THE 29 JULY 2004 FLASH FLOOD ON THE REEDY RIVER: A CASE STUDY

1. THE29 JULY 2004 FLASH FLOOD ON THE REEDY RIVER: A CASE STUDY

2. RADAR TELLS THE STORY . . . .

3. 6-8 inches in < 4 hours • Reedy River gage reaches 19’ • (highest level since 1908)

4. INTRODUCTION TO WEATHER MODELING---------------------------------------------------- National Centers for Environmental Prediction Nested Grid Model (NGM) Global Forecast System (GFS) North American Model (NAM – formerly Eta) ECMWF, NOGAPS, etc…….. Regional Mesoscale Models MM5 RUC WRF WS-ETA

5. CHAMPS Overview Multidisciplinary application of numerical weather prediction * Real-time mesoscale modeling for winter and tropical storms * Regional climate modeling for seasonal forecasting * Coupled land surface-hydrologic modeling for water resource management and flood forecasting Collaborative effort to enhance preparedness and response during weather-related emergencies * SC Department of Natural Resources * SC Emergency Management Division * National Weather Service (GSP, CAE, CHS, ILM) * USC-Columbia, Clemson University, and others

6. INTRODUCTION TO ENVIRONMENTAL PREDICTION SYSTEMS SCAN, COOP, WBAN, ASOS, other observations Gridded Model Analyses & ForecastsMM5 Real-time, Ensembles, Climate Model, Tropics RegCM3 Climate Model Air Quality, Hydrologic, Smoke/Fire Weather, Water Resource Management SCO Website, FTP access Observations and Model Inputs Local Atmospheric Modeling Systems Regional Applications Web Pages and Distribution Approaches

7. MODEL ARCHITECTURE Using the NCAR/PSU fifth generation mesoscale modeling system (MM5) Why choose the MM5 ? 1) Scope of Use: Globally re-locatable 2) Nesting Capability: Multiple and Flexible 3) Coupling with other NWP models 4) Various Computer Platforms 5) WELL DOCUMENTED, USER SUPPORT

8. HOW DOES GRID-SIZE AFFECT FLOOD FORECASTING??? 27km 27km 9km 9km Total Precipitation 3km 1-2”2-4”4-6” 3km

9. MITIGATION THROUGH PREPARATION Improve weather forecasting techniques *Lead-time factor *Post-landfall flooding *Anticipate travel problems Long-term, climate forecasting tools *Initiate water restrictions *Agricultural, economic considerations Research/Case Studies *What if…hydrologic (river) modeling *What went wrong…post-event insight

10. NAM FORECASTS FOR THE EVENT

11. NAM MODEL BASICS DOMAIN: North America (west to Hawaiian Islands) GRID: 12-km grid (40-km output on 212-grid) ***BEST MESOSCALE MODEL OPERATIONALLY*** MM5 MODEL BASICS DOMAIN: Southeast US / Upstate SC GRID: 12-km / 4-km (output on same grid)

12. MM5 MODEL OUTER DOMAIN 12-kilometer resolution 10-minute USGS terrain

13. MM5 MODEL INNER DOMAIN 4-kilometer resolution 2-minute USGS terrain

14. MM5 MODEL HI-RES TERRAIN

15. MM5 SENSITIVITY TESTS What is a sensitivity test?? Changes to the design of the model or input information to investigate how the output changes (i.e. rainfall) Why do a sensitivity test?? To determine which model configuration performs best in a given scenario Are we guaranteed to get better results than operational models? Not a chance…..we should see improved results but not always. So what did MM5 show for this particular event?? Let’s see…….

16. MM5 SENSITIVITY TESTS #1 MRF PBL scheme, Simple Ice Microphysics (MRFSI) What does this mean? MRF is often used in summer months and typically produces higher precipitation amounts Simple Ice usually generates more tropical precipitation #2 BLK PBL scheme, Reisner 2 Microphysics (BLKR2) What does this mean? BLK has been shown to be preferred in summer months and reduced spurious convection along the coasts Reisner 2 includes additional ice phases in clouds such as hail and larger ice particles and may perform best in summer-convection

17. SYNOPTIC OVERVIEW • - Low-level southeasterly winds off the Atlantic Ocean • - Persistent Upslope Flow for 24 hours prior to the event • Old frontal boundary stretched across the Upstate • GSP OPERATIONS • Briefing highlighted upslope component and slow motion • Flood statements issued as early as 8:38 AM • 51 operational phone calls

18. CHRONOLOGY OF THE 29 JULY FLOOD EVENT 840LST T-storms converged over W Greenville Co Reedy River gage reaches 6.0 feet 900LST Two inch per hour rainfall near Berea Reedy River gage reaches 8.0 feet 904LST Flood Warning Issued 1037LST Flash Flood Statement highlighting flooded yards and local fishing pond over-running dam 1039LST River stage up to 12.21 feet, homes under water to roofs along Long Branch; Warning extended til 1500LST 1226LST Gage reads 16.1 feet 1330LST Rain ending over the Reedy Basin Gage at 16.84 feet Gage fails due to submersion by water– post-storm survey reveals 19 feet 2 inch height. FLOODING CONTINUES THROUGH EVENING ON LOCAL RIVERS AND CREEKS

19. REMEMBER THE RADAR . . . . 8AM 9AM 10AM 11AM NOON 1PM

20. PRECIPFORECASTS 5AM 8AM 11AM

21. PRECIPFORECASTS 2PM 5PM 8PM

22. SENSITIVITY TEST #2 INDICATES MESO-LOW

23. INVESITIGATING THE WINDS….. INITIAL 8AM 3AM 3PM 8PM

24. WHAT DID THE MM5 RUNS SHOW??? Sensitivity test #1 failed to produce significant precipitation until later in the period Sensitivity test #2 produced early precipitation but moved the precipitation south instead of holding it stationary Sensitivity test #2 performed best in comparison Both MM5 model runs generated precipitation in regions that precipitation occurred- unlike the NAM Wind forecasts and frontogenesis forecasts show a meso-scale low pressure system across the Upstate in the vicinity of the heaviest rainfall

25. CONCLUSIONS AND REMARKS Further testing is necessary with the MM5 model to discern the best model design for this particular event Incorporation of local mesonet data into the initial conditions Flash flood events may be better indicated by local, mesoscale models in short-term forecasting than in the operational NCEP products Mitigation of future flood events may become possible through use of short-term regionally-specific model products for additional guidance. Future work will entail selection of multiple Reedy River flood events and MM5 runs to maximize performance level for all events