FOUS30 KWBC 090758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 9 2024

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

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

Overall 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 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 decided to maintain the previous
SLGT risk inheritance with only some minor modifications made to
the overall risk area. 

Latest 00z HREF neighborhood probabilities for >3" of rainfall are
within the low-end 20-25% range with highest prospects likely 
between 1-2" when taking relevant CAMs into account. This is 
similar to the previous forecast prob fields from the 12z HREF 
iteration, and with coordination from the local WFOs along the Gulf
Coast earlier for the SLGT risk, there was no need to deviate from
what was proposed in the last update. Best chance 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. 

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...

As the previous shortwave trough kicks to the northeast, a stronger
mid-level trough will dig quickly out of the Rockies into the
Southern Plains by Tuesday morning with sights downstream on areas
east of the Mississippi. 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 Tuesday 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 profile 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 D2 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 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...

As the longwave pattern evolves further, the neutral-tilted trough
from previous D2 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 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 location 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 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
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