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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1558Z Jul 09, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Tue Jul 9 2024

Day 1
Valid 16Z Tue Jul 09 2024 - 12Z Wed Jul 10 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST, MID MISSISSIPPI AND OHIO VALLEYS, AS WELL AS FOR THE
SOUTHEASTERN LOUISIANA...

...16z Update Summary...

Two primary updates to the previous D1 to reflect changes in
guidance and to update on the recent radar reflection going
forward. The SLGT risk across the center of the country into MI was
expanded to the east over southern IN and further northeast across
much of southern MI. The second update was across the Central Gulf
Coast where the previous SLGT risk was scaled back in size to
mainly encompass the New Orleans metro and surrounding locales
given the recent radar trends and CAMs output through the period.
More details were added to the respective sub-headings below....

Elsewhere over the CONUS, the other notable change exists across FL
where the MRGL was cut over much of the western coast and over far
southern FL due to the setup for flash flood potential being
primarily confined to the FL Panhandle, FL Big Bend, and the
eastern coast given the flow pattern conducive for localized
enhancement from the Atlantic sea breeze. Spotty max totals over 3"
are forecast within CAMs, but the placement is very variable, so
wanted to maintain some coverage to account for the threat, but
local uncertainty. The setup across New England, Southern
Appalachians, and portions of the Southwestern US are all fairly
similar with regards to the threat being localized for flash
flooding within the respective terrain. The best threat remains
over the burn scars in NM where any convective impact over the
affected areas will be susceptible to flashy returns. The other
areas within New England and the Southern Appalachians lie within
the lower-end of MRGL, but non-zero considering the environment and
terrain focus.

Kleebauer

...Mississippi Valley and Midwest...

..16Z Update..

Increasing signals for heavy rainfall from the remnants of Beryl
will impact some of the urbanized areas extending through the
Detroit Metro with a greater convective concern located over
southern IN. Southern IN will be within the best instability axis
in the Ohio Valley with several small mid-level perturbations
advecting overhead this morning through the evening around the
broad periphery of Beryl's circulation. Recent HREF mean QPF across
the area has really ramped up with storms already impacting the
southern fringe of the state with locally heavy rain as rates top
1.5-2"/hr in the recent radar QPE output. A second round is
expected this evening in wake of better instability leading to a
multi-wave threat of convection within portions of the area.
Considering the priming this morning and eventual impact later, the
threat for flash flooding has increased enough to warrant the SLGT
expansion. Across southern MI, the forward speed of Beryl's
remnants will allow for a solid 925-700mb moisture flux through the
region with sights on the Detroit metro by the end of the period.
HREF neighborhood probabilities for at least 3" have shot up to
40-60% across portions of Southeast MI, including Detroit with
signals over Southwest MI growing to 80-90% for at least 3" and
25-40% for potential of 5". This was a significant enough signal to
warrant the SLGT expansion across Southern MI through the Detroit
Metro with agreement from the local Detroit WFO.

Kleebauer

..Previous Discussion..

Tropical moisture associated with Beryl will spread a swath of
heavy rain across the Mississippi Valley and into the Midwest as a
cold front advances east through the region. The last few runs of
guidance have trended toward a more narrow axis setting up right
along the front and QPF values increasing further northeast across
Illinois/Indiana and southern Michigan. A sizable swath of 3+
inches continues to span from the Missouri Ozarks to northwest
Indiana with local maximums upwards of 8 inches.

A Slight Risk extends from central Arkansas to southern Michigan.
A Marginal Risk stretches from the central Arkansas northward to
southern Michigan and central Ohio.

Campbell

...Central Gulf Coast and Southeast...

..16Z Update..

Recent trends in the enhanced QPF focus across the Central Gulf
coast have led to the New Orleans metro and surrounding locales
over Southeast LA as the focal point for heavy rainfall today into
the evening. Considering very high FFGs located over the coastal
portions of LA/MS/AL, the threat will be mainly an urban flood
threat with the New Orleans metro easily being the target for the
current setup. HREF probabilities for at least 2"/3-hrs is highest
across Southeast LA with values mainly between 25-40% over a span
of several hrs beginning now through 06z with the spatial coverage
centered right over NoLA and surrounding Parishes. This allowed the
previous SLGT to shrink in size to mainly encompass the New Orleans
metro and Parishes just off to the south and southwest.

Kleebauer

..Previous Discussion..

Deep, onshore flow will continue to drive convection to form along
the Gulf Coast, with the potential for heavy down pours. Areal
average rainfall is expected to be in the 1 to 2 inch range however
the CAM guidance suggests very localized maximums of 3 to 5 inches
possible. A Slight Risk is in effect for portions of the Gulf Coast
from eastern Louisiana to the Florida Panhandle.

Campbell

...Previous Discussions...

...Southwest...

Isolated to widely scattered convection is likely to form along
the mountains once during the afternoon and evening, which could
very well move over flood-sensitive areas. A Marginal Risk remains
in effect for portions of Arizona, New Mexico, southern Colorado
and extreme western Texas.

...Rio Grande of Texas...

Convection that forms along the mountains of Mexico will have
enough of an eastward push that some may cross into the Del Rio and
Eagle Pass areas. Isolated instances of flash flooding may develop,
therefore a Marginal Risk remains in effect.

...Southern Appalachians...

Storms are likely to form in the deep tropical moisture indirectly
associated with Beryl with the mountains acting as the primary
forcing. Numerous flood sensitive areas in western NC may support
flash flooding with any slow moving or stationary storms. A
Marginal Risk area remains in effect.

...Northeast...

Deep tropical moisture pumping north well ahead of Beryl will make
for anomalous PWATs across New England. The Appalachians of this
region may also act as forcing for widely scattered thunderstorms
capable of heavy rainfall. A Marginal Risk area is in effect for
across this region.

Campbell


Day 2
Valid 12Z Wed Jul 10 2024 - 12Z Thu Jul 11 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES REGION INTO THE NORTHEAST AND FOR CENTRAL NEW
MEXICO...

...Northeast...

Training of storms will likely develop along the west-east
orientated front across New York, Vermont and New Hampshire. The
heaviest rains are expected into the Adirondacks primarily...where
the mountains will provide the greatest additional lift being
oriented largely orthogonal to the WSW flow. PWATs will be more
than 3 sigma above the mean, which compared with GFS climatology
would break the daily record. Thus...there will be unusual amounts
of moisture available for Beryl's remnants to convert to rainfall.

A Slight Risk is in effect from northern Indiana to Maine while a
Marginal Risk area spans from Illinois to Maine. Currently, much of
northern New York and Vermont will be a higher end Slight Risk. As
details resolve and get closer in time, there may be the need to
upgrade to a Moderate Risk.

There will be similarly abundant atmospheric moisture all along
the Eastern Seaboard Wednesday and Wednesday night, but the forcing
further south will be much more limited, resulting in less
widespread convection. However, given the amount of moisture in
place...even local forcings should be enough to result in some
storms.

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

Continuation of the monsoonal moisture and diurnal heating will
keep convection possible across portions of the Central and
Southern Rockies and southwest New Mexico during this period.
Higher QPF is expected to concentrate in the vicinity of the
Sacramento Mountains where there have been a few recent wildfires.
The burn scar complex (Blue-2, South FOrk, Salt and McBride)
is very sensitive to anything more than a gentle, light rain. QPF
is forecast to range from 0.25 to 1 inch across portions of central
New Mexico which elevates the threat for flash flooding and debris
flows. A Slight Risk was raised for this part of the state given
the aforementioned sensitivity.

...Central Gulf Coast...

A stalled frontal boundary along the Gulf Coast may continue to
provide focus for convection capable or producing heavy rainfall. A
Marginal Risk area remains in effect.

Campbell


Day 3
Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE NEW ENGLAND...

...East Coast...

The low pressure system and associated tropical moisture will
progress through New England during this period with its trailing
cold front sweeping through the Mid-Atlantic. Lingering showers and
thunderstorms will persist over New England while maintaining an
elevated threat for excessive rainfall and flash flooding. A
Slight Risk remains in effect for much of Vermont, New Hampshire
and central Maine. There will be a better concentration of
convection producing higher QPF across southeast Virginia and
eastern North Carolina. Model consensus had areal averages of 1 to
2 inches for this area although local maximums up to 5 inches may
be possible. A Marginal Risk spans from South Carolina northward to
Maine for this period.

...Southwest...

The monsoonal pattern of diurnal convection will persist thus
maintaining an elevated threat for localized flash flooding. A
Marginal Risk remains in effect for portions of eastern Arizona and
much of New Mexico.m

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