FOUS30 KWBC 090758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Fri Aug 9 2024

Day 1
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 Northeast...

Debby and it's remnant plume of tropical moisture are forecast to 
lift northward today from the central Appalachians to the Interior 
Northeast, while producing a swath of heavy rainfall and the 
potential for numerous flash floods from near the West Virginia 
Panhandle early this morning to Upstate New York this afternoon. 
Precipitable water values of 2-2.5" stream along and just east of 
the system will equate to above the 95th climatological percentile 
and +3.0 standard deviations above the climatological mean. 
Additionally, ample shear surrounding the low center and MUCAPE 
values up to 1000 J/kg nearby along the Atlantic Seaboard being 
advected northward will allow for efficient tropical downpours 
within scattered to numerous showers and thunderstorms. A focus in 
the heaviest rainfall is likely on the northern and northeastern 
quadrant of the low pressure system as it transitions to an 
extratropical phase, becomes frontal, and begins to increase 
forward speed to the northeast by Friday night. A warm front 
lifting northward will add increasing frontogenetic forcing to the
environment and lead to a more widespread precipitation shield 
today across parts of PA and Upstate NY, with embedded rainfall 
rates likely up to 2-3"/hr at times. The trend for this axis of 
heavy rainfall has been for it to occur slightly westward than 
previously forecast, now across far northwest NY. Additionally, the
trend is for faster northward progression with this system. 12z 
NAEFS IVT valid 18z Fri is forecast to max all values in the 
1979-2009 climo period, amplifying the moisture surge fueling 
today's heavy rainfall event. 00z HREF neighborhood probs for at 
least 3" in 6-hrs are high (60-80%) from central PA to the Finger 
Lakes region and northwest portions of NY State, which would also 
exceed local FFG for much of this region. As a result, the Moderate
Risk was maintained from the previous forecast with only minor 
cosmetic changes. Numerous instances of flash flooding are expected
in this area, with locally significant flash flooding possible. 
Elsewhere in the Northeast, convection developing along and ahead 
of a cold front should have enough forward motion to lead to only 
scattered to localized flash flooding concerns closer to the I-95 
corridor, but with rainfall rates of 2"/hr possible. Areas that 
received recent rainfall and poor drainage locations could have a 
relatively greater flash flood risk, which supports the SLGT Risk 
extending to parts of New England and the Mid-Atlantic.

...Eastern Carolinas...

A separate area along the East Coast at risk for flash flooding
today will be associated with a convergence axis along 
coastal/eastern sections of North and South Carolina this afternoon
and evening. PWs elevated and above 2.25", along with ample 
instability and mean winds parallel to the boundary/coastline will 
support slow-moving and/or training thunderstorms across saturated
terrain. CAMs and the associated 00z HREF suite depict the 
potential for 5"+ amounts throughout eastern North Carolina and 
coastal South Carolina, but the expectation remains that these 
totals would be pretty localized. The SLGT Risk was expanded from 
the previous forecast across South Carolina given recent CAM output
and very saturated antecedent conditions in place. Will monitor 
this region for upgrades in the future outlooks.

...Four Corners, Central Rockies, Central High Plains, and Southern
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 southern Utah, south/central
Colorado, and northern New Mexico, where a SLGT 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 Intermountain West and the 
Mogollon Rim in Arizona. Convection (possibly organized) will also 
spread eastward into western Kansas and the panhandles of Oklahoma 
and Texas, as well as northeast NM. Here, 00z HREF neighborhood 
probs are modest (20-40%) for at least 3" in 6-hrs. A broad 
MRGL Risk covers much of the High Plains.

...Central Texas...

A MRGL Risk was introduced across parts of central Texas as
convection develops along the tail-end of a lingering cold front
and ahead of a stronger frontal boundary dropping south from the
central Plains. Shear will be weak, but steep low-level lapse rates
and sufficient moisture (PWs around 2" will allow for intense
rainfall rates over relatively wet ground conditions and a 
historically flash flood prone region. 00z HREF neighborhood
probs have increased and now show elevated chances (30-50%) for
rainfall amounts exceeding 3" this evening.

Snell


Day 2
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 THE COASTAL CAROLINAS...

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

A rather broad MRGL Risk was maintained extending from the
Southwest to the central/southern Plains as diurnal heating and 
monsoonal moisture will keep showers and thunderstorms developing 
during the afternoon and evening hours for much of the Four Corners
region. Meanwhile, an area of more organized convection triggered 
by shortwave energy from the D1 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 1-3 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.

...Coastal Carolinas...

A lingering frontal boundary and diurnal sea breeze convection will
continue chances for scattered thunderstorms across the Southeast
and more specifically the eastern Carolinas on Saturday. Soils
should remain saturated after prior heavy rainfall, increasing the
flash flood threat across an area that isn't usually as prone to
flash flooding. CAMs depict localized totals from pulse
thunderstorms could reach 1-3", which is supported by PWs over 2".

Snell


Day 3
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, FOUR CORNERS, CENTRAL ROCKIES INTO THE PLAINS, AND
THE EASTERN CAROLINAS...

...Four Corners, Southwest, and Central Rockies...

A deepening western U.S. trough by D3 will allow for another round
of diurnal monsoonal convection across the Southwest and Four
Corners region, with GEFS and ECENS depicting PWs above the 75th
climatological percentile. Opted to maintain the MRGL Risk for now
given the limited QPF signal in available guidance and the
possibility for fast storm motions.

...Plains and Mid-Mississippi Valley...

Northwest flow aloft and a gradually lifting warm front over the
Southern Plains will support MCS potential into the overnight
period on D3 along with increasing mid-level flow, which could 
produce training and intense rainfall rates within convection. 
Relatively high uncertainty remains with the exact placement of 
heaviest rainfall (greatest potential currently near the western 
Ozarks into northeast OK and southeast KS), which prompted the MRGL
Risk as opposed to a SLGT for now. Additional convection is 
possible over SD and NE, with similar uncertainty in placement.

...Eastern Carolinas...

Another day of diurnal convection along a nearby frontal boundary
could be enhanced by an approaching shortwave diving over the Ohio
Valley around a deep upper-level low over SE Canada. Similar to D2,
rainfall amounts of 1-3" appear possible and are likely to occur
over saturated soils.

Snell
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