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=20
OHIO AND TENNESSEE VALLEYS ALONG WITH THE SOUTHEAST FLORIDA COAST...

...Parts of the Mid-South to Lower Ohio Valley...=20
Maintained the Marginal Risk area across parts of western and central
Tennessee into southern Kentucky as models still advertise an=20
upper-level trough amplifying to the west which allows for a more=20
curved upper level jet streak on the lee side of the trough. That=20
configuration may result in some enhanced rainfall rates...an idea=20
supported by the 18/00Z HREF neighborhood probabilities showing=20
potential for rainfall rates exceeding an inch per hour between=20
12-18Z today (mainly in Tennessee). With rainfall blossoming to=20
the west of the outlook area in the pre-dawn hours this morning=20
helping to prime the soils a bit more and a general slowing of the=20
eastward progression across the Tennessee Valley compared with=20
earlier forecasts...opted to extend the Marginal risk area a bit=20
farther westward than previously indicated. The HREF probability of
exceeding flash flood guidance remained low but did reflect=20
somewhat higher probabilities west of the same guidance from the=20
17/12Z and 17/18Z runs. The decision to expand westward slightly=20
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=20
coming down and the 0-40km soil moisture percentiles per NASA SPoRT
have climbed to 60-80% for most areas.=20

...Southeast FL...=20
Post frontal easterly low level flow off the Atlantic will linger=20
allowing for a possible repeat of convective clusters over and=20
near the southeast Florida coastline today. The 00Z runs of the=20
high-res CAMs continued to show onshore low-level moisture=20
flux/convergence and sufficient deep-layer instability to support=20
localized downpours. The 18/00Z HREF supports rates of 2+ inch per=20
hour peaking around 10 percent during the afternoon with and a=20
50-70 percent 40km neighborhood probabilities of at least 5 inches=20
of additional rain and 30-50 percent probabilities of 8+ inches.=20
Given that these numbers are comparable with the 17/12Z and 17/18Z=20
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.=20


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


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!_VGHg43JVbddWsGyChhnr8-fx97u_pymZDz3gX1SY7Kd=
dlOcg__DFsi6EXZOzhmDQhXigylvL1PrE30Ftfyc_-3_3qM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!_VGHg43JVbddWsGyChhnr8-fx97u_pymZDz3gX1SY7Kd=
dlOcg__DFsi6EXZOzhmDQhXigylvL1PrE30Ftfycs51r0AU$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!_VGHg43JVbddWsGyChhnr8-fx97u_pymZDz3gX1SY7Kd=
dlOcg__DFsi6EXZOzhmDQhXigylvL1PrE30FtfycVJoHKhQ$=20

=3D =3D =3D
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