 

(6d) mean lead time between 2008-2016 and percentage of WFOarea covered by NEXRAD.

18-20 
6 

I. Executive Summary

The Weather Research and Forecast Innovation Act of 2017 (Public Law 115-25) (aka the 

Weather Act) directs the Secretary of Commerce to conduct a study to assess the impact of 

limited radar coverage as it pertains to warning performance for hazardous weather events, 

identify other sources of observations for high impact events, and determine the feasibility of 

integrating radar data other than Next Generation Weather Radar (NEXRAD) data into 

operations. The agency submits this report on the findings of that study to the Committee on 

Commerce, Science, and Transportation of the Senate and the Committee on Science, Space, 

and Technology of the House of Representatives. This report is also in response to similar 

House Report 114-605 language accompanying P.L. 115-31, the Consolidated 

Appropriations Act, 2017, directing a NEXRAD Coverage Report, which directs the National 

Oceanic and Atmospheric Administration (NOAA) to examine warning performance 

associated with radar coverage at a range where the beam is 6,000 feet (ft.) above ground 

level (AGL) and higher, and other items similar to the language in the Weather Act. 

What the Study and Report Considered

The study examined the impact of radar coverage on warnings for tornadoes and flash floods, 

although it should be noted that radar also provides necessary observations to support other 

weather warnings (e.g., hail/severe thunderstorms, winter weather, hurricanes, etc.,) that are 

not addressed by the study and this report. Tornadoes and flash floods are rare events in 

general, and only a small fraction of those events cause fatalities. For instance, tornadoes 

causing deaths comprise only about two percent of all tornadoes. To warn the Nation of 

these rare events, trained forecasters use a variety of tools and datasets. Data that come from 

the NEXRAD network are critical for tornado and flash flood warnings, and forecasters also 

use information from satellites, surface observations, storm-spotters, and high resolution 

forecast models (among other sources) to aid in the forecast and warning process. 

What the Study Found 

The study upon which this report is based indicates there is not a significant negative impact 

to warning performance tied directly to radar coverage where the beam is higher than 6,000 

ft. AGL (i.e., where there is little coverage below 6,000 ft.). That evidence does not mean 

radar is unimportant, rather it suggests that there is significant, usable radar data above 6,000 

ft. AGL and other information employed by forecasters to issue warnings. This report 

provides explanations as to why the tornado and flash flood performance statistics behave as 

they do. Forecasters, public or private sector, will say that arbitrarily increasing NEXRAD 

coverage would improve their ability to warn. This appears to be contrary to what the 

study’s key findings say, and the reason is that humans issue the warnings, not the radar. For 

tornadoes that result in deaths, damage, and injuries, trained forecasters are able to overcome 

aspects of reduced radar coverage, resulting in warning performance that shows no 

statistically significant difference between inside or outside the 6,000 ft. AGL range. The 

NOAA study found that there are minor differences between the numbers of “misses” (i.e., 

unwarned events) within the range covered by radar beam at 6,000 ft. AGL compared to 

“outside” (i.e., beyond) the range across the contiguous United States (CONUS). 
7 

The study also examined the difference between the most and least intense tornadic events. 

For unwarned tornadic events causing fatality, injury, and/or significant damage, the study 

found weak to no statistical dependence on radar coverage in a linear model. This lack of 

correlation is especially true with the strongest tornadoes, Enhanced Fujita (EF) scale3 EF3–

EF5 events. The bottom line is that radar coverage, even outside of the coverage by radar 

beam at 6,000 ft. AGL, allows forecasters to rarely, if ever, miss warnings for EF3–EF5 

tornadoes because of their ability to use all available data, including data far from the 

operational NEXRAD radar. NOAA’s National Weather Service (NWS) warns for about 

98% of these events. Conversely, weak, short-lived tornadoes are unwarned in many 

locations. Our ability to detect and warn for these tornadoes is consistent for areas covered 

by radar within 6,000 feet and those areas with lesser coverage. Short-lived tornadoes rarely 

cause fatalities or significant damage. Of the tornadoes in the study that went unwarned, 

only 2% of those caused injuries. The study also determined that tornado false alarm rate – 

issuing a warning for a storm that does not eventually reach warning criteria - is insensitive 

to radar coverage. Thus, radar coverage itself bears no discernible impact on false alarm rate. 

Overall, the study showed that flash flood warning performance is more sensitive to radar 

coverage than tornado warning performance. However, the overall number of events seems 

to be a more important determinant of warning skill than radar coverage, and there are more 

tornado and flash floods events where there is more relative radar coverage. This is not 

because the number of recorded tornado and flash floods events is larger, unsurprisingly, 

where radars are present to observe them. Indeed, the radars were sited based in part upon 

historical occurrences of severe weather events. Since no statistical method is necessarily 

perfect or without limitations, the study acknowledges that additional geospatial analyses 

related to radar coverage may provide other information about NOAA’s NWS performance, 

in particular about impact-based events. Moreover, the study is in general agreement with 

similar research published in peer-reviewed articles. 

Future Considerations 

While the study did not reveal significant areas of warning service deficiency due to reduced

radar coverage, the report – per direction – does examine the availability and use of other 

observational data sources and analyzes options to improve hazardous weather detection. In 

summary, NOAA recommends an “all of the above” approach, using ongoing research and 

development to improve the prediction and detection of hazardous weather. These efforts 

include improved numerical model development, analysis of new weather satellite data, 

developing advanced radar technology, employing novel observation systems such as 

Unmanned Aircraft Systems (UAS), and others. NOAA will continue to strive to improve 

weather prediction at all scales.

 

3 The Enhanced Fujita scale (EF-Scale) is a scale for rating tornado intensity based on damage surveys: EF0 (65-85 

mph), EF1 (86–110 mph), EF2 (111-135 mph), EF3 (136-165 mph), EF4 (166-200 mph), F5 (>200 mph).
8 

Figure 1. NEXRAD coverage at the bottom of the radar beam of 4,000, 6,000, and 10,000 ft. AGL.

II. Background 

Tornadoes and flash floods are two of nature’s most violent events, sometimes leaving incredible 

damage and casualties in their wake. In the 1980s, the Department of Commerce, the 

Department of Defense (DoD), and the Federal Aviation Administration (FAA) strategically 

designed the NEXRAD network with geographical locations of radars to provide extensive radar 

coverage, with overlapping radars where possible east of the Rockies, to detect the precursors to 

tornadoes, increase tornado warning lead time, and help improve rainfall estimates which 

contribute to flash flooding. Radar coverage over the western and intermountain areas (defined 

as western Montana, Wyoming, western Colorado, western New Mexico, and all states to the 

west) was increased dramatically through the NEXRAD program, although areas of poor 

coverage remain, especially in the complex terrain of the western United States. Within, 

CONUS the NEXRAD network consists of 142 radars that provide coverage of about 73 percent

of the land mass and 94 percent of the population at the 6,000 ft. AGL range. An additional 17 

NEXRAD sites are deployed in Alaska, Hawaii, U.S. territories, and military bases. The 

deployment of the NEXRAD network achieved its goal: tornado warning lead times increased

from four minutes in the 1980s before NEXRAD to about an 11 minute average over the past 10 

years; and flash flood warning lead time increased from 18 minutes in the 1980s to over 70 

minutes today. 
9 

This study and report examine warning performance in the context of current Government 

Performance and Results Act goals, not future performance as described in Section 103 of the 

Weather Act. With the ongoing Service Life Extension Program, the NEXRAD network should 

be operational for approximately the next 20 years. Efforts are underway now in NOAA to 

define and refine the future requirements for hazardous weather detection post-NEXRAD. A key 

component during the requirements process for a potential follow-on to NEXRAD is to ensure 

that NWS continues to lead the way in hazardous weather warnings. There is a real opportunity 

to improve observation and prediction capabilities, as stated in the Weather Act, far beyond 

current network capabilities, and as highlighted in this report and its attendant study. As is

shown in this report and via data in the study, improving NEXRAD coverage alone is not likely 

to significantly improve warning performance for tornadoes and flash flooding. That is not to 

say that NEXRAD is not important to warning performance, because quite the opposite is true. 

Numerous studies (including this one) have clearly demonstrated the value of NEXRAD for 

saving lives and property, and it cannot be understated that NEXRAD is the single most 

important source of data used by public and private sector forecasters in the tornado and flash 

flood warning decision processes. It is expected that the next significant leap forward in lead 

time for tornadoes and flash flooding will be the result of the Warn-on-Forecast paradigm that 

has been discussed in the weather community, potentially leading to 60 minute lead time for 

tornadoes, and described in Section 103 of the Weather Act: Tornado Warning Improvement 

and Extension Program, “...to reduce the loss of life and economic losses from tornadoes 

through the development and extension of accurate, effective, and timely tornado forecasts, 

predictions, and warnings, including the prediction of tornadoes beyond 1 hour in advance.” A 

follow-on to NEXRAD would necessarily provide the foundational observational needs of Warnon-Forecast. 

This report summarizes a comprehensive, nationwide study performed by NOAA to examine 

warning performance for instances in which no, or insufficient warnings, were given for 

hazardous weather events, including tornadoes. It also looks at whether limited or no NEXRAD 

coverage in areas in the United States has resulted in degraded forecasts for hazardous weather 

events that resulted in fatalities, significant injuries, or substantial property damage. 

An important definition used in the study is where to draw the line between “optimal” and 

“poor” radar coverage. NOAA used the definition from the House Report for the Consolidated 

Appropriations Act, 2017, of the radar beam at 6,000 ft. AGL as the demarcation between 

“optimal” and “poor” radar coverage. This level was selected to address concerns that some 

forms of severe weather, such as tornadoes, can occur far from the radar, escape detection

(because the beam shoots over the top of the phenomena) and, therefore, are more likely to be 

unwarned. The horizontal distance this corresponds to is about 71 miles from the radar, though 

useful radar data are available to about 144 miles in range. 

Another key distinction in this report is how to characterize “hazardous weather”, other than 

tornadoes as stated in the Weather Act. The impetus for the study was congressional concern 

over “missed” tornados and “missed” flash floods and the impact on loss of life, injuries and 

property damage. Warnings for these short fuse events can benefit most from radar coverage, 
10 

because forecasters use the radar to detect features in real-time, such as precursors to tornado 

formation and heavy rainfall. There are many other weather conditions that can be considered 

hazardous, such as winter storms, high winds, hurricanes, coastal flooding, etc. While radar will 

help in overall situational awareness during these types of events, in general the most critical 

data source for issuing watches and warnings for these longer time-scale events come via 

numerical weather prediction. Therefore, this report will focus on tornados and flash floods as 

the relevant “hazardous weather” under review. Also for this study, “degraded forecasts” or 

“insufficient warnings” are considered the same and defined as “unwarned events.” In order for 

a tornado or flash flood event to be considered significant, there must be one or more injuries 

caused by the event. Significant tornado property damage is about $1.6 million per event, while 

for flash floods significant property damage is defined at and above about $416,000 per event. 

A detailed explanation is warranted regarding language in the Weather Act where it states 

“identify areas in the United States where limited or no NEXRAD coverage resulted in—(i) 

instances in which no or insufficient warnings were given for hazardous weather events, 

including tornadoes; or (ii) degraded forecasts for hazardous weather events that resulted in 

fatalities, significant injuries, or substantial property damage.” This and similar language in the 

House Report for the Consolidated Appropriations Act, 2017, implies that limited radar 

coverage, or a “gap”, is a primary factor for missing warnings for short fused events. This is 

typically not the case. There are numerous factors that play a role in any missed warning, and 

radar coverage is one of many in an extremely complex decision process. No one argues the 

importance of radars and that optimal coverage is beneficial to hazardous weather detection, but 

humans are responsible for issuing hazardous weather warnings, not the radar. Humans 

routinely integrate numerous sources of information, often times conflicting information, from 

storm spotters/chasers, webcams, satellites (especially the high resolution data coming from the 

new geostationary satellites), the Multi Radar-Multi Sensor (MRMS) products, surface 

observations like those available through the Mesonet Program, Probsevere, the High Resolution 

Rapid Refresh forecast model, and other high-resolution modeling systems, prior to issuing 

warnings. The operational version of MRMS includes NEXRAD and Canadian radar networks,

numerical weather models, precipitation gauges, background climatologies, and lightning 

mapping arrays to generate severe weather and quantitative precipitation estimates products for 

the forecasters.

Forecasters also use the FAA Terminal Doppler Weather Radars (TDWR), installed near many 

major airports, to aid the warning decision process. In some cases, these data have been 

instrumental to pinpointing the threat of a tornado, especially in areas where the TDWR offers 

better radar coverage than the NEXRAD. Trained forecasters understand that when a storm is 

farther from the radar, signatures will not be as clear cut or perhaps not present at all, and they 

rely on other sources of information to make the warning decision. As the study shows, they are 

skilled at making these difficult decisions with sometimes sub-optimal radar data. Additionally, 

an informal examination of unwarned events, including tornadoes, was gathered as part of a 

training exercise covering 15 years. Forecasters were asked to list the various factors they felt 

played a role in an event being unwarned. There were tens of individual factors identified during 

the root cause analyses, each of which, in isolation, did not cause an unwarned event. However, 

when several factors cascade together, unwarned events can occur. Poor radar coverage was a 
11 

contributing factor for some unwarned events, but the majority of the root causes were related to 

human factors. Poor radar coverage is never the single contributing factor to an unwarned event. 

To identify unwarned events where poor radar coverage played a role, each individual event 

would need to be closely scrutinized, including interviews with all the forecasters on shift, and it 

would need to be done within days or weeks of the event given the need to capture minute details 

of the decision process. This kind of post-event work is well beyond the scope of the study and 

this report. Instead, the report will examine the bulk statistics associated with radar coverage and 

warning performance conducted in the study. 

Much of the previous discussion is related to the tornado warning process, but NEXRAD is also 

an important piece of the flash flood warning decision process. This is because the radars are 

able to estimate the amount and rate of rainfall, which, combined with the knowledge of the local 

hydrological response to heavy rain, allows forecasters the ability to issue warnings with 

significant lead times. The precipitation estimates from the radar have vastly improved NWS 

flash flood warning prediction, and the lead time has increased from about seven minutes in the 

late 1980s, to near 50 minutes or more after NEXRAD became operational nationwide in the late 

1990s. As with tornadoes, NWS forecasters use other tools in the flash flood warning process 

such as satellites, rain gauges, storm spotters, and high-resolution forecast models both in good 

and sub-optimal radar coverage. 

Finally, no single observational source, such as the NEXRAD, will mitigate warning challenges 

and eliminate the potential for loss of life, given that every observational system has inherent 

limitations. Even if perfectly detected, predicted and warned, these events can still cause 

damage, injuries, and fatalities. Ask any forecaster and he or she will tell you NEXRAD is the 

preferred source for issuing warnings for hazardous weather, but no single data source is used 

exclusively, whether near or far from a NEXRAD radar. The availability and usage of multiple 

sources of data in the warning decision process is the backdrop for the study and will help 

explain some of the study’s findings in this report. 

III. Study Methodology 

The study examines National Weather Service (NWS) warning performance for both tornado and 

flash flood events over a 10-year period.

4

 Each type of hazardous weather event is tallied by all 

events (warned or not) using the definition of radar coverage with the beam below 6000 ft. AGL. 

NOAA examined over 12,000 tornado events and over 30,000 flash flood events from 2008-

2016. The source of performance data is the NWS Local Storm Reports.5

As discussed previously, NWS forecasters use several sources of data, including additional 

Federal Government radar assets like the TDWRs, when available, to make warning decisions. 

Data from the 45 TDWRs provide radar coverage in many locations, including Charlotte, North 

Carolina, Columbus, Ohio, Las Vegas, Nevada, and Puerto Rico. In general, TDWR coverage is 

 

4 A 10-year period was used to determine damage, injuries and deaths for tornadoes. An 8-year period (2008-2016) 

was used for GIS analysis of tornadoes of polygon tornado warnings, which began in 2008.

5 Available at https://www.spc.noaa.gov/exper/reports/?&all&date=20200311.
12 

supplemental to the NEXRAD network and overlaps existing NEXRAD coverage for many 

major U.S. cities. It would not be possible to isolate the impact of TWDR data usage on tornado 

warning performance without examining every warning decision in detail, thus, NOAA assumes 

any positive impacts from the usage of TDWR would not change the overall statistical results in 

the study. 

The study also examined integrating data from other FAA radars and from non-Federal radars. 

Other FAA radars, such as the Air Surveillance Radars, are not useful to NWS operations to 

improve detection and prediction of hazardous weather events like tornadoes and flash floods 

due to optimizations that are made to track aircraft. Non-Federal radars have considerable cost 

and technical challenges to deliver the data to NWS offices, and are not generally viable, 

especially in light of the fact that most of these “additional” radars are primarily in regions that 

are covered by the existing NEXRAD and/or TDWR networks. Non-Federal radars may not 

have the same rigorous maintenance and calibration standards, and may not be scanning at all 

times during the day and night. 

 

While there are gaps evident on maps depicting coverage by radar beams at 6000 ft. or 10,000 ft. 

AGL, the radar data above those levels still provide important information for the forecasters to 

use as they develop their forecasts and issue warnings for hazardous weather events. The 

possible exception is the intermountain west, where radar data coverage is reduced and the 

hazardous events are limited, and in the case of strong tornadoes, are very rare. However, even 

in the intermountain region, the benefits would be limited to potentially improving flash flood 

warnings, with some impacts (reduced deaths and property damage) limited here as well. The 

tornadoes that occur in the intermountain region are typically of the weaker variety, EF0 or EF1, 

and the statistical correlation to damage, fatalities, and injuries is very low. 

 

IV. Tornado Warning Performance 

Tornado events (both warned and unwarned) which occurred within radar coverage (e.g., areas 

of coverage at 6000 ft. AGL or lower) were compared with events that occurred outside of 

optimal radar coverage (no coverage at beam centerpoint 6000 ft. AGL). Further analysis 

compared radar coverage at beam centerpoint 6000 ft. as a percent of state (i.e., Colorado) 

covered compared against the national average of warning skill. A similar measure of percent 

coverage below 6000 ft. for each NWS Weather Forecast Office’s (WFO) area of warning 

responsibility was conducted to see if there was any correlation between good and sub-optimal 

radar coverage related to warning performance. Nationwide, the mean coverage of NEXRAD at 

6000 ft. AGL is about 60 percent of each state. Western states, except California, have coverage 

at or below average. Preliminary national analyses of tornado warnings between 2008 and 2016 

show similar performance among events inside, outside, or those that cross the boundaries of 

radar coverage, with a consistent majority of events having been warned, regardless of radar 

coverage (Figure 2). 

The analysis in Figure 2 shows high parity in the fractions of events warned and unwarned 

nationally inside or outside of radar coverage, regardless of whether supplemental TDWR 

information is added. In some cases, being far from the radar, with the uncertainty and 
13 

ambiguity involved because of the poorer radar coverage, leads a forecaster to lower his or her 

internal threshold to issue a tornado warning (The finding of a higher false alarm ratio overall 

beyond 150 km (93.7 miles) in range by Brotzge et al., 2011, supports this interpretation.). A 

lower warning threshold may explain why there was little difference in performance, in a bulk 

sense, inside and outside the range covered by a radar beam at 6000 ft. AGL. 

Figure 2. Tornado event fraction (from all events) as a function of tornado events for 2008-2016. The red color bars 

denote events inside NEXRAD and TDWR radar coverage threshold at 6000 ft. AGL that are warned (left red bar) 

and unwarned (right red bar). The cyan bars are the corresponding warned events outside (left bar) and unwarned 

outside (right bar). The numbers atop the color bars denote the event counts out of a total of 12,296. The bars are 

ratios. Note the ratio of warned and unwarned events inside coverage and outside coverage is virtually the same.

The evidence that only minor improvement occurs regarding tornado warning performance 

within 6,000 feet AGL radar coverage should not be misconstrued or misunderstood as a reason 

to downplay the importance of the NEXRAD network. On the contrary, the radar network is the 

most important tool for short fuse, high impact warnings and without it the loss of life from 

severe weather would be extraordinarily higher (The Economic and Societal Impacts of 

Tornadoes by Simmons and Sutter, 2011). The fact is that human forecasters know to rely on 

other data sources in poor radar coverage areas to make warning decisions. Overall confidence 

in those decisions are enhanced when the event is near the radar, a fact that cannot be overlooked 

because as discussed earlier, human factors constitute a majority of the reasons behind unwarned 

events. Parity is also seen in terms of tornado intensity. Violent tornadoes (EF3–EF5) are

properly warned for at higher rates than weaker tornadoes (EF0–EF2), and the weaker tornadoes 

are much more numerous than the stronger ones. Some tornado metrics have a moderate 

sensitivity to the degree of radar coverage, an exception is that there is a strong correlation 

between the number of tornado events and the number of warned events per WFO area of 

responsibility. As the number of tornado events increases to close to 200 per 8 years, the percent 

of warned events increases rapidly, leveling off at about 80 percent. This implies that the offices 

that have the most tornadoes typically perform the best in terms of warning performance. The 

reason this is the case is primarily because of forecaster experience. NWS offices that issue 
14 

more tornadoes warnings have better experience and more training for events that are relatively 

more frequent, particularly across the plains and southeastern states. The study also found that 

on average, there are very few tornado events in most western states (Figure 3). This means that 

the region with the poorest radar coverage also experiences the fewest tornadoes. This is not 

surprising because radar coverage was designed to be maximized in the regions with the most 

severe convective weather and where beam blockage due to terrain is not as severe, both of 

which are true east of the Rocky Mountains. 

Tornadoes that cause deaths are rare and comprise about two percent of all tornadoes. There 

were about 1,000 fatalities that occurred in 270 tornado events, out of over 12,000 tornado events 

over the 8 years. Trained forecasters do an excellent job in detecting violent tornadoes, in part 

because NEXRAD radars likewise can better detect tornadic signatures associated with violent 

tornadoes, and this is true whether there is radar coverage below 6,000 ft. or not. The data show 

there is virtually no sensitivity relative to radar coverage for the killer tornadoes. About half of 

the fatalities occurred in the southeastern United States and Oklahoma, which generally have 

very good radar coverage. 

Injuries are also generally related to strong or violent tornadoes. Fewer than 10 percent of events 

had any injuries, and fewer than two percent of all injuries occurred during unwarned events. 

The same relationship applies for the tornado fatality statistics; NEXRAD radars and human 

forecasters generally detect and warn for the vast majority of stronger tornadoes which result in

injuries. In most cases, strong and violent tornadoes are well warned everywhere, and injuries 

have virtually no sensitivity to relative radar coverage. Both fatalities and injuries are highly 

variable and concentrated in spikes due to large-scale severe weather outbreaks (April and May 

of 2011 account for about half of the fatalities and injuries in the study). About 85 percent of 

fatalities are due to violent tornadoes (EF3, EF4, and EF5). A similar pattern is seen in the few 

unwarned fatal events, also mostly due to strong to violent tornadoes. In summary, the study’s 

results show that radar coverage is statistically not a significant factor in tornado warning 

performance regarding unwarned events that resulted in fatalities or injuries. 

A similar pattern is seen in terms of warned and unwarned tornadoes that result in significant 

damage, defined as at or above the mean damage of about $1.6 million per event. Four percent 

of all the tornado events in the study generated at or above $1.6 million in damages. Most of the 

damaging tornadoes were concentrated in 2008, accounting for 20 percent of all such events in 

the study. 
15 

Figure 3. Warned tornado event fraction (from all events) as a function of tornado events per state for 2008-2016. 

There are very few tornado events in western states. The percent of events warned increases rapidly as the event 

rate grows and stays uniformly high past 200 events/state. 

Significant property damage has some correlation with radar coverage, but with large variance. 

Seventy-two percent of the most damaging tornadoes (based on a threshold of $1.6 million in 

damages) are accurately warned inside the coverage of a radar beam at 6,000 ft. AGL, while 68 

percent are accurately warned outside the same threshold. The unwarned numbers for the most 

damaging tornadoes are 28 percent inside the threshold compared to 32 percent outside, a minor 

difference in performance but not statistically significant due to the low occurrence of such 

tornadoes. 

One of the tools used to measure the effectiveness of the NWS tornado warning program is to 

use metrics called “probability of detection” and “false alarm ratio” (FAR). A FAR is 

determined by the number of tornado warnings that were issued against the number of warnings 

that were associated with no tornado occurrence. Analysis from this study found no correlation 

with regard to radar coverage regarding FAR or fatalities, and nearly no correlation with 
16 

unwarned injury event ratios regardless of range of the warned event from the radar (Figure 4). 

Figure 4. Tornado false alarm ratio (FAR) for 2008-2016, and percentage of WFO-area covered by NEXRAD. 

Fitted line is essentially insensitive to radar coverage.

This analysis reinforces the results from the coarser inside/outside 6,000 ft. AGL spatial binning 

that show no significant systematic warning degradation for tornadoes due to variations in radar 

coverage. There is a slight degradation in FAR beyond the range of a radar beam at 6,000 ft. 

AGL, but it is extremely small and not statistically significant. 

It is thus not surprising that radar coverage in and of itself is a weak predictor of tornado warning 

performance overall, because as discussed at length earlier in this report, warning skill draws 

from multiple sources, some of which cannot be easily measured. Very high-resolution (time 

and space) numerical weather models (called storm-scale models) are expected to replace today’s 

warn-on-detection practice with warnings based on model predictions, such as those being 

developed under the NOAA Warn-on-Forecast project. This report details the significance of 

Warn-on-Forecast advances in Section VII. 

V. Flash Flood Warning Performance

NWS WFOs assess and monitor the threat of flash flooding 24 hours a day, 7 days a week, to 

provide timely and accurate life-saving flash flood forecasts, watches, and warnings. The 

issuance of flash flood warnings helps the NWS meet its mission by providing advance 

notification of dangerous, short-fused flood events. This allows users to take immediate 

mitigation actions, such as evacuating to higher ground, helping to protect life and property. 

Toward this end, WFOs integrate a wide range of numerical model guidance, radar-based 

precipitation estimates, satellite-based precipitation estimates, and real-time telemetered 

precipitation and stream gauge observations to provide critical decision support services, 
17 

including issuing flash flood warnings when precipitation capable of causing flash flooding or 

debris flows is indicated by radar, rain gauges, and/or satellite. 

Precipitation estimates from radar have vastly improved NWS flash flood warning prediction and 

the lead time has increased from about seven minutes in the late 1980s to around 50 minutes or 

more after NEXRAD became operational nationwide in the 1990s. Accurate radar-rainfall 

estimates are of critical importance and are shown to be improved when the range from storm to 

radar is less than 60 miles, and when there are at least two overlapping radars within 142 miles. 

In general, storm-total radar-rainfall measurement accuracy decreases farther from the radar 

because at that height, frozen precipitation is typical, and it is much harder to estimate the liquid 

content once the precipitation reaches the ground (Rogalus and Ogden, 2013). 

As with tornadoes, forecasters use multiple sources of information during the flash flood warning 

decision process, which minimizes the impact and/or reliance on a single source of information. 

Radars are the most important source of information for detecting heavy rainfall, but in poor 

radar coverage areas forecasters are for the most part able to incorporate other sources of 

information to help make warning decisions. Seasonal changes and climatological patterns play 

a more significant role in flash flood warning skill compared to tornadoes. During the course of 

a year, if the majority of flash flood events result from widespread, highly predictable rainfall 

events, such as large scale, well-forecasted land falling tropical or winter storms, the overall 

effect of improved national average performance metrics for flash flood warnings is observed. 

Conversely, lower lead times and accuracy would be expected when there is a general lack of 

these widespread, highly predictable rainfall events. Additional forecasting and verification 

challenges arise in areas of steep terrain or where wildfires can leave burn scars across the land 

surface, conditions notably observed in the western United States. In these situations and 

regions, warning lead time and accuracy is often diminished, as hydrologic response is very 

rapid, and oftentimes not that much rainfall is needed to produce life-threatening impacts. 

Preliminary national analyses of flash flood warnings between 2008 and 2016 show similar 

performance among events inside, outside, or those that cross the boundaries of radar coverage, 

with a consistent majority of events having been warned, regardless of radar coverage (Figure 5). 

There is a more significant increase in the ratio of unwarned flash flood events outside the 

coverage of radar beams at the 6,000 ft. AGL level compared to tornadoes, but the differences 

are still quite small (12 percent inside 6,000 ft. AGL v. 18 percent outside). National analysis 

shows a slight increase in probability of detection, critical success index, and lead time and a 

decrease in false alarm rate with increasing WFO area under the coverage radar beams at 6,000 

ft. AGL. 
18 

Figure 5. Ratio of warned inside coverage (red), warned cross (purple), and warned outside (cyan) to unwarned 

inside (red), unwarned cross (purple), and unwarned outside (cyan). These are for flash flood events from 2008-

2016 normalized by the total number of events inside or outside coverage area of the 6 kft. beam bottom altitude 

according to the radar coverage type (within radar coverage, outside of radar coverage, or crossing limit of radar 

coverage). The two purple bars are “crossed” events that fell partially inside and outside the 6 kft. beam bottom 

altitude range. Flash floods events are generally warned at the same proportion regardless of radar coverage. Most 

flash floods events (68.4 percent) occur inside radar coverage.

The ratio of warned flash flood events to total number of events shows most states have a high 

warned ratio for all levels of events and little apparent difference between the warned event ratio 

for events inside radar coverage as compared to those outside. The exception is a set of western 

states, which show a smaller decrease in warned event ratio with increasing radar coverage as 

compared to states with similar numbers of events. Further analysis could better differentiate 

regional trends and relationships among radar coverage, event type, population, warning 

performance, and other variables. 

From 2008 to 2016, 17 fatalities and 13 injuries occurred for every 1,000 flash flood events. 

Injury statistics, already a small dataset, are dominated by a single event encompassing nearly a 

quarter of all injuries for the 8-year period. This strong bias makes trend analysis less 

meaningful for these data. Nevertheless, there is some positive correlation between the number 

of events per state as well as injuries per state and state population. There is very little 

relationship between flash flood events, fatalities, injuries, damages, or significant damages 

compared to radar coverage for the CONUS. Around 4,300 flash flood events went unwarned 

during that period (12 percent of all events), with a rate of 28 fatalities and 17 injuries per 1,000 

unwarned events. Most fatalities in unwarned events occurred in events inside of or very close 

to the coverage threshold of a radar beam at 6,000 ft. AGL. Overall, radar coverage does have 
19 

some relationship to NWS flash flood warning performance statistics at a national level (e.g., 

detection of a flash flood) (Figure 6). 

Figure 6a
20 

Figure 6b

Figure 6c
21 

Figure 6. Flash flood (a) warned-event ratio (probability of detection), (b) false alarm ratio, (c) critical success 

index, (a performance measure including warned and unwarned events), and (d) mean lead time 2008-2016 and 

percentage of WFO-area covered by NEXRAD. Probability of detection, critical success index, and mean lead time 

are all positively correlated with increasing percentage of WFO area covered by radar with a coefficient of 

determination of around 0.1. False alarm rate is negatively correlated with increasing percent of WFO area covered 

by radar with a coefficient of determination (R2

) of around 0.1 also. 

VI. Other Observational Sources Available 

On the day before enactment of the Weather Act,

6 there were many observational systems 

operating across the Nation that are used for high-impact weather, some in areas of limited 

NEXRAD coverage below 6,000 ft. AGL. These included mesonets, rain gauges, river gauges, 

lightning data, FAA TDWR radars, research X-band radars (e.g., Collaborative Adaptive Sensing 

of the Atmosphere (CASA) in the Dallas-Fort Worth, TX, area), and the NOAA geostationary 

weather satellites. For this report, satellite data from polar-orbiting weather satellites are not 

considered because while extremely useful for longer range forecasting, the data are not frequent 

enough to aid in the warning decision process for tornadoes or flash flooding that require new 

data on the order of minutes. NWS uses other observational systems to obtain additional 

information on severe weather hazards, precipitation estimates, or even help adjust precipitation 

rates in sub-standard or non-existent radar coverage areas. The different platforms are described 

below. 

Other Observational Data 

 

6 See Weather Act at 131 Stat. 114(a)(2)(ii) requiring the Secretary to “identify additional sources of observations 

for high impact weather that were available and operational for such areas on the day before the date of the 

enactment of this Act. . .”.

Figure 6d
22 

Satellite Observations: The most important source of data for severe convective warnings other 

than radar comes from space. Instruments on geostationary orbiting satellites provide a way to 

assess environmental and storm characteristics that can improve severe weather and heavy 

rainfall detection. Precipitation estimation from orbiting satellites can provide rainfall 

information in areas of complex terrain that cannot be sampled by the NEXRAD network. The 

high-resolution data from Geostationary Operational Environmental Satellite – R (GOES-R) 

Series currently in orbit (GOES East and West), although just recently made operational, are 

identifying features that have never been seen before. NOAA believes the improved temporal 

and spatial resolution with the GOES-R Series will improve the short-fuse warning decision 

process. When combined with other sources of information, data from geostationary orbit 

increases forecaster confidence in the severity of a storm. There is promising, ongoing research 

related to the utility of GOES-R Series data for rainfall estimation, particularly important in the 

data-sparse regions of the western United States. Overall, NOAA anticipates that continued 

research using the GOES-R Series data should continue to have a significantly positive impact 

on NWS tornado and flash flood warnings. 

Total Lightning Data: Lightning mapping arrays can detect intra-cloud and cloud-to-ground 

lightning within convective storms. There are lightning mapping arrays across the country, and a 

new Geostationary Lightning Mapper is employed on the GOES East and West satellites. 

Ground-based networks also provide NOAA with total lightning data, purchased from vendors. 

Studies have shown that increased lightning activity, more commonly known as “lightning 

jumps,” can be a precursor to severe weather events (e.g., tornadoes) by up to tens of minutes. 

NOAA is encouraged about the potential impact this information may have on NWS severe 

weather warnings, including for tornadoes, but research still needs to be done to validate the role 

of lightning jumps as precursors to severe weather and tornadoes. From a precipitation 

perspective, studies have shown some correlation between lightning frequency and precipitation 

rates, which can be beneficial for adjusting precipitation rates of convective storms occurring in 

areas of inadequate radar coverage. 

Multi-Radar Multi-Sensor System: Each of the observational platforms have strengths and 

challenges regarding severe weather and heavy rain/flash flood detection. The development 

effort to incorporate multiple platforms/systems together to improve the short-term prediction 

and detection of hazardous weather became operational in the NWS in the last few years. 

NOAA’s National Severe Storms Laboratory (NSSL) developed a Multi-Radar Multi-Sensor 

system (MRMS) (Zhang et al., 2016). The current operational version of MRMS includes the 

following in its product generation for severe weather and quantitative precipitation estimates 

(QPEs): NEXRAD and Canadian radar networks; numerical weather models; precipitation 

gauges; background climatologies; and lightning mapping arrays. All data from MRMS are

provided at a 1 km × 1 km spatial resolution with most radar-based products generated every 5

minutes. Longer duration QPEs and those that are gauge-derived or gauge-adjusted are available 

every hour. NOAA is exploring options to include non-federal radar assets into MRMS. This 

could allow NOAA to use data from other radar assets in the private and academic sectors to 

improve the overall product. Finally, MRMS data help drive an NSSL-developed system called 

“Flooded Location and Simulated Hydrographs” (FLASH), which shows promise for helping 
23 

detect flash floods at the small hydrologic basin scale. FLASH is being tested and evaluated by 

NWS forecasters.

Precipitation Gauges: Various precipitation gauge networks provide accumulated liquid 

precipitation values on different temporal scales. Gauges are commonly referred to as “ground 

truth”; however, they do not have the adequate coverage densities to capture the spatial gradients 

and variabilities of precipitation. Precipitation observations from gauges are provided by such 

entities as the NWS, United States Geological Survey (USGS), local departments of 

transportation, city/county networks, etc. States, such as Oklahoma, Kentucky, and New York, 

even have their own statewide mesonet systems, which generally places at least one observing 

system in each county. National collections of gauge networks include the Hydrometeorological 

Automated Data System (HADS) and Meteorological Assimilation Data Ingest System (MADIS) 

allow for a compiled, more direct access to multiple data sources. These “ground truth” 

observations are also used to calibrate the precipitation estimates from the NEXRADs and for 

validation of research satellites such as the NASA-Global Precipitation Mission.

USGS Stream Gauges: For flash flood events, point observations from USGS stream gauges can 

provide information on local waterways in 15-minute increments. Significant jumps in discharge 

and stage height alerts forecasters to rising waters relating to flash flooding. 

Unmanned Aerial Weather Systems: Remotely controlled aircraft can carry weather 

instrumentation, including radars and dropsondes, which can be flown over weather features to 

improve data collection and forecasting. More details on NOAA unmanned aircraft systems can 

be found at http://uas.noaa.gov/. Currently, this is entirely in the research phase and nonoperational. Other organizations are also experimenting with weather drones.

Non-NOAA Radars 

Terminal Doppler Weather Radars: TDWRs owned and operated by FAA, are strategically 

placed near airports and provide radar observations at a greater spatio-temporal resolution than 

the NEXRAD network. TDWR data are processed similarly to the NEXRAD radars and are 

viewable by NWS personnel in WFOs. The low-elevation angles of the TDWR’s radar beam 

combined with a more frequent temporal resolution and higher spatial resolution provide 

supplemental data that are valuable to the detection of short-fused severe weather hazards, 

especially tornadic circulations. However, current limitations with TDWR data make it 

challenging to obtain accurate precipitation information operationally. Data from TDWRs is 

supplemental to NEXRAD radar coverage at or near airports, surveilling below the radar beam 

of the NEXRAD and providing wind and precipitation data to forecasters. 

CASA Radars: The Collaborative Adaptive Sensing of the Atmosphere (CASA) project uses 

small-scale Doppler radars for observing the lower part of the atmosphere where weather occurs. 

The CASA project (http://www.casa.umass.edu/) started with the National Science Foundation 

(NSF), and was subsequently supported with limited NOAA funding, around 2004. NSF funding 

ended in 2013. After operating in Oklahoma, the current CASA experiment in the Dallas-Fort 

Worth Metro area began in 2012 with the installation of the first radar. A total of seven
24 

networked radars were installed by 2015 at a cost of approximately $3.5 million with annual 

operating costs of the network around $600,000. These radars have a shorter range (around 25 

miles) and are much more susceptible to attenuation, or degradation of the beam, than the 

NEXRAD or the TDWR. The advantage of this system is that the beam is never very far off the 

ground; the disadvantage is that you need many radars in close proximity to cover an area. The 

CASA experiment has provided NWS forecasters in Fort Worth rapid updates of high-resolution 

data (both spatial and temporal) during severe weather events in the metropolitan area. This 

supplemental radar has been modestly beneficial to the WFO’s warning decision making and 

their impact-based decision support services. The high-resolution CASA radar data has, 

however, expedited NWS post-event storm damage surveys by providing additional information 

regarding tornado location and track. Additional benefits of the radar network, such as the urban 

flash flood modeling and warning project, may still be realized, but are currently unknown due to 

infrequent hazardous weather events and a delayed integration of data into the NWS display 

system. CASA receives partial funding and support from NWS via the National Mesonet 

Program. 

Other non-NOAA Radars: There are some non-NOAA radars deployed across the country that 

are accessible by NOAA offices. For instance, some universities have development radars that 

may be of use to NWS. One example is the University of Louisiana-Monroe that has a radar 

built to similar specifications as the NEXRAD radar fleet that is located between Shreveport, 

Louisiana, and Jackson, Mississippi, where the lowest available data from the NEXRAD 

network can exceed 6,500 ft. AGL. Another example is the University of Missouri radar 

between Kansas City, Missouri, and St. Louis, Missouri, that can provide additional coverage 

over northern Missouri. Environment Canada also has a radar network providing additional 

coverage across the northern border of the United States. Local forecast offices have access to 

these university and Canadian radars. There are a large number of TV weather radars having 

widely different operating characteristics; these detailed data are not directly available to NOAA

at this time. DoD radars were not considered in this report because they do not provide weather 

detection capabilities that meet the mission of the NWS. 

The study examined integrating data from non-Federal and non-academically supported radars. 

This scenario has considerable cost and technical challenges, and is not seen as a viable 

direction, given these radars are primarily in regions that are covered by the existing NEXRAD 

and/or TDWR networks. The discussion of feasibility of integrating and upgrading the 

NEXRAD network with non-NOAA weather radars should be separated into two categories: 1) 

integrating data from weather radars that are owned, operated, and maintained by third parties; 

and 2) integrating and upgrading the NEXRAD network with non-NOAA weather radars to be 

owned, operated, and maintained by NOAA. 

In both categories, there are starting and recurring costs associated with every connection. For 

example, communication lines and bandwidth would need to be upgraded and maintained to 

support effective delivery and display of the data. Integrating radars that are operated and 

maintained by third parties would encompass NOAA receiving the data from state or university 

owned radars, local community radar networks (similar to the CASA experiment), or television 

radars. While the maintenance and sustainment of these radars would not be the responsibility of 
25 

NOAA, there would be non-trivial recurring and one-time costs associated with every connection 

to third party radars. Integrating the data into NOAA’s Advanced Weather Interactive 

Processing System would require external weather radar data to be processed through a 

supplemental radar product generator, which would require maintenance and sustainment of that 

software and hardware infrastructure. Ingesting data from non-NEXRAD weather radars 

requires security updates and monitoring. Requirements for any third-party weather radars must 

be established prior to ingesting the data, including data availability, quality, and reliability 

standards consistent with the operational needs of the NWS. These requirements include 

accepting data from weather radars only in NEXRAD Level II format, data quality of sufficient 

level (as determined by NOAA), reflectivity and velocity data available and dual polarization as 

high priority. These are neither inexpensive nor technically simple solutions, especially in light 

of the fact that NEXRAD data are the gold standard across the world for operational weather 

radar data.

VII. Options to Improve Hazardous Weather Detection and Forecasting Coverage 

Even ideal or perfect radar coverage does not, and will not, mitigate against all key weather 

events, as found in the results of this study and from basic scientific considerations, such as 

limitations in predictability and complex human responses in the high stress environment of 

warning decision making. There are still gaps in the understanding of meteorological 

phenomena as well as intrinsic and practical limits to numerical prediction. Improvements in 

warning and forecasting, especially for high-impact and hazardous weather conditions, remain a 

top priority for NOAA. This section will highlight the cutting-edge research that could one day 

transform the NWS warning decision process. 

There are many projects underway within NOAA, including the Hurricane Forecast and 

Improvement Project (HFIP), tornado detection and prediction (undergoing development), 

Unmanned Aircraft Systems Program (UAS) and other specific event-focused projects. There 

are also efforts underway in NOAA to change the warning paradigm from “Warn-on-Detect,” 

where binary warning decisions are made with 0-10s minutes of lead time, to “Warn-onForecast”, where probabilistic information on hazardous weather informs decisions 1-2 hours 

ahead of the event. Research and development on higher resolution and improved numerical 

models is expected to improve NWS prediction and warning capabilities for these high-impact 

events. Social science is also a critical component of “Warn on Forecast,” as it is essential that 

the information is communicated in a way for people to understand and take appropriate action. 

Additional and frequent observations, particularly in the lower few thousand feet of the 

atmosphere, may provide the data needed for the next increase in predictive capability. These 

are options that can be enhanced or accelerated to improve key weather event performance. 

Candidate observational systems can be tested for impact in various numerical models. A 

significant option under research is the potential use of phased array weather radar, technology 

developed initially for military applications, shown by NOAA to be capable of sampling weather 

more rapidly. This rapid scan capability is essential for storm-scale modeling efforts and 

increasing warning lead time for hazardous weather events. Below is a summary of some 

options to improve hazardous weather detection and coverage, but much more research and 

development is being done. 
26 

1. New Technology to Expand Coverage with the Existing NEXRAD Network: Following 

the completion of the study (and thus not addressed therein), the NWS has recently begun

using innovative technologies to improve the radar coverage within the existing 

NEXRAD network. Over the last two years, the NWS tested and has now begun to 

implement lower angle operations to extend the range of the radar. Once it was 

determined that NEXRAD could physically and electronically operate at an angle lower 

than the standard base angle of 0.5 degrees (above horizontal), NWS lowered the angle at 

ten sites, with an additional eight sites planned in 2020. This low-cost enhancement has 

or will expand the effective low-level coverage at these 18 sites. [These sites and 

improved coverages are reflected with stars in Figure 1] 

2. Advanced Radars: NOAA has recognized the potential benefit of new radar technology 

and has been conducting necessary research and development in Phased Array Radar

(PAR) that includes high temporal data with a goal of not sacrificing the high-quality 

Doppler and dual-polarimetric data currently available with the NEXRAD network. 

High-resolution temporal data, such as those available from a PAR-type radar network, 

are needed to improve the high-resolution modeling necessary for attaining the prediction 

of tornadoes one hour in advance, as stated in the goal of Section 103 of the Weather Act. 

Initial results from these forecast models look promising. Increased high-resolution radar 

coverage, strategically located and as planned for the next generation of radars, could 

also improve the prediction of flash floods, as these data feed forecast models that may 

provide improved precipitation forecasts that can be provided to run-off and flooding 

models. 

3. Advanced Models: NOAA, and the weather enterprise continue to work on highresolution convective-allowing numerical model with 1km or higher horizontal resolution 

and associated continual improvements. This may be an under-appreciated option. This 

technology is rapidly evolving toward achieving a more complete representation of 

weather systems by ingesting high-resolution observations, especially in the lower 

atmosphere. These high-resolution modeling systems are also incorporating ensemblebased solutions, which provide a measure of forecast certainty, which is an important part 

of Impact-based Decision Support Services from NWS. 

4. Advanced Paradigms: Incremental improvements, on the order of a few minutes, are 

possible using the present warn-on-detection concept the NWS has employed since 

weather radars became operational. NOAA is approaching that limit in lead time and 

accuracy because there are limits to warning based on detecting precursor signatures and 

favorable atmospheric conditions. The next dramatic increase in severe storm warning 

performance is likely to come from a change to Warn-On-Forecast paradigm. Combining 

the efforts of advanced radar and high-resolution models, the Warn-On-Forecast program 

could one day result in accurate, 1-2 hour lead time probabilistic tornado warnings. 

Numerical weather prediction has made giant leaps forward in terms of predicting winter 

storms and hurricanes over the past 15-20 years. The next frontier for numerical 

prediction is storm-scale modeling, which requires high-resolution observations (like 

PAR and mesonets) in both horizontal and vertical dimensions. Warn-On-Forecast aims 

to accurately predict the location and severity of severe storms hours in advance, and 
27 

despite this being a long-term goal, any improvements in the prediction and evolution of 

severe weather along the way will be beneficial to NWS forecasters.

5. Advanced Satellites: The new suite of NOAA polar and geostationary satellites is 

providing greater space-based multi-band (visible, infrared, microwave) imagery with 1-

km horizontal resolution or better, including winds. The new GOES East and West 

satellites are providing unprecedented spectral coverage and spatial resolution. GOES

East and West imagery is a main backup for radar estimation of precipitation, and 

certainly a valuable operational source for NWS warnings. We are just learning about 

the information that can be derived operationally from the new GOES-R Series data. 

6. Complementary sensors: Greater coverage of precipitation sensors and hydrographs 

measuring water level in basins not currently covered. This amounts to greater coverage 

by surface-based sensor networks. No information is available at this time on the key 

weather event performance of these systems in areas of limited radar coverage. 

7. Emerging Technologies: Adoption of both unmanned and manned aerial systems to 

sample particular areas and weather features in an operationally targeted fashion. This is 

an active area of research.

VIII. Cost and Timeline for Hazardous Weather Detection and Forecasting Coverage 

There are significant costs associated with many of the above options that must be balanced with 

sustaining the existing operations infrastructure including NEXRAD. Some of the work 

described in this report is work already being done within NOAA. As the detailed analysis 

shows, increasing radar coverage in and of itself is not the best way to improve hazardous 

weather detection and forecasts. It takes an integrated effort for all components and sources of 

data that will improve overall services to the Nation. Much ongoing work at NOAA is intended 

to improve forecast and warnings for the Nation, and is being accomplished through NOAA’s 

continued research and development (programs such as MRMS, HFIP (http://www.hfip.org/), 

Warn-On-Forecast, and tornado warning improvement project), improvements in atmospheric 

modeling, water modeling, observations (satellites and in situ), new radar design (or PAR), and 

computing capacity. The largest cost item would be a new and upgraded radar system. These 

ongoing programs will address improving detection, and more importantly prediction of the 

hazardous weather and water events. 

IX. Summary

NOAA conducted a comprehensive national-scale study assessing the impact of limitations in 

radar coverage for warning for hazardous weather events (tornadoes and flash floods), 

identifying other sources of observations for high impact events, and addressing the feasibility of 

integrating radar data other than NEXRAD data into operations. 

This analysis supports that the original rationale for the geographical distribution of the national 

NEXRAD network coverage remains valid, as warning performance holds fairly steady under 

the complete envelope of radar coverage shown in Figure 1. While there are gaps evident on any 

map depicting radar coverage of beams at 6,000 or 10,000 ft. AGL, the data above those levels 

still provide important information for forecasters to use as they issue their forecasts and 
28 

warnings for hazardous weather events. Radars are required and very necessary to issue 

lifesaving warnings, however, variations in radar coverage do not lead to significant variations in 

warning services provided by the NWS, especially when considering events that kill or injure 

people. Even ideal or perfect radar coverage does not, and will not, mitigate against all key 

weather events, as found in the results of this study and from basic scientific considerations such 

as limitations in predictability, detection, resolution, verification methods, and in complex 

human responses. 

The study finds no significant statistical difference between warned and unwarned tornadoes at 

the national scale inside the coverage area of radar beams at 6,000 ft. AGL and outside radar 

coverage. Preliminary national analyses of flash flood warnings also show similar performance 

among events inside or outside radar coverage. The number of tornado or flash flood events, in 

contrast, has a strong correlation with the ratio of warned events overall. The number of events 

across an NWS warning area of responsibility seems to be a more important determinant of 

warning skill than radar coverage. The exception is a set of western states, where data show a 

slight decrease in warned flash flood event ratio under radar coverage as compared to states with 

similar numbers of events. It is important to recognize that the sample size in these states is 

relatively small, so any conclusions must be done carefully. Tornado and flash flood warning 

and detection rely on many other sources of information, including satellites, mesonets, forecast 

model output, and spotter reports, which aid forecasters, especially where there is minimal radar 

coverage, which could explain the parity. 

NOAA research and development is ongoing to improve the prediction and detection of 

hazardous weather. These efforts include numerical model development, analysis of the new 

GOES-R Series satellite data, advanced radar technology through the phased array, other 

observation systems such as UAS, and scientific research and development within NOAA, and

with partners in the academic and private weather community. These advances and new data 

will help meet the direction of Section 103 of the Weather Act: Tornado Warning Improvement 

and Extension Program, “...to reduce the loss of life and economic losses from tornadoes 

through the development and extension of accurate, effective, and timely tornado forecasts, 

predictions, and warnings, including the prediction of tornadoes beyond 1 hour in advance.” 

NOAA will continue to strive to improve weather prediction at all scales. There are still 

significant gaps in the understanding and detection capability of meteorological phenomena, as 

well as intrinsic and practical limits to numerical prediction. These are active areas of research 

the value of which should be not overlooked in any rigorous effort to significantly raise weather 

warning performance.