FOUS30 KWBC 091546
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Fri Aug 9 2024

Day 1
Valid 16Z 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...

...16Z Update...

Debby's remnants continue to press rapidly to the north as the
remaining core analyzed at 15z is rushing northward at just over 30
kts (35 mph). Bands of heavier rainfall continue to push north out
of the Central Mid Atlantic with the heaviest rain focused near the
remnant center of now Post-Tropical Cyclone Debby. The current
forecast remains on track with little variance in the QPF and
associated probabilities as rainfall rates of 1-2"/hr will be seen
within the heaviest cores as the storm drives northward out of
Northern New England by late this evening. The previous MDT risk
was trimmed on the southern edge of the previous forecast to
reflect the radar trends of the heaviest precip shifting north with
more of a focus across Northern PA through Upstate NY with the max
for the period likely situated across the North Country of NY
state. A trailing cold front will allow for pockets of heavy rain
to develop this afternoon across portions of the Central Mid
Atlantic, but will quickly vacate eastward as low-level convergence
along the front will sweep east with a degrading threat of heavy
precip in wake of the boundary. Some of the SLGT risk across the
interior Central Mid Atlantic was also adjusted given the
environment improving with much of the heavier precip likely
falling along and east of the Blue Ridge. 

The Eastern Carolinas exhibited very little in the way of variance
from the previous forecast issuance with the afternoon and evening
time frame remaining the period of interest with the convective
development anticipated. Hi-res was in strong agreement on the
placement of a line of convection extending from coastal SC up
through Eastern NC with much of the heavier cores likely to fall
east of the I-95 corridor. Embedded heavy thunderstorms within the
main convective axis will likely produce a few cells that can reach
upwards of 5" with a modest signature on the 12z HREF neighborhood
probability for the threshold settling between 30-50% over a large
area encompassing most of Eastern NC. HREF EAS signals for at least
2" is relatively high as well (20-30%) over the same areas, so the
threat is likely on the higher-end of the SLGT risk threshold,
especially given the antecedent saturated grounds after a multi-day
onslaught from Debby. This led to an easy decision to keep
continuity with locally significant wording reserved for any
urbanized areas within the above region. 

Finally, out west the pattern remains on track with scattered
thunderstorm development during peak diurnal max with a complex
initiating over the High Plains of Southeast CO and Northeast NM,
shifting east with the mean flow focused along the northern
periphery of the mid-level ridge. Some pretty good signals for at
least 2" given a solid EAS probability between 30-50% over a small
zone between the NM/OK/TX borders. Neighborhood probability was
bullish for the >3" signal running between 60-70% over the
aforementioned area. There's little signal for >5", however so the
threat is likely capped between 3-5" at peak QPF. That's still
fairly prevalent to a localized flash flood concern within that
portion of the Southern High Plains, so the risk is likely within
the higher-end of the SLGT comparatively to what will occur over
NM. Much of the impact will be focused in those burn scar areas
where heightened sensitivity drives much of the risk. This was
sufficient to maintain what was forecast previously with only some
minor adjustments based on the latest HREF mean QPF footprint. 

Kleebauer

...Previous Forecast...

...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
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