Louisville KY August 4 2009 Flash Flood Frank

Louisville, KY August 4, 2009 Flash Flood Frank Pereira NOAA/NWS/NCEP/Hydrometeorological Prediction Center

Motivation Recent high-profile flood events highlight the need for situational awareness of low probability, yet high impact events. Louisville: Aug. 21, 2009 Nashville: May 1, 2010 Atlanta: Sept. 21, 2009

Outline • • • Event Summary Model and HPC Performance Any Indication of Impending Event? Spatial Density Plots Conclusions and Discussion

Louisville Impacts Rainfall amounts up to 6 inches fell between 1100 Z and 1400 Z across central Louisville • Five inches fell in 90 minutes from 1145 Z to 1315 Z • In Louisville, nearly 200 people rescued from the tops of cars and houses. • No fatalities or injuries. • Courtesy of NWS WFO – Louisville, KY

SPC Analysis 04 Aug 12 Z 500 mb Height and Vorticity & 700 -400 mb Differential Vorticity Advection 850 mb Height, Temperature, Wind & Temperature Advection

SPC Analysis 04 Aug 00/12 Z Precipitable Water, Upwind Propagation Vectors & 1000 -500 mb Thickness

04 Aug 12 Z Nashville, TN Sounding PWAT = 1. 74 in.

Radar and HPC Surface Analysis

Verification 24 -hr Amounts Ending 04 Aug 12 Z observed Courtesy of NWS/OCWWS – National Precipitation Verification Unit HPC Ohio RFC NAM GFS

Verification 24 -hr Amounts Ending 05 Aug 12 Z observed HPC NAM Courtesy of NWS/OCWWS – National Precipitation Verification Unit Ohio RFC GFS

HPC Excessive Rainfall Graphics • Displays the probability that precipitation will exceed the flash flood guidance values issued by the River Forecast Centers (RFCs) Valid 04/12 Z – 05/12 Z Issued ~1000 Z

HPC Excessive Rainfall Graphics • Displays the probability that precipitation will exceed the flash flood guidance values issued by the River Forecast Centers (RFCs) Valid 04/15 Z – 05/12 Z Issued ~1400 Z

Moisture and Weak Warm Advection 12 -hour GFS Forecast Valid 04 Aug 12 Z Louisville 850 mb wind and PWATS Louisville 850 -700 mb Q-vector divergence & 850 mb warm air adv

SDF Forecast Sounding 09 -hour NAM Forecast Valid 04 Aug 09 Z Courtesy of NWS WFO – Louisville, KY

High-Res Model Guidance • HPC investigating utility of high-res model guidance to anticipate heavy rainfall threats • Example from 4. 0 km WRF-NMM (run @ EMC) initialized with the 04/00 Z NAM

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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”

Neighborhood / Spatial Density Approach • Create binary field where threshold exceeded (Flash Flood Guidance) • Smooth the resulting binary (1 or 0) distribution (using a Gaussian Smoother) Model 1 h QPF 1” Binary Schwartz et al. (2009)

Creating the Exceeding FFG Density Plot Raw data (1 s & 0 s) are run data through a Gaussian Weighted Filter to create an index of values from 0 -100 Model 1 h QPF > FFG Density Schwartz et al. (2009)

Density 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 usingle models • Not calibrated (30% does not necessarily occur 30% of time) SPCWRF 4 (30 -HR QPF) QPF > 3 -hr FFG in 3 hrs

Conclusions Convection initiated ahead of warm front in a moist atmosphere along a weak low level jet • Event was poorly handled by lower resolution deterministic models and manual forecasts • High-resolution model input and spatial density plots may have been used to raise situational awareness •