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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0811Z Jul 10, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Wed Jul 10 2024

Day 1
Valid 12Z Wed Jul 10 2024 - 12Z Thu Jul 11 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
UPSTATE NEW YORK THROUGH MUCH OF VERMONT AND NORTHERN NEW
HAMPSHIRE...

...Northeast...

Remnant moisture and mid-level energy from Beryl will translate
northeastward out the Ohio Valley and Great Lakes creating a
heightened threat for widespread heavy rain and flash flooding
across Southeast Michigan through New York and Central/Northern
New England. At the surface, a warm front will bisect much of New
York through Central New England, marked very well by a sharp
theta-E gradient. This will be a focal point for convection prior
to the main circulation moving overhead, as well as the axis of
where the highest PWATs will be confined during the unfolding of
the synoptic scale event. Low- level instability will be highest
within the confines of the warm front and points south with the
best upper level forcing likely along and north of the boundary
creating a dynamic scenario with a zone of highest heavy rain
potential where all three characteristics overlap. The highest
heavy rain prospects lay within the Adirondacks and points east
into north-central Vermont and New Hampshire, including the Green
and White Mountains in the respective states.

12z HREF EAS probabilities were much more aggressive in the signals
for at least 2" and 3" across the aforementioned areas with a
50-80% probability for at least 2" within the Adirondacks to just
south of the Champlain Valley, a strong signal for higher totals
given the necessary overlap of CAMs to exhibit such a larger
probability. Historically, when an EAS signal is above 70%, the
expectation for widespread coverage of that value of rain or more
is very certain and regardless a higher risk consideration if
the areal FFG indices allow. HREF EAS for 3" was not as strong, but
still manages a large coverage of 20-35% with the highest potential
across the Adirondacks and northern Mohawk Valley. This is right
within the inflection of where the warm front is forecast to
reside, creating a zone of higher confidence for heavy rainfall.
The main prospects for flash flooding will occur between 18z
Wednesday until about 06z across New York state until the end of
the period for points further north and east.

Heavy rain will be possible all the way into Maine where the
elevated PWAT anomalies between +2 to +3 standard deviations will
be recognized leading to a higher end SLGT risk residing from
Southeast Michigan all the way into western and central Maine. Totals
of 2-4" are anticipated with local maxima to 6" plausible within
the above areas in the Moderate Risk. 1-3" will be possible as far
south as central and northeastern PA up through the NY Capital
District due to convection developing along and ahead of the
trailing cold front moving through the area tomorrow afternoon and
evening.

Kleebauer


...Central Rockies and portions of the Southwest...


A Slight Risk for excessive rainfall remains in effect across the
higher terrain of central New Mexico. During this period monsoonal
moisture and diurnal heating will maintain convection across
portions of the Central and Southern Rockies and southwest New
Mexico. Some of the highest forecast QPF is expected to focus in
the vicinity of the Sacramento Mountains where there have been a
few recent wildfires. he burn scar complex
(Blue-2, South Fork, Salt and McBride) are highly sensitive to
rainfall and can easily lead to flooding and debris flows,
especially with the 0.25 to 1 inch QPF that is forecast.

...Central Gulf Coast...

Training convection expected to continue along the stalled frontal
boundary. Very high FFGs within the area highlighted by a Marginal
Risk which will limit flash flooding to very isolated and mainly
within larger urban zones like New Orleans and towns based in the
southeastern Parishes. Given the limited threat, should the
convection shift further south offshore there may be the need to
downgrade in future updates.

Campbell


Day 2
Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST THROUGH THE MID ATLANTIC, AS WELL AS THE SOUTHWEST
U.S...

...East Coast...

Much of the remnant moisture and ascent from Beryl will be out of
the region however additional rainfall of upwards of 1 to 1.5
inches will be possible across New England, mainly during the
morning hours. Further south, the trailing cold front will be the
focus for convective potential with the highest risk of flash
flooding aligning within the Del-Mar-Va Coastal Plain down through
the eastern Carolinas. An upper level disturbance of the Atlantic will
approach the region and enhance upper forcing and moisture as it
moves closer to the Carolina coast. Meanwhile, a area of
convergence in proximity to a slow-moving surface front will help
trigger the development of widespread convection. The deep layer of
moisture over this region will help bolster rainfall and with the
expected training of cells, 2 to 4 inches may spread over the Mid-
Atlantic. Should the guidance persist with this set up and/or
trend up with amounts, there may be the need to upgrade Risk level.

Kleebauer/Campbell

...Southwest...

The persistent monsoonal pattern of diurnal convection will
maintain an elevated threat for localized heavy rainfall and flash
flooding concerns. Local totals of up to 1" signal a lower-end
threat, however it remains within the Marginal Risk threshold.

Campbell


Day 3
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND NORTH CAROLINA...

The training of thunderstorms capable of producing several inches
of rain will continue through this period. Rain from the day 2
period will have helped to raise soil saturation along the Mid-
Atlantic and Carolina region thus lowering FFG. Most of the
guidance is suggesting additional amounts of 1 to 3 inches from
South Carolina to southern New Jersey with a couple showing local
maximums up to 5 inches along this corridor. A Slight Risk was
maintained for this part of the East Coast. A Marginal Risk spans
from South Carolina northward to southern New hampshire.

Campbell


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt