FOUS30 KWBC 091955
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Mon Dec 9 2024

Day 1
Valid 16Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

Current radar and surface observations indicate the ongoing
forecast on track with southwest-east oriented bands of showers
and storms impacting the central Gulf Coast with brief periods of
training and hourly rainfall rates up to 2". Overall the pattern
evolution will continue to point to a benign instability axis
(MUCAPE ~500-1000 J/kg) being confined closer to the Gulf with
sufficient deep layer shear and moisture presence (PWATs of
1.5-2.0" and b/w +1 to +2 deviations) allowing for a small area of
convective development capable of locally enhanced rainfall rates
for flash flood potential. Given the overall antecedent dry
conditions prior, the prospects of flash flooding will likely be
relegated to more urban settings along the Central Gulf coast,
including places such as New Orleans, Biloxi, and Mobile. A few
other Parishes within Southeastern LA also contain areas that
experience a bit easier ability to flash flood given some of the
above urbanization factors, so the previous SLGT risk was
maintained with only some minor modifications made to the overall
risk area.

Latest 12z HREF neighborhood probabilities for >3" of rainfall are
in the 20-40% range with highest prospects likely between 1-2"
when taking relevant CAMs into account. With coordination from the
local WFOs along the Gulf Coast earlier for the SLGT risk, there
was no need to deviate much from what was proposed in the previous
forecasts. Best chances will reside from training echoes on
persistent west- southwest flow during the diurnal instability
maximum, carrying into the evening before the setup decays prior to
the next period of relevant convection for D2.

Snell/Kleebauer


Day 2
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
ALABAMA TO UPSTATE SOUTH CAROLINA...

...1930 UTC Update...
Minimal (cosmetic) changes made to the Day 2 ERO based on the
latest guidance suite. Within the Slight Risk area, blend of
operational guidance continues to show areal average totals between
2-3 inches, with locally higher totals per the CAMs. Continues to
look like a low-end Slight however (neighborhood exceedance
probabilities closer to 15% vs. 40%), considering the lack of
deep-layer instability which will limit the short-term rainfall
rates.

Hurley

...Previous Discussion...
As the previous shortwave trough kicks to the northeast, a stronger
mid-level trough will dig quickly out of the Rockies into the=20
Southern Plains by Tuesday morning with sights downstream on areas=20
east of the Mississippi. Increasing meridional flow ahead of the=20
digging trough will aid in the advection of deep moisture poleward=20
with the initial surge propagating north out of the Gulf through=20
the Southeast by Tuesday afternoon, eventually spreading northward=20
into the Southern and Central Mid Atlantic the second half of the=20
period. The advection pattern will be sufficient in increasing=20
regional mid-level instability as profile moisture saturates deeper
into the profile across the areas above. PWAT anomalies on the=20
order of 2-3 deviations above normal will be common over parts of=20
GA spreading up through the neighboring Carolinas, eventually=20
surging into the Central Mid Atlantic and Northeastern U.S by the=20
end of the D2 cycle. This will prime several areas for heavy=20
rainfall prospects as the final ingredients come to together for a=20
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 Tuesday afternoon 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, 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 Virginia with some lighter
precip making headway into areas further north. Places across
Southwestern VA down to the I-40 corridor in NC will see anywhere
from 1-2" of rainfall with locally higher further south through the
zone. 2-3" with local totals to 4" will be plausible for the area
extending from Southeastern AL through Central GA into Upstate SC,
an area more prone to flash flood prospects thanks to the
complexity of the local terrain.

This signal has been prevalent over the past series of
deterministic model outputs and is now firmly within the ensemble
mean as bias corrected ENS now supports 2.5-3" through much of the
area outlined above in the 2-3" forecast. The saving grace in this
setup is the time of year allowing for lower surface based
instability parameters and very dry antecedent conditions leading
into the event. This allowed for a SLGT risk to be sufficient for
the setup, a carry over from the previous forecast. The MRGL risk
still extends into VA where the northern periphery of the heavy
rainfall is expected.

Kleebauer


Day 3
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...

...1930 UTC Update...
Minimal changes made to the Day 3 ERO based on the latest guidance
suite. One of the more notable tweaks was to extend the Marginal
Risk a little farther s-sw to include more of the NC Piedmont and
the Charlotte metro region. This is because the event (heavier
rainfall) will be ongoing Wed morning in that area. TPW anomalies
across New England during Day 3 peak between 4-4.5 standard deviations
above normal per the NAEFS, where areal-average totals between 2-3"
are still expected within the Slight Risk area. Once again, as with
the Slight Risk area on Day 2, the main inhibitor for flash
flooding will be the lack of elevated instability and thus cap on
short-term rainfall rates.=20

Hurley

...Previous Discussion...
As the longwave pattern evolves further, the neutral-tilted trough
from previous D2 time frame will enhance further with the mean=20
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=20
deviations above normal will quickly progress northward=20
encompassing much of New England by Wednesday afternoon, even=20
getting up towards +4 deviations as we move into Wednesday night=20
over Northern New England from NH through ME. This pattern is=20
textbook for a corridor of heavy rainfall to spread from the Mid=20
Atlantic through all of the Northeast with the primary areas of=20
interest location along and ahead of the cold front that will sweep
eastward during the forecast period. Portions of Western NY state=20
and Northern PA will eventually change to winter precipitation=20
allowing for a decay in flash flood concerns, however areas=20
downstream will be under a respectable deluge of rainfall prior to=20
the rain threat ceasing in wake of the front.

Totals of 1.5-2.5" 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" is 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 very low at this point.

The setup 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 Eastern NC
as the early portion of the forecast period will still see
moderate to heavy rainfall Wednesday morning before pulling
northward.

Kleebauer

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!5p4ZIKz1GdsVc9JBavSpwL6p9Cu5wM9ksJBBkdB2iBDr=
PFEy4To3GNv9tTLjwx8o3Nb9MZSQAQ1FRtmUQN1rgdB-8hA$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5p4ZIKz1GdsVc9JBavSpwL6p9Cu5wM9ksJBBkdB2iBDr=
PFEy4To3GNv9tTLjwx8o3Nb9MZSQAQ1FRtmUQN1r3TR_tT0$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5p4ZIKz1GdsVc9JBavSpwL6p9Cu5wM9ksJBBkdB2iBDr=
PFEy4To3GNv9tTLjwx8o3Nb9MZSQAQ1FRtmUQN1rp4IniKA$=20

=3D =3D =3D
To unsubscribe from WX-STORM and you already have a login, go to
https://lists.illinois.edu and use the "Unsubscribe" link.  Otherwise email
Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM.

--- SBBSecho 3.20-Linux