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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0822Z Jul 11, 2024)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Thu Jul 11 2024

Day 1
Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHEAST
VIRGINIA AND EASTERN CAROLINAS...

...Coastal Mid Atlantic...

Stalled frontal boundary will bisect portions of the Eastern
Seaboard with elevated PWATs mirroring the alignment of the front
to points east. Local environment will be primed for convective
potential, but will have some assistance in upper level support as
a meandering wave off the Southeast coast lifts northwest on the
western fringe of the ridge in the Atlantic, acting as a beneficial
forcing mechanism to enhance regional rainfall potential. The best
chance will lie from the Southern Delmarva through the Virginia
Tidewater down into the Eastern Carolinas. Latest HREF
probabilities are fairly aggressive in their signature for local
totals between 2-5" as neighborhood probs for 2-3" totals are
running between 60-90% with an area of 5" probabilities between
50-70% across Eastern North Carolina. This area has been very dry
as of late, and is well reflected within the FFG indices present
for 1/3/6 hour markers. Isolated to scattered urbanized flooding
and/or flash ponding possible for 2-4 inches/hour rates as per the
latest HREF mean hourly QPF output. A Slight Risk was raised for
the North Carolina coast and a very small potion of southeast
Virginia and northeast South Carolina. A Marginal Risk spans from
South Carolina to southern New Jersey.

Kleebauer/Campbell

...Northern Maine...

Lingering convective threat across Northern Maine could induce
some local flash flood concerns, mainly for northern and Downeast
Maine. The latest guidance continues to signal the highest QPF across
Downeast Maine where the highest PWAT anomalies remain leading to
some deterministic output indicating 1-2" of rainfall within the
first 3-6 hours. The Marginal Risk was kept for this issuance, but
may be considered for removal is convection dissipates sooner than
forecast.

...Midwest...

Trailing shortwave trough pivoting southeast around the backside of
the mean trough over the Great Lakes and Ohio Valley will
strengthen as it moves across Iowa into Illinois by tomorrow
afternoon with large scale forcing increasing across the Central
Midwest and adjacent Mid-Mississippi Valley. Convection is expected
to fire up across Illinois, Missouri and Indiana and within these
storms some will be stronger and capable to producing hourly rates
of 1 to 2 inches/hour. Much of the region was soaked by Beryl as is
passed through and soils have not fully recovered. Antecedent
moisture in the top layer of the soils is well-defined with NASA
SPoRT moisture percentiles relaying the 0-10cm layer running
between 75-90% leading to higher runoff capabilities. Given the
sensitive nature of soils and the potential for a few inches, a
Marginal Risk for excessive rainfall was maintained for this
period.

...Southwest...

The persistent monsoonal pattern will continue to support highly
isolated convection with the primary areas of interest being the
burn scars in New Mexico and small urban threat of towns within
complex terrain across Southeast Arizona through much of New
Mexico. A Marginal Risk remains in effect for southeast Arizona and
a majority of New Mexico.

Campbell


Day 2
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 THE CAROLINAS...

A second round of slow moving convection is expected near the
stalled frontal boundary. The greater impacts of scattered
instances of flash ponding and flooding be primarily be focused
over urbanized areas and areas that see repeat heavy rains from
the period prior. The ridge will assist the convection spreading
rains further inland. It's possible the northern extent of the
Slight Risk could be scaled back given some small trends in lower
QPF focused north of the Virginia Tidewater. Trends in that stretch
of the coast will be monitored with future updates. The Eastern
Carolinas will remain the primary target for heavier rainfall with
some areas in the two day period potentially receiving over 5
inches from the evolving event. The Slight Risk spans from
northeast South Carolina to southern New Jersey and the Marginal
stretches from South Carolina to southern New Hampshire.

Campbell

Wanted to also make mention of monitoring the progression and
convective evolution within a migrating inverted trough axis
through South Texas into the Rio Grande. Current ensemble forecast
is relatively tame within the realm of QPF, however the
environment is more than favorable for higher convective impacts,
including heavy rain within the Rio Grande and points just inland.
First Guess fields do pinpoint a small MRGL located within the
corridor from Eagle Pass down through Laredo, so perhaps if
guidance ramps up the QPF signal in future updates, there could be
a targeted Marginal Risk area added to the central RGV.

Kleebauer


Day 3
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024

...THERE ARE MARGINAL RISKS IN THE SOUTHWEST AND ACROSS NORTH
DAKOTA AND MINNESOTA...
be dropping south from
...Southwest...

There will be a resurgence of monsoonal flow back into the
Southwest. Diurnal heating will help fire up convection during the
afternoon and evening hours with the potential for higher rainfall
fall rates. Maintained the Marginal Risk area for much of Arizona,
northwest New Mexico and southwest Colorado for this period.

...North Dakota and Minnesota...

A shortwave trough will be tracking east across Canada and will
send a surface cold front south-southeast through the Canadian
Prairie and into the northern tier U.S.. Showers and thunderstorms
are expected to form along and ahead of this feature which will
predominately focus over North Dakota and northern Minnesota
during this period. This part of the country has been above/well-
above normal on moisture for the past few months and remain
sensitive to additional rainfall. A Marginal Risk is in effect for
eastern North Dakota and northern Minnesota.

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