Louisville, KY August 4, 2009 Flash Flood

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

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

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

4. Louisville Impacts • Rainfall amounts up to 6-inches fell between 1100Z and 1400Z across central Louisville • Five inches fell in 90-minutes from 1145Z to 1315Z • 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

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

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

7. PWAT = 1.74 in. 04 Aug 12Z Nashville, TNSounding

8. Radar and HPC Surface Analysis

9. observed HPC Ohio RFC NAM GFS Courtesy of NWS/OCWWS – National Precipitation Verification Unit Verification24-hr Amounts Ending 04 Aug 12Z

10. observed HPC Ohio RFC NAM GFS Courtesy of NWS/OCWWS – National Precipitation Verification Unit Verification24-hr Amounts Ending 05 Aug 12Z

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

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

13. 850 mb wind and PWATS 850-700 mb Q-vector divergence & 850 mb warm air adv Moisture and Weak Warm Advection 12-hour GFS Forecast Valid 04 Aug 12Z Louisville Louisville

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

15. 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 04/00Z NAM

16. 0700 UTC

17. 0800 UTC

18. 0900 UTC

19. 1000 UTC

20. 1100 UTC

21. 1200 UTC

22. 1300 UTC

23. 1400 UTC

24. 1500 UTC

25. 1600 UTC

26. 1700 UTC

27. 1800 UTC

28. 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”

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

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

31. 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 using single models • Not calibrated (30% does not necessarily occur 30% of time) QPF > 3-hr FFG in 3 hrs SPCWRF4 (30-HR QPF)

32. 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