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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
741 PM EST Tue Dec 10 2024

Day 1
Valid 01Z Wed Dec 11 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE=20
WESTERN CAROLINAS...

...01Z Update...

The Slight Risk area was trimmed to portions of the western
Carolinas, while the southern end of the Marginal Risk area was
trimmed from the west where the rain has ended and to the east
where expected rainfall overnight is expected to be light enough to
preclude any flooding concerns.

The Slight Risk area was reduced to the western Carolinas this
evening due to diminishing instability favoring mostly stratiform
rainfall across the Southeast tonight. Isolated areas of heavy rain
in the western Carolinas has made a few areas more flood prone,
while topographic concerns can still result in flooding in the
adjacent valleys. Thus, in coordination with GSP/Greer, SC forecast
office the Slight Risk was maintained for this update.=20

Wegman

Previous Discussion...

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.
As the mean trough pivots across the Southern Plains near OK/TX,
increasing meridional flow ahead of the digging trough will aid in
the advection of deep moisture poleward with the initial surge
propagating north out of the Gulf through the Southeast by the
afternoon, eventually spreading northward into the Southern and
Central Mid Atlantic the second half of the period. The advection
pattern will be sufficient in increasing regional mid- level
instability as moisture saturates deeper into the profile across
the areas above. PWAT anomalies on the order of 2-3 deviations
above normal will be common over parts of GA spreading up through
the neighboring Carolinas, eventually surging into the Central Mid
Atlantic and Northeastern U.S by the end of the D1 cycle. This
will prime several areas for heavy rainfall prospects as the final
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
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through central GA into portions of east-central AL, aligning
with the cold front as it moves through the region. The eventual
heavy rain footprint will spread northeastward into the Southern
Mid Atlantic with locally heavy rainfall impacting areas as far
north as southwestern Virginia with some lighter precip making
headway into areas further north.

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
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
putting the Atlanta metro squarely within the SLGT risk forecast.
The SLGT remains firmly in-place across Upstate SC with a general
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
Greenville/Spartanburg). The MRGL risk remains generally in a
similar place compared to previous forecast, but some minor
adjustments to the northwest across AL and northward a bit further
into western VA with the most notable inclusion of the Birmingham
metro where the urban flash flood threat will lie right on the edge
where guidance is depicting the heaviest rainfall.

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...

20z Update: The below discussion still describes the situation
well, and only minimal changes were made to the inherited
ERO. Southern VT into central NH and ME will see the overlap of
1-3" of rain and significant snow melt, likely leading to some
flooding concerns. HREF supports hourly rainfall over this area
approaching 0.5", which combined with the 0.25"-0.5" of hourly
snow water equivalent melt shown in NOHRSC forecasts, could be
enough to result in some areas of rapid onset flooding.

Chenard

...Previous Discussion...

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
neutral- tilted trough from previous D1 time frame will enhance
further with the mean trough taking on a more negative tilt
allowing for enhanced surface low strengthening as the surface low
progresses northward into New England and eventually Quebec. PWAT
anomalies on the order of 2-3 deviations above normal will quickly
progress northward encompassing much of New England by Wednesday
afternoon, even getting up towards +4 to +5 deviations as we move
into Wednesday night over Northern New England from NH through ME.
This pattern is textbook for a corridor of heavy rainfall to spread
from the Mid Atlantic through all of the Northeast with the
primary areas of interest located along and ahead of the cold front
that will sweep eastward during the forecast period. Portions of
Western NY state and Northern PA will eventually change to winter
precipitation allowing for a decay in flash flood concerns, however
areas downstream will be under a respectable deluge of rainfall
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
up through Southern New England, eventually moving up through ME as
we close out the forecast period. 1-2" with locally higher amounts
are forecast for points further west, but those areas also have
the added snow melt factors that will play into the flooding
concerns as high dewpoints and heavy rain will promote rapid snow
melt for places in the interior over VT/NH and the neighboring
Berkshires in MA. The expectations are for some creeks and smaller
streams to become problematic in the setup and could cause
localized flood concerns during the peak of the heavy rainfall. The
rivers across New England can thankfully take a decent surge of
moisture after a very dry fall, so the prospects of significant
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
zones in the interior where rapid snow melt has historical
precedence for localized flash flood concerns. A MRGL risk will
encompass the SLGT risk across Northeast with an extension down
into Central and Northeastern NC as the early portion of the
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.

Chenard

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!-X8P93eWw1Bs3GgGcl-M71PMEVEzVvVJ1rHcpNB4ck4L=
lI6Dyo9H5GVUCG2LVd2uZ9xNTAxaH96ngDyk-c9Y3fWLdb4$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-X8P93eWw1Bs3GgGcl-M71PMEVEzVvVJ1rHcpNB4ck4L=
lI6Dyo9H5GVUCG2LVd2uZ9xNTAxaH96ngDyk-c9YP9T7Fqg$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-X8P93eWw1Bs3GgGcl-M71PMEVEzVvVJ1rHcpNB4ck4L=
lI6Dyo9H5GVUCG2LVd2uZ9xNTAxaH96ngDyk-c9Y3k_8ENI$=20

=3D =3D =3D
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