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
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Event Summary
Model and HPC Performance
Any Indication of Impending Event?
Spatial Density Plots
Conclusions and Discussion
Louisville Impacts
•
Rainfall amounts up to 6inches fell between
1100Z and 1400Z across
central Louisville
• Five inches fell in 90minutes 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
SPC 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 00/12Z
Precipitable Water, Upwind Propagation Vectors &
1000-500 mb Thickness
04 Aug 12Z Nashville, TN
Sounding
PWAT = 1.74 in.
Radar and HPC Surface Analysis
Verification
24-hr Amounts Ending 04 Aug 12Z
observed
Courtesy of NWS/OCWWS –
National Precipitation Verification Unit
HPC
Ohio RFC
NAM
GFS
Verification
24-hr Amounts Ending 05 Aug 12Z
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 04/12Z – 05/12Z Issued ~1000Z
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
Moisture and Weak Warm Advection
12-hour GFS Forecast Valid 04 Aug 12Z
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 09Z
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
04/00Z 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”
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 (1s & 0s) 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 using single models
•Not calibrated (30% does not necessarily occur 30% of time)
SPCWRF4 (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