FOUS30 KWBC 080845
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Thu Aug 8 2024

Day 1
Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH CAROLINA AND SOUTHWEST VIRGINIA...

...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY...

...Southeast and Mid Atlantic...

There was a trend with the latest guidance for an eastward shift
given the Debby's forward motion and expected heavy rain
footprint. Debby will continue to move inland with a westward jog
over towards the Carolina Piedmont during the morning and afternoon
hours. To the northwest, an upper trough will scoot eastward
through the Great Lakes and Ohio Valley with a surface cold front
progged to move quickly to the east with frontal approach into the
Mid Atlantic by the end of the period. The remnant circulation of
Debby will hold steady through the morning, but the cyclone will be
undergoing a transitional state to a more extra-tropical cyclone
with more synoptic mid-latitude characteristics customary once
inland and tracking northward. The combination of Debby's moisture
flux, primary core, and the trough/front approach will allow for a
robust swath of heavy rainfall to occur in-of the Carolinas,
eventually moving north into the central Mid-Atlantic with sights
on the Piedmont from southwestern Pennsylvania down through
Maryland/West Virginia and western Virginia. The High Risk area
covers portions of the southeast North Carolina, north-central
North Carolina and much of central/northern Virginia where areal
average of 6 to 10 inches are forecast.

A Moderate Risk extends along the eastern South Carolina Piedmont
near I-77 up through much of Southwest Virginia into the
Shenandoah and adjacent Blue Ridge, stopping in Southwest
Pennsylvania just north of the Mason Dixon line. Heavier rains
will expand further away from the primary axis of heavier
precipitation, but the coverage will be lower due to the confined
low to mid- level moisture flux associated with Debby along the
leading edge of the trough and advancing cold front. Some heavier
tropical downpours will be forecast along the far western side of
the setup due to convergent upslope flow against the Appalachians.
With reduced forcing closer to the coastline the threat for heavier
rain will be less than areas to the west, but non-zero.

...Four Corners, Central Rockies and High Plains...

The monsoonal pattern should yield a targeted focus for heavy rain
across portions of the Central and Southern Rockies. The primary
threat will be across the Sangre de Cristos thanks to strengthening
upslope component as a cold front banks against the terrain with
strong upward motion along a prominent theta-E ridge situated
across the Southeast CO Front Range into the adjacent terrain.
Heavy thunderstorms capable of 1-1.5"/hr will cause issues within
the complex terrain and the remnant burn scars that persist within
the confines of the Sangre de Cristos. A Slight Risk remains in
effect for portions of central Colorado and northern New Mexico
given the increased risk for isolated to scattered flooding
concerns. A Marginal Risk covers much of the Four Corners region,
western Kansas and Nebraska, southwest South Dakota, eastern
Wyoming and extreme southeast Montana.

Campbell/Kleebauer


Day 2
Valid 12Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC AND NORTHEAST...

...Mid-Atlantic to the Northeast...

The core of the heaviest rainfall is anticipated to occur along
the convergent area of the cold front as Debby's remnants get
absorbed into the mean trough to the northwest and shunted
eastward; from the Piedmont region of the Maryland northward to
Upstate New York, Vermont and New Hampshire. Debby's remnants will
be moving rapidly to the north-northeast with the associated
moisture streaming well ahead of any persisting circulation. The
storm will have gone through full extra-tropical transition once
into central PA and will advance all the way into Canada by the end
of the forecast period. Widespread 2-4" of rainfall are forecast
over central and western Pennsylvania through western New York
state and the western side of Northern New England (North Country
over into VT) before exiting into the Canadian Maritimes and
eastern Quebec. Rainfall rates of 1-2"/hr are forecast within the
zone of highest impact relayed above with the eastern flank of the
storm becoming drier as it gains latitude.

The Moderate Risk was maintained with very minor adjustments to
account for the latest WPC forecast. There continues to be some
uncertainty in the exact track of the remnant cyclone as it reaches
the New York state border, but if the trends continue, the eastern
side of the risk areas will likely be brought further west to
tighten the corridor of heaviest rainfall as is expected with the
setup. With rain diminishing across the Appalachians and western
portions of the Carolinas and Virginia the Slight and Marginal
Risks were significantly trimmed eastward. Some heavier bands are
expected to setup over coastal areas of North Carolina that will
keep an elevated threat for flooding concerns during this period.

...Four Corners, Central Rockies and High Plains...

Showers and thunderstorms are expected to fire up across the
Southwest and Rockies as shortwave energy passes through and
encounters the ongoing monsoonal moisture in place. There will be
increased focus for more organization and higher amounts
concentrating near the higher terrain of south/central Colorado and
northern New Mexico, where a Slight Risk area remains in effect.
Accumulations of 1-2" will be possible in the strongest cells, a
stronger correlation to flash flooding concerns compared to
historical precedence. The rest of the Southwest will still see
periods of storms capable of localized flash flood concerns with
the best threat located more into the Inter Mountain West and the
Mogollon Rim in Arizona. Convection will also spread eastward into
western Kansas and the panhandles of Oklahoma and Texas. A broad
Marginal Risk covers much of the Four Corners region, east into
western Kansas and northward into south-central Montana.

Campbell/Kleebauer


Day 3
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS AND FOR MAINE...

...Southwest, Central Rockies and Plains...

Diurnal heating and monsoonal moisture will keep showers and
thunderstorms developing during the afternoon and evening hours for
much of the Four Corners region. The area of more organized
convection triggered by shortwave energy from the D2 period will
be making its way eastward into the foothills and spreading out
into the high Central/Southern Plains. There is a fair amount of
uncertainty with how far east into the plains the convection
reaches, location of heaviest amounts and the overall
accumulations. Most of the consensus is suggesting 1 to 3 inches
however a couple models have maxes near/over 4 inches. A Marginal
Risk covers much of the Four Corners, eastern Wyoming, western
Nebraska and Kansas, Oklahoma, northern Texas, southwest Missouri
and northwest Arkansas. If amounts trend up then a Slight Risk area
may be need with further updates.

...Maine...

The absorbed remnants of Debby will continue to track through the
Canadian Maritimes meanwhile there will be lingering showers and
thunderstorms. Recent heavy rain will have lowered FFGs
significantly thus making the area sensitive to additional
rainfall. Maintained a Marginal Risk for this period; however if
the timing speeds up there may be an opportunity to downgrade with
further updates.

Campbell
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