FOUS30 KWBC 172000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Tue Dec 17 2024

Day 1
Valid 16Z Tue Dec 17 2024 - 12Z Wed Dec 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE EAST CENTRAL TO SOUTHEAST COAST OF FLORIDA...

...16Z update...

12Z soundings from MFL and KEY showed PW values of 1.5 and 1.7
inches, respectively, or near the 90th percentile for mid-
December. Low level easterly flow is expected to support
showers/thunderstorms moving inland during the day today with an
increasing threat for slow moving/stalled cells late today and
overnight along an expected coastal/near-coastal convergence axis.

925-850 mb winds of 15 to 20+ kt are forecast by the latest
guidance with a shift toward more of a southeasterly direction
later tonight. The 12Z HREF indicated neighborhood probabilities
for 5 inches in 24 hours (ending 12Z Wednesday) of 50-60 percent
and while these probabilities have come down and are largely=20
driven by the more bullish 12Z NAM_nest and ARW2, a low end threat=20
for localized flash flooding will remain through 12Z Wednesday for=20
the eastern and southeastern FL Peninsula.

Otto

...previous discussion follows...


Model consensus is for increasingly active showers late Tuesday
afternoon into Tuesday night/early Wednesday along the east central
to southeast coast of Florida. Low level easterly to southeasterly
flow in an axis of PW values 1.5 to 2+ standard deviations above
the mean expected to persist into the east central to southeast
Florida coast, with models showing an axis of defined boundary
layer convergence in this low level inflow axis parallel to the
east central to southeast coast. There will be potential for very
slow moving cells along the coastal regions, supporting locally
very heavy rainfall amounts and an urban flash flood threat. HREF
neighborhood probabilities are high (50-90%) for 3"+ amounts from
Cape Canaveral, south to Miami, while the 5"+ neighborhood
probabilities are high (60-90%) from near West Palm Beach, south to
Miami. The best hi res overlap, evident in the greatest EAS
probabilities for 2"+ amounts, is from near West Palm, south to
just north of Miami.

Oravec


Day 2
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF
THE MID-SOUTH AND LOWER OHIO VALLEY ALONG WITH THE SOUTHEAST=20
FLORIDA COAST...

...TN into southern KY...
Have included a targeted Marginal Risk in the updated Day 2 ERO
across parts of western-central TN (especially northern portions)
into southern KY, which would include the Nashville metro area.
Amplifying upper trough to the west will allow for a more curved
upper level jet streak on the lee side of the trough -- which will
enhance the upper divergence and low-level moisture transport/flux
into the outlook area. The latest (12Z) HREF show more elevated
probabilities of 1"/hr rainfall rates between 12-18Z. In addition,
especially when considering recent heavy rainfall over the past=20
24+ hours (1-2 inches over parts of nw TN and sw KY), the latest 1
and 3 FFG values have come down while 0-40km soil moisture
percentiles per NASA SPoRT have climbed to 60-80% for most areas.
As such, the HREF neighborhood probabilities of 3 hourly QPF=20
exceeding 3 hour FFG peaks between 25-40% between 12-15Z Wed, when
in that short 3-hour window most areas should see additional=20
rainfall of 1-1.5" while some (isolated) locations receive closer=20
to 2+ inches.


...Southeast FL...
Basically carrying over the Marginal Risk area from Day 1 into the
Day 2 period, as much of the 12Z high-res CAMs show lingering
convective clusters along the se FL coast (mainly south of Vero
Beach). Continued favorable (onshore) low-level moisture
flux/convergence and sufficient deep-layer instability on Wed will
maintain elevated short-term rainfall rates, likely 1-2+ in/hr at
times during the day 2 period per the HREF probabilities. The 12Z=20
HREF meanwhile also indicates 50-70% 40km neighborhood=20
probabilities of at least 5 inches of additional rain during Day 2
(12Z Wed-12Z Thu), with 30-50% probs of 8+ inches. The NAM-CONUS=20
Nest, ARW1, and 12Z HRRR in particular are leading to these high=20
HREF probabilities.=20

Hurley


Day 3
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

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

Hurley


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!99ChtqScNzFHFt1Sx9hi6HGttWsZb6Y3RkAhHODBf4BK=
pdTtu8mLOa6H8YwUEV07SWB1lJstCx9wuldJSg7GODxh8XI$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!99ChtqScNzFHFt1Sx9hi6HGttWsZb6Y3RkAhHODBf4BK=
pdTtu8mLOa6H8YwUEV07SWB1lJstCx9wuldJSg7G19Fvcv0$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!99ChtqScNzFHFt1Sx9hi6HGttWsZb6Y3RkAhHODBf4BK=
pdTtu8mLOa6H8YwUEV07SWB1lJstCx9wuldJSg7GIIpOF0M$=20

=3D =3D =3D
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