# Presentation: 850091

## Flash Flood Forecasting

- DLOC Workshop 2008
- Paul Schlatter
- Warning Decision Training Branch

## Outline

- Hydrologic ingredients
- Meteorologic ingredients
- DLOC Simulations flash flood forecasting pre-brief (final 2)

## Learning Objectives

- Understand basic hydrology and meteorology forecast ingredients that contribute to flash flooding
- Identify important Flash Flood ingredients specific to the final 2 DLOC scenarios

## A. Hydrologic Ingredients

- Size, shape, topography of basin and stream channel
- Urban Effects
- Vegetation, soil type, and soil moisture
- Season

**Notes:**
These conditions should be known as best as possible before any heavy rainfall begins.  Hydrologic ingredients are every bit as important to flash flooding as meteorological ingredients.  We’ll examine each one briefly.

## Hydrologic Factors Quantified in FFG Products

- Definition
- Most RFC’s have 12 hr precip window
- Resolution leaves much to be desired across some RFCs

## Basin Characteristics:  Size

**Basin Characteristics:  Size**

_**Size**_, shape, topography of basin and stream channel

**Notes:**
The majority of flash floods occur in very small basins, mainly because the scale of the heaviest rainfall is also quite small.  

## Basin Characteristics: Size

-  3-10 mi2 basin can be easily inundated, while basins 50-100 mi2 require training of some sort
-  Small Basins delineated vital to detect FF

- Adapted From Davis (2001)

**Notes:**
These basins are 38-44 km from KPBZ, and color equates to flash flood threat, with red meaning signficant.
Evening of July 1, 1997
The image on the left clearly shows the threat in Dirty Camp run, which did in fact verify after greater than 2 inches fell in 2 hours.  Aber’s Creek in the left doesn’t appear to have much of a threat since just 1.25 inches fell in 2 hours...until you subdivide it further into smaller tributaries it is clear Thompson’s run is at high risk, and indeed significant flash flooding occurred in that small basin

## How a Large Basin Might Flash Flood

- Adapted From Davis (2001)

**Notes:**
On the left, with little core movment, typically ONLY small basins are under a flash flood threat since the footprint of the heaviest rainfall occurs in a very small areas.  However, on the right, with very fast storm movement, the rainfall footprint is spread out, such that a basin of any size could be in danger of flash flooding depending on several factors.
Fast cell movement is problematic:  not obvious where flash flooding will occur if at all, and fast movement means strongly sheared environment and thus a forecaster may be more concerned with other forms of severe weather
With training, fast individual cell movement can lead to flash flooding with repeated development over the same areas

## Basin Characteristics: Shape

- Size, _**shape**_, topography of basin and stream channel

- Change storm motion with identical basin

## Basin Characteristics: Terrain

- Size, shape, _**topography**_ of basin and stream channel

## Basin Characteristics: Terrain

- Size, shape, _**topography**_ of basin and stream channel
- -- width, steepness

**Notes:**
Steep terrain and narrow channels are all conducive to flash floods, meaning far less rainfall can produce significant flash flooding in basins with such characteristics.  The above is from Crack Canyon in Southeast Utah:  probably a worst case scenario for stream channel condition and propensity for flash flooding. On the right is a corn field in Nebraska.  Basin and stream channel is very flat, thus tremendous amounts of rainfall would be required to produce any flash flooding in these basin types.

## 2. Urban Effects

-  Rainfall easily converted to runoff

-  Inadequate drainage systems

**Notes:**
Vegetation intercepts rainfall, inhibits surface runoff, allowing more time for infiltration
Las Vegas is one of the worst cities for flash flooding because the soil surrounding the city has very poor infiltration with next to no vegetation:  thus nearly all rainfall is converted to runoff.  Add to the fact that all the natural washes within the city are now paved over with city streets and the problem is exacerbated.
Cities in general are very flash flood prone with all the concrete and poorly constructed drainage systems

## L

**Girtys Run**
**Millvale, PA**

**L**

**K**

**Auto Body**
**Shop**

**BP**

**Road Crossings L & K**
**On Girtys Run**

-  Courtesy Bob Davis, PBZ

## L

**Auto Body Shop**

**Girtys Run**

**L**

**Road Crossing L**
**On Girtys Run**

-  Courtesy Bob Davis, PBZ

## Remember Topic 5?

## FFMP 1 and 3 Hour Diff Display

- QPE = 1.0-1.5”

- _FFMP Diff_ = Radar QPE - FFG

- Question to all of you:  Why do these numbers miss the Flash Flood in New York City?

- QPE=1.5-2.0”

## Urban Areas and Flash Flood Guidance

-  FFG 0.75 – 1.50 in/hr

-  Forced Flash Flood Guidance Tool with FFMP

**Notes:**
PBZ has found that setting FFG at ~1in/hr for all urban areas has worked well and is consistent with past events.  Other WFOs should examine their areas for past events to see how much rainfall in a hour is required to initiation urban flooding.  Forced FFG GUI is how you can permanently change flash flood guidance for urban drainages.

## 3.  Vegetation, Soil Type, and Soil Moisture

-   ALL related to runoff

-   Soil type affects infiltration capacity
-   Saturated soils just as efficient as rock at converting rainfall to runoff
-   Vegetation reduces runoff

**Notes:**
Soils that are heavily clay have a low infiltration capacity and thus more prone to runoff.  Rocky soils likewise are prone to high levels of runoff.  By the same token, soils that are saturated from previous rainfall are prone to high levels of runoff.

## Soil Type vs. Infiltration Capacity

**Notes:**
A small portion of ABR may be lost due to evaporation, transpiration, and depression storage, but compared to the ABR rate related to flash flood producing rainfall, these losses are minimal

## Soil Moisture and Antecedent Precipitation

- Moral:  Soil moisture/saturation strongly influences surface runoff.  Antecedent precipitation important to account for, and many times FFG doesn’t capture it

**1****st**** burst, complete saturation**

**12-hr later, 2****nd**** burst nearly same amount & intensity, but all converted into runoff**

**Notes:**
2 distinct periods of intense rainfall separated by 12 hours, claimed 25 lives in Kansas City.  First rainfall saturated the soil, while the second rainfall went entirely into runoff, producing disastrous flash flooding.
Much more will be discussed about the importance of antecedent precipitation and how FFG may not be representative in IC Severe 3 in the lesson on Flash Flood Guidance Modification by Bob Davis of the PBZ office.

## Soil Moisture and Antecedent Precipitation

- Doswell et al. (1996) present a case of 180 mm (7 inches) rainfall in less than a day in Iowa Sep 1989, with only minor flooding
-  Tail end of 2-year drought, soil could handle 	precipitation

## Lack of Vegetation—Burn Scars

-  Buffalo Creek

- Waterman Canyon 

**Notes:**
All rainfall converted to runoff in burn areas, and the problem is especially severe in steeper terrain
Buffalo Creek:  2 injuries but substantial structural/road damage in the area
Fei et al. 2001 also showed though numerical simulation that the burn area was actually preferred location for convection due to enhanced insolation
Lytle Creek killed 15 after 2 days of moderate/heavy rains over a recent burn area led to tremendous flash flooding

## Soil and Vegetation vs. FFG

- Soil type important
- Soil moisture, specifically antecedent precipitation reduces FFG
- Sparse vegetation decreases FFG
- Burn scars should drastically reduce FFG

## 4.  Seasonal Effects

-   Mainly impacts surface runoff potential
- 	 Vegetation growth stage
- 	 Lower temperatures mean lower 	infiltration
- 	 Frozen and/or snow-covered 	ground worst case

**Notes:**
Dormant vegetation leads to less rainfall interception, less evapotranspiration (soil moisture high to begin with)

## 4.  Seasonal Effects

-  Early January
-  Some snow-cover
-  Frozen ground
- _[ KILX Flash Loop](http://www.wdtb.noaa.gov/courses/dloc/workshop/temp/radar_2008Jan07ILX.swf)_

## B. Meteorologic Ingredients

- Important insofar as they increase:
-   Duration
-   Rate*
- * Most important

## Increase Duration

- Storm motion:  Corfidi
- Quasi-stationary source of lift
- Ample supply of moisture
- _[KFWS Radar Loop](https://www.wdtb.noaa.gov/courses/dloc/workshop/temp/radar_2007Jun18FWS.swf)_

**Notes:**
Will not rehash Topic 7 here

## Duration and Wind Profiles

- Weak steering layer flow
- 	-- 850-300 mb

- Strong vertical wind shear can increase dry air entrainment
-  	-- Upwind propagation needed

**Notes:**
These two soundings have characteristics of an environment very supportive of warm rain process dominated convection:  Very high PW, especially relative to climatology, little to no CIN, thin CAPE and the CAPE shouldn’t be very high (1000 J/kg or less), absence of dry air at any level, low wind shear, very deep warm cloud depth. You can look at real-time data to assess whether storms are feeding off the moisture rich environment and becoming warm rain dominant. This environment could certainly be characterized as tropical even hours before any rainfall occurs, preparing the forecaster for the possibility of switching to tropical Z/R.

## Rate Increase

- Recall importance of high rate to FF
- Deep moist convection (Topic 7), especially HP/CL supercells, have sufficient rates
- Forecast potential for highly efficient precipitation production

## Skew-T Flash Flood Composite

- 1

- 2

- 3

- 4

- 5*

## Deep Warm Cloud Layer = Warm Rain Process

## High Rate Potential

**Notes:**
These two soundings have characteristics of an environment very supportive of warm rain process dominated convection:  Very high PW, especially relative to climatology, little to no CIN, thin CAPE and the CAPE shouldn’t be very high (1000 J/kg or less), absence of dry air at any level, low wind shear, very deep warm cloud depth. You can look at real-time data to assess whether storms are feeding off the moisture rich environment and becoming warm rain dominant. This environment could certainly be characterized as tropical even hours before any rainfall occurs, preparing the forecaster for the possibility of switching to tropical Z/R.

## Meteorologic Ingredients Checklist

- High PW, no dry air at any height
- Low LCL
- Moderate CAPE not required
- Deep warm cloud layer
- Ample supply of moisture
- Weak steering flow (unless upwind propagation)
- Weak synoptic forcing (unless cell training occurs)
- Quasi-stationary forcing mechanism

## C.  Flash Flood Briefing for Final 2 Simulations

- Simulation Wednesday Afternoon:
- Pittsburgh, PA CWA in August
- Run down Checklists

## 1.  Topography

## 2.  Urban Areas

- Utilize “Urban Boundaries” map overlay on rainfall and FFMP products

## 2.  PBZ Urban Areas

- Combination of hills and poor drainage makes PBZ’s urban areas particularly vulnerable
- Forced FFG 0.7” 1-hr

## 3.  Soil Conditions

- No burn areas
- Significant rainfall 4 hrs prior to simulation start time
- Spotter reports of 2-3 inches

## Season

- Summer
- Peak of growing season
- Near theoretical max of evaporation and transpiration

## 1 Hr FFG 1200 UTC

- Doesn’t incorporate much of 1st event
- Recall Forced FFG areas
- Expect an 1800 UTC FFG

**> 3.5 km,****(12 kft)**

## Pros/Cons for Flash Flood Threat from both sources

| Pros | Cons |
| --- | --- |

## Pros/Cons for Flash Flood Threat from both sources

| Pros
Terrain
Antecedent precip
PW/Moisture source
Deep warm cloud layer | Cons
CAPE too high?
Corfidi vectors 20-25 kts
LCLs when combined with CAPE point to decent cold pools
 |
| --- | --- |

## C.  Flash Flood Briefing for Final 2 Simulations

- Simulation Thursday Afternoon:
- Kansas City CWA in late February
- Run down Checklists

## 1.  Topography

## 2.  EAX Urban Areas

- KC has recent history of significant flash floods
- NO Forced FFG available in this case
- 1”/hr should be adequate

## 3.  Soil Conditions

- Long term (2 yr+) drought
- Last precipitation event was over 3 days ago

## 4.  Season

- Tail end of winter, nearly all vegetation dormant
- Ground *not *frozen, no snow on ground

## 1-hr 0000 UTC FFG

- Some non-urban areas < 1”/hr
- May be too low many places outside of KC metro
- 3-hr FFG far too low (1.25”-1.75”)

## ~ 2.2 km,(7.5 kft)

**~ 2.2 km,****(7.5 kft)**

## Pros/Cons for Flash Flood Threat from both sources

| Pros | Cons |
| --- | --- |

## Pros/Cons for Flash Flood Threat from both sources

| Pros
Season
Surface Boundary
Low LCL
*Supercells | Cons
Terrain
Drought
High wind shear
Fast storm motion
Mid-level dry air, low PW
Weak moisture conv.
Shallow warm cloud layer
 |
| --- | --- |