FOUS30 KWBC 180823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Wed Dec 18 2024

Day 1
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF THE 
OHIO AND TENNESSEE VALLEYS ALONG WITH THE SOUTHEAST FLORIDA COAST...

...Parts of the Mid-South to Lower Ohio Valley... 
Maintained the Marginal Risk area across parts of western and central
Tennessee into southern Kentucky as models still advertise an 
upper-level trough amplifying to the west which allows for a more 
curved upper level jet streak on the lee side of the trough. That
configuration may result in some enhanced rainfall rates...an idea 
supported by the 18/00Z HREF neighborhood probabilities showing 
potential for rainfall rates exceeding an inch per hour between 
12-18Z today (mainly in Tennessee). With rainfall blossoming to 
the west of the outlook area in the pre-dawn hours this morning 
helping to prime the soils a bit more and a general slowing of the 
eastward progression across the Tennessee Valley compared with 
earlier forecasts...opted to extend the Marginal risk area a bit 
farther westward than previously indicated. The HREF probability of
exceeding flash flood guidance remained low but did reflect 
somewhat higher probabilities west of the same guidance from the 
17/12Z and 17/18Z runs. The decision to expand westward slightly 
was also driven by the observed rainfall during the past day or two
over parts of northwest Tennessee and southwest Kentucky which has
resulted in the latest 1-hour and 3- hour flash flood guidance 
coming down and the 0-40km soil moisture percentiles per NASA SPoRT
have climbed to 60-80% for most areas. 

...Southeast FL... 
Post frontal easterly low level flow off the Atlantic will linger 
allowing for a possible repeat of convective clusters over and 
near the southeast Florida coastline today. The 00Z runs of the 
high-res CAMs continued to show onshore low-level moisture 
flux/convergence and sufficient deep-layer instability to support 
localized downpours. The 18/00Z HREF supports rates of 2+ inch per 
hour peaking around 10 percent during the afternoon with and a 
50-70 percent 40km neighborhood probabilities of at least 5 inches 
of additional rain and 30-50 percent probabilities of 8+ inches. 
Given that these numbers are comparable with the 17/12Z and 17/18Z 
runs of the HREF...saw little reason to make too many changes other
than nudges to the boundary of the previously issued Marginal risk
area. 


Bann

Day 2
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Fri Dec 20 2024 - 12Z Sat Dec 21 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann
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