FOUS30 KWBC 100756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Tue Dec 10 2024

Day 1
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL ALABAMA
THROUGH PORTIONS OF THE WESTERN CAROLINAS...

The overall synoptic pattern remains on track to deliver a swath of
heavier rainfall to portions of the Southeast and Southern Mid
Atlantic during the period. A strong mid- level trough is currently
analyzed over the Four Corners region, digging quickly to the
southeast with sights on the Southern Plains by later this morning.=20
As the mean trough pivots across the Southern Plains near OK/TX,=20=20
increasing meridional flow ahead of the digging trough will aid in
the advection of deep moisture poleward with the initial surge=20
propagating north out of the Gulf through the Southeast by the=20=20
afternoon, eventually spreading northward into the Southern and=20
Central Mid Atlantic the second half of the period. The advection=20
pattern will be sufficient in increasing regional mid- level=20
instability as moisture saturates deeper into the profile across=20
the areas above. PWAT anomalies on the order of 2-3 deviations=20
above normal will be common over parts of GA spreading up through=20
the neighboring Carolinas, eventually surging into the Central Mid=20
Atlantic and Northeastern U.S by the end of the D1 cycle. This=20
will prime several areas for heavy rainfall prospects as the final=20
ingredients come to together for a locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into the afternoon hours and=20
beyond. The main trigger for a line of enhanced convection will=20
form at the hands of a strong surface cold front that will shove=20
eastward beneath an intensifying surface low that develops in=20
conjunction to the favorable upper level evolution. Current=20
deterministic sfc to 850mb wind field indicates a corridor of=20
backed flow across much of GA into the Carolinas during Tuesday=20
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front=20
swings eastward by late Tuesday night through early Wednesday, the=20
peak rainfall rates will be occurring across the Carolina Piedmont=20
down through central GA into portions of east-central AL, aligning
with the cold front as it moves through the region. The eventual=20
heavy rain footprint will spread northeastward into the Southern=20
Mid Atlantic with locally heavy rainfall impacting areas as far=20
north as southwestern Virginia with some lighter precip making=20
headway into areas further north.=20

Recent 00z HREF output remains steadfast with the prospects of a
widespread 1-3" of rainfall across a large chunk of the Southeast
into the Highlands of the Western Carolinas with the northern
extension of any maxima peeking into southwestern Virginia where
some of the terrain will make use of the southeasterly upslope
component to the wind field. Probability fields in both the
neighborhood and EAS realm are pretty solid when it comes to the
potential with the latter of which indicating a well-defined axis=20
of 30-50% for at least 2", a signal prevalent historically for
favored SLGT risks or higher for the current impact areas.
Neighborhood probabilities are of course higher in percentile given
how the prob field is calculated, however the most notable output
is the very high 60-80% coverage for >3" and low-end probs of
20-25% for up to 5" across the area southeast of Birmingham up
through the Atlanta metro. This was a small shift northward in
terms of the heaviest QPF axis from previous forecasts and align
well with the latest ECMWF and GFS ML outputs, a testament to
increasing agreement within the deterministic on where the heaviest
rainfall will occur. The saving grace from this setup being a more
prolific outcome is the lack of sufficient surface based
instability that tends to add a greater convective element to the
rainfall rates that shift the signal to a higher-end risk threat.
Thankfully, that is not the case here, so the threat does remain a
bit capped on the upper bound of potential.

The previous SLGT risk was adjusted a bit further northwest across
AL/GA to reflect the change within the axis of heaviest rainfall=20
putting the Atlanta metro squarely within the SLGT risk forecast.=20
The SLGT remains firmly in-place across Upstate SC with a general=20
northward extension into the Appalachian front of southwestern NC
as well as small eastward expansion into the Charlotte metro to
atone for the higher QPF being signaled as of the latest 00z
guidance (All in coordination with the local WFO in=20
Greenville/Spartanburg). The MRGL risk remains generally in a=20
similar place compared to previous forecast, but some minor=20
adjustments to the northwest across AL and northward a bit further=20
into western VA with the most notable inclusion of the Birmingham=20
metro where the urban flash flood threat will lie right on the edge
where guidance is depicting the heaviest rainfall.=20

Kleebauer

Day 2
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

The previous forecast remains largely unchanged as the synoptic
regime anticipated for mid-week continues to trend favorable for
heavy rainfall across much of the Northeast and Mid Atlantic. The=20
neutral- tilted trough from previous D1 time frame will enhance=20
further with the mean trough taking on a more negative tilt=20
allowing for enhanced surface low strengthening as the surface low=20
progresses northward into New England and eventually Quebec. PWAT=20
anomalies on the order of 2-3 deviations above normal will quickly=20
progress northward encompassing much of New England by Wednesday=20
afternoon, even getting up towards +4 to +5 deviations as we move=20
into Wednesday night over Northern New England from NH through ME.=20
This pattern is textbook for a corridor of heavy rainfall to spread
from the Mid Atlantic through all of the Northeast with the=20
primary areas of interest located along and ahead of the cold front
that will sweep eastward during the forecast period. Portions of=20
Western NY state and Northern PA will eventually change to winter=20
precipitation allowing for a decay in flash flood concerns, however
areas downstream will be under a respectable deluge of rainfall=20
prior to the rain threat ceasing in wake of the front.

Totals of 2-3.5" of rain are forecast across much of Long Island=20
up through Southern New England, eventually moving up through ME as
we close out the forecast period. 1-2" with locally higher amounts=20
are forecast for points further west, but those areas also have=20
the added snow melt factors that will play into the flooding=20
concerns as high dewpoints and heavy rain will promote rapid snow=20
melt for places in the interior over VT/NH and the neighboring=20
Berkshires in MA. The expectations are for some creeks and smaller=20
streams to become problematic in the setup and could cause=20
localized flood concerns during the peak of the heavy rainfall. The
rivers across New England can thankfully take a decent surge of=20
moisture after a very dry fall, so the prospects of significant=20
flooding are lower than normal at this point.

The previously inherited forecast was largely unchanged as the
setup and subsequent forecast is still worthy of a large SLGT risk
encompassing much of New England with the best threats likely over
urbanized areas in LI and Southern New England and over those=20
zones in the interior where rapid snow melt has historical=20
precedence for localized flash flood concerns. A MRGL risk will=20
encompass the SLGT risk across Northeast with an extension down=20
into Central and Northeastern NC as the early portion of the=20
forecast period will still see moderate to heavy rainfall Wednesday
morning before pulling northward.

Kleebauer


Day 3
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.=20

Kleebauer

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!7S5fGphiS3QezPr1KU0yMpAccyf4DGGKKWMoUK2BPBoH=
V_vUTOJbRUjFzljWbAdmgg0i55ZD_xulKEtBtVZO1B1pxXA$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!7S5fGphiS3QezPr1KU0yMpAccyf4DGGKKWMoUK2BPBoH=
V_vUTOJbRUjFzljWbAdmgg0i55ZD_xulKEtBtVZOtOAM464$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!7S5fGphiS3QezPr1KU0yMpAccyf4DGGKKWMoUK2BPBoH=
V_vUTOJbRUjFzljWbAdmgg0i55ZD_xulKEtBtVZOHeDvYJ8$=20

=3D =3D =3D
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