Louisville, KY Louisville, KY August 4, 2009 Flash FloodAugust 4, 2009 Flash Flood

Frank PereiraFrank PereiraNOAA/NWS/NCEP/Hydrometeorological Prediction CenterNOAA/NWS/NCEP/Hydrometeorological Prediction Center

MotivationMotivation

Recent high-profile flood events highlight the need for situational awareness of low probability, yet

high impact events.

Nashville: May 1, 2010 Atlanta: Sept. 21, 2009

Louisville: Aug. 21, 2009

OutlineOutline

• Event SummaryEvent Summary• Model and HPC PerformanceModel and HPC Performance• Any Indication of Impending Event?Any Indication of Impending Event?• Spatial Density PlotsSpatial Density Plots• Conclusions and DiscussionConclusions and Discussion

Louisville ImpactsLouisville Impacts• Rainfall amounts up to 6-Rainfall amounts up to 6-

inches fell between inches fell between 1100Z and 1400Z across 1100Z and 1400Z across central Louisvillecentral Louisville

• Five inches fell in 90-Five inches fell in 90-minutes from 1145Z to minutes from 1145Z to 1315Z1315Z

• In Louisville, nearly 200 people rescued from the from the tops of cars and houses.

• No fatalities or injuries.

Courtesy of NWS WFO – Louisville, KY

SPC Analysis 04 Aug 12ZSPC Analysis 04 Aug 12Z

500 mb Height and Vorticity &700-400 mb Differential Vorticity Advection

850 mb Height, Temperature, Wind &Temperature Advection

SPC Analysis 04 Aug 00/12ZSPC Analysis 04 Aug 00/12Z

Precipitable Water, Upwind Propagation Vectors &1000-500 mb Thickness

04 Aug 12Z Nashville, TN04 Aug 12Z Nashville, TNSoundingSounding

PWAT = 1.74 in.

Radar and HPC Surface AnalysisRadar and HPC Surface Analysis

VerificationVerification24-hr Amounts Ending 04 Aug 12Z24-hr Amounts Ending 04 Aug 12Z

observed HPC Ohio RFC

NAM GFS

Courtesy of NWS/OCWWS –National Precipitation Verification Unit

VerificationVerification24-hr Amounts Ending 05 Aug 12Z24-hr Amounts Ending 05 Aug 12Z

observed HPC Ohio RFC

NAM GFS

Courtesy of NWS/OCWWS –National Precipitation Verification Unit

HPC Excessive Rainfall GraphicsHPC Excessive Rainfall Graphics

Valid 04/12Z – 05/12Z Issued ~1000ZValid 04/12Z – 05/12Z Issued ~1000Z

•Displays the probability that precipitation will exceed the flash flood guidance values issued by the River Forecast Centers (RFCs)

HPC Excessive Rainfall GraphicsHPC Excessive Rainfall Graphics

Valid 04/15Z – 05/12Z Issued ~1400ZValid 04/15Z – 05/12Z Issued ~1400Z

•Displays the probability that precipitation will exceed the flash flood guidance values issued by the River Forecast Centers (RFCs)

Moisture and Weak Warm AdvectionMoisture and Weak Warm Advection

Louisville Louisville

12-hour GFS Forecast Valid 04 Aug 12Z

850 mb wind and PWATS 850-700 mb Q-vector divergence &850 mb warm air adv

SDF Forecast Sounding SDF Forecast Sounding 09-hour NAM Forecast Valid 04 Aug 09Z

Courtesy of NWS WFO – Louisville, KY

High-Res Model GuidanceHigh-Res Model Guidance

•HPC investigating utility of high-res HPC investigating utility of high-res model guidance to anticipate heavy model guidance to anticipate heavy rainfall threatsrainfall threats•Example from 4.0 km WRF-NMM Example from 4.0 km WRF-NMM (run @ EMC) initialized with the (run @ EMC) initialized with the 04/00Z NAM04/00Z NAM

0700 UTC

0800 UTC

0900 UTC

1000 UTC

1100 UTC

1200 UTC

1300 UTC

1400 UTC

1500 UTC

1600 UTC

1700 UTC

1800 UTC

 

Integration of Hi-Resolution WRF guidance

• High-resolution models are not accurate on the scale of individual grid points

• However, high-resolution models can capture realistic amplitude of events

• Use neighborhood approach (e.g., Schwartz et al. 2009) to give credit for the correct event/phenomenon, even if the placement is not perfect

•Also known as “Spatial Density”

Model 1 h QPF 1” Binary

• Create binary field where threshold exceeded (Flash Flood Guidance)

Neighborhood / Spatial Density Approach

Schwartz et al. (2009)

• Smooth the resulting binary (1 or 0) distribution (using a Gaussian Smoother)

Model 1 h QPF

Creating the Exceeding FFG Density Plot

Schwartz et al. (2009)

> FFG Density

Raw data (1s & 0s) are run data through a Gaussian Weighted Filter to create an index of values from 0-100

Density PlotDensity Plot•Used to raise forecaster’s situational awareness•Diagnostic Available from NCEP High Res Window runs and experimental EMC run•Not a silver bullet –

•Limitations of using single models•Not calibrated (30% does not necessarily occur 30% of time)

SPCWRF4 (30-HR QPF) QPF > 3-hr FFG in 3 hrs

ConclusionsConclusions

• Convection initiated ahead of warm front Convection initiated ahead of warm front in a moist atmosphere along a weak low in a moist atmosphere along a weak low level jetlevel jet

• Event was poorly handled by lower Event was poorly handled by lower resolution deterministic models and resolution deterministic models and manual forecastsmanual forecasts

• High-resolution model input and spatial High-resolution model input and spatial density plots may have been used to raise density plots may have been used to raise situational awarenesssituational awareness