The Influence of Basin Size on Effective Flash Flood GuidanceThe Influence of Basin Size on Effective Flash Flood Guidance

David SlayterCooperative Institute for Mesoscale Meteorological Studies

(CIMMS)University of Oklahoma

MotivationMotivation

Flash floods are a significant threat to lives and property across the U.S.– The 30-year average for annual flood deaths in the

U.S. is 143 (NOAA Office of Meteorology) There were 128 flash floods in Arizona reported

during 1998 and 1999. In Arizona, during 1998, the National Climatic and

Data Center reported four deaths occurred from flash flooding and, during 1999, 5 injuries and approximately $13 million in property damage.

MotivationMotivation

Implementation of AMBER (Areal Mean Basin Estimated Rainfall) functionality in AWIPS Flash Flood Monitoring and Prediction (FFMP).

FFMP version 2.0 now uses the resolution of the Digital Hybrid Reflectivity (DHR) product on a 1° x 1 km polar grid.– With DHR, rainfall accumulation is at the volume

scan level. FFMP uses two square mile basins developed at

NSSL. (http://www.nssl.noaa.gov/western/basins)

ObjectiveObjective

RESEARCH QUESTION: – Are the two square mile threshold basins the most

suitable basin size for FFMP in central Arizona? OBJECTIVE:

– Using various basin sizes, assess which size best represents flash flood warnings in AMBER when compared to known flash floods.

Data SourcesData Sources Flash Flood Events

– Forty-nine NCDC-reported flash floods occurring under KIWA or KFSX during 1998 and 1999.

– These flash floods occurred during the summer months (July through early September).

NEXRAD

– Archived Level II NEXRAD data from KIWA and KFSX.

– DHR generated using WATADS version 10.2. Basins

– One-arc-second (~ 30-meter) National Elevation Dataset DEMs.

Location of Flash Floods, Flash Flood “Areas”, and WSR-88D Installations

Location of Flash Floods, Flash Flood “Areas”, and WSR-88D Installations

LimitationsLimitations

Radar Limitations1. Blockage of lower-tilt radar scans by terrain and

subsequent over-sampling.Terrain-based hybrid scans used to

accommodate for this.2. Uncertainty resulting from the Z-R relationship.

Flash Floods– Reporting of only flash floods affecting a

population versus those that occur unnoticed.

MethodologyMethodology

The flash flood events used were selected based on availability of NEXRAD data covering the time frame of the flash flood and the six hours prior to the event.

Basins were delineated at accumulation thresholds of 2 mi2, 10 mi2, 100 mi2 and for the extent of the combined KIWA / KFSX radar umbrella.

MethodologyMethodology

Baseline 1, 3, and 6-hour FFG for Arizona supplied by the Colorado Basin River Forecast Center (CBRFC). – FFG values used in this study are uncorrected for

antecedent soil moisture conditions.– According to CBRFC the FFGs may vary +/- 0.5

inch depending on soil moisture conditions. AMBER/GIS (authored by Paul Jendrowski) used to

determine accumulation warnings.

MethodologyMethodologyArizona FFGs and Forecast Areas

MethodologyMethodology

H0: There is no difference between the occurrence of AMBER warnings between 2, 10, and 100 sq mile basins.

HA: There is a significant difference between the occurrence of AMBER warnings between 2, 10, and 100 sq mile basins.

Hypothesis

MethodologyMethodology

AMBER warnings were compared to reported flash floods to assess the degree of association.– There were no 100 mi2 basins in this analysis

that had a flash flood warning. Three flash floods cases defined in this analysis:

– Case 1: Rainfall >= FFG and FF occurred.– Case 2: Rainfall >= FFG and no FF occurred.– Case 3: Rainfall < FFG and FF occurred.– These cases were analyzed by basin size.

Example of 2, 10, and 100 sq mile basins. Flash flood reported in Washington Park on 7/11/99

15:40 MST

Example of 2, 10, and 100 sq mile basins. Flash flood reported in Washington Park on 7/11/99

15:40 MST

ResultsResults

Case 1: Rainfall >= FFG and FF occurred.Case 2: Rainfall >= FFG and no FF occurred.Case 3: Rainfall < FFG and FF occurred.

YES NO

>= FFG 8 112< FFG 41

YES NO

>= FFG Case 1 Case 2

< FFG Case 3

Observed FF

YES NO

>= FFG 17 626< FFG 32

2 sq miles

10 sq miles

POD / FAR / CSI

POD = Case 1 / (Case 1 + Case 3) = 35%

FAR = Case 2 / (Case 1 + Case 2) = 97%

CSI = Case 1 / (Case 1 + Case 2 + Case 3) = 3%

POD = 16%

FAR = 93%

CSI = 5%

Flood Observed?

Flood Observed?

ConclusionsConclusions

With a POD of 35%, two square mile basins detected flash floods much better than 10 square mile basins (POD = 16%).

The range of probability of association between FFG and reported flash floods for two square mile basins, at a 95% confidence level, is 21% to 48%.

ConclusionsConclusions

Ten and 100 square mile basins are too large to be effectively used for flash flood warnings using current FFGs.– POD for 10 square mile basins of 16%– No 100 square mile basins had an accumulation >

an FFG. Using AMBER and current FFGs, the basins

delineated at the 2 square mile threshold are the best hope for prediction of flash floods.

RecommendationsRecommendations

Improve Flash Flood Guidance to the basin level, incorporating basin physiographic characteristics.– Research is currently underway to associate the

physiographic characteristics of basins used in this study to show statistical differences between basins where flash floods were reported or unreported.

Considering the improvement in precipitation estimates, the next step may be a near real-time assessment of soil conditions and updating of FFG. – A distributed model?