FOUS30 KWBC 110803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Dec 11 2024

Day 1
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...

Potent longwave trough is currently aligned over the Mississippi
Valley with a tilt closer to neutral as of the latest UA analysis.
A vigorous 160kt speed max at 250mb was analyzed over the Ohio
Valley on the leading side of the trough with sampled winds from
the 00z KILN sounding indicating those 160kt winds are verified.
This signals a very potent upper level evolution with an
anticipated holding of strength the next few hrs before a further
strengthening is anticipated later this morning and afternoon as
the trough axis begins tilting more negative allowing for a jet
buckling downstream as the speed max enters New England and
Quebec. At the surface, multiple waves of low pressure are centered
north to south from the eastern Great Lakes, down through the
Southern Appalachians as they lie within the elongated cold front
extending down from the northern low to the Central Gulf coast. The
forecast is for these waves of low pressure to propagate northward
during the next 3-6 hrs before the surface low over the lakes
becomes the dominant SLP feature, strengthening rapidly as the
trough buckles and the low pressure centered gets positioned firmly
within the right entrance region of the very powerful jet over
northeast North America. 

The evolution above will lead to a large swath of rainfall to
encompass much of the area along and east of the Appalachian front
with a trailing axis of convection situated over the southern half
of the Carolinas down into FL as the precip aligns along and just
ahead of the surface cold front. Frontal progression will lead to
rainfall shifting eastward overtime, however the environment ahead
of the front will be significantly enhanced with deep layer
moisture advecting poleward well ahead of the mean trough. PWAT
anomalies are already over +2 deviations across a good portion of
the Northeast U.S down through the Southeast with anomalies
expected to climb closer to +3/+4 deviations as me move into the
late-morning afternoon hours over the Northern Mid Atlantic into
New England. Assessment of local bufr soundings across Southern New
England from Central LI up through CT/MA/NH/ME signal a deep, uni-
directional wind field with modest theta_E advection occurring 
during the morning and afternoon time frame as the warm-conveyor 
points directly through the area with a well-defined warm front
bisecting the area by 18z. 

The time frame between 18-00z will be the most significant time
frame with regards to heavy rainfall potential across much of New
England as the cold front begins racing eastward and the warm front
tilts southwest to northeast from Western CT up through Southern
ME. This alignment will become a focus of a strong, cold rain band
that is textbook in these evolving inland synoptic pressure falls
leading to a period where rainfall rates will be a solid 0.5-1"/hr
extending within that convergence zone. The line will advance
eastward eventually as the cold front approaches and kicks the
convergence axis downstream leading to the heavy rain threat
shifting more towards the coastal regions with sights on Eastern
MA/NH/ME towards the 00-03z time frame. Backed flow off the
Atlantic will aid in maintaining the convergent field along and
ahead of the advancing front with heavy rains lingering until the
front back finally pass through and the winds shift west to
northwesterly ushering in colder air and stabilizing the mid-levels
enough to limit the heavier precip. Prior to this occurring, a
solid 2-3" with locally as high as 4" of rainfall are anticipated
in the zone encompassing Central and Eastern LI, much of MA/NH and
the western half of ME. Rapid snow melt will be a problem for 
areas located in the Northern Berkshires and Worcester Hills 
towards the MA/NH border extending northward as NOHRSC snow depth
analysis pins a solid 2-4+" on the ground currently within those
zones with over a foot in parts of NH/VT/Western ME. This will
allow for potential small stream cresting and ice jam concerns in
the interior that would exacerbate flood concerns during the
heaviest of the precip time frame later this afternoon. 

An expanse of Flood Watches are positioned across much of the 
above areas, including all of VT as complex terrain and higher snow
depths in the Green Mountains and surrounds leading to higher than
normal flood potential, despite the heaviest rain footprint likely
a bit further east. Considering the above variables and in
coordination with local WFOs, the inherited SLGT risk was
relatively unchanged with the only shift being an extension a bit
further southwest into CT to cover for urban flood concerns within
the axis of expected heavy rainfall during the cold frontal band
being forecasted across much of the 00z CAMs suite. The MRGL risk
still extends around the SLGT and further south into the Central
Mid Atlantic and Carolinas with the primary concerns likely
isolated urban flood potential, especially in the larger metro
corridors like Raleigh, DC/Balt, and Philadelphia. 

Kleebauer


Day 2
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

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

Kleebauer


Day 3
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

Fairly robust disturbance over the Pacific will translate 
southeastward towards the North-Central CA coast by the end of the 
week with a persistent IVT pulse oriented southwest to northeast 
into the coastal plain over the course of the forecast period 
leading to waves of moderate precipitation to impact areas north of
the Bay area. Current progs indicate precip totals on the order of
2-4" with locally higher embedded within the coastal ranges 
spanning north of Santa Rosa. Current rates anticipated to be 
modest with upwards of 0.5-0.75"/hr at peak intensity lending to a 
lower end threat for flash flooding, however totals are significant
enough to warrant a low-end threshold MRGL, mainly for the coastal
urban areas of Northwest CA up through Southwestern OR where 
totals will be on the order of 2-4", especially into the windward areas
of the Klamath Range. MRGL was generally maintained from previous
forecast with only some minor modifications to align with the
latest guidance. 

Kleebauer
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