FOUS30 KWBC 080935
QPFERD

Excessive Rainfall Discussion...Corrected Day 3 discussion
NWS Weather Prediction Center College Park MD
534 AM EDT Fri Sep 08 2023

Day 1
Valid 12Z Fri Sep 08 2023 - 12Z Sat Sep 09 2023

...A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR MUCH OF
THE APPALACHIANS, THE I-95 CORRIDOR FROM DC TO NYC, AS WELL AS
EAST-CENTRAL FLORIDA...

...Appalachians Through Southern New England...

A digging and increasingly-negatively-tilted trough will move over
the OH and TN Valleys today. Meanwhile, deep southerly flow will
advect tropical moisture northward up the I-95 corridor. This
pattern will increase divergence aloft, allowing storms taking
advantage of the abundant moisture to organize and train along the Appalachians. Thus, a marginal flash flooding threat exists for
much of the Appalachians, Mid-Atlantic, and into southern New
England. The axis of heaviest rainfall is likely along and in the
immediate eastern foothills of the Appalachians.

The Marginal Risk area was expanded eastward to include the I-95
corridor from DC to NYC. In addition to a small eastward shift in
the guidance of the heaviest rainfall, the inherent lower FFGs in
the cities will allow even the isolated to widely scattered storms
to be capable of 1 to 2 inch per hour rainfall rates, causing
isolated flash flooding.

For the southern Appalachians, rainfall from storms yesterday
(Thursday) caused isolated flash flooding in the Roanoke Valley.
With potential for similar coverage of storms again today, the
Marginal Risk area was expanded to include more of southwest VA
and northwest NC.

...East-Central Florida...

Sea breeze convergence will initiate a round of heavy
thunderstorms within a narrow corridor near the Melbourne area
this afternoon. Hi-res deterministic has been consistent over the
past succession of runs for a line of 2-3"/hr rainfall rates to
run along the eastern FL coast within the more urbanized corridor
extending from Melbourne to up near Daytona beach. There will be a
little more enhancement in the upper-levels thanks to a digging
shortwave trough forecast to the enter the area, providing a bit
more ascent compared to the typical mesoscale boundary
convergence. This will lead to additional coverage of storms over
the central part of the state, including the Orlando metro. The
Marginal Risk area was expanded westward to include potential for
isolated flash flooding from very efficient rain-making storms,
especially in urban areas.

Wegman


Day 2
Valid 12Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE WESTERN
CAROLINAS AND ADJACENT PORTIONS OF VIRGINIA AND GEORGIA...

...Western Carolinas...

In coordination with RNK/Blacksburg, VA; GSP/Greer, SC; and
FFC/Peachtree City, GA forecast offices, a Slight Risk area was
introduced for much of the western Carolinas and small portions of
adjacent VA and GA with this update. A newly cutoff upper level
low will be stuck over eastern TN Saturday and Saturday night.
Continued southerly flow up the Gulf Stream will continue
advecting a 1.5 to 1.75 inch PWAT northward up the east side of
the Appalachians. The combination of upper level divergence over
the western Carolinas, plentiful moisture that will be
replenished, an upslope component, and antecedent storms in recent
days over portions of the area will all favor the development of
widely scattered instances of flash flooding, with the heaviest
rain likely in the foothills of the Appalachians. Rainfall totals
of 1-3 inches are likely with the heaviest rains, but much of that
will be from storms producing 1-2 inch per hour rainfall rates.
Training and slow-movement of the storms are expected, since the
upper level forcing will be nearly stationary. This too will favor
flash flooding, particularly in any flood-sensitive and urban
areas.

...Mid-Atlantic into the Northeast...

Another afternoon of showers and thunderstorms is expected
Saturday along the I-95 corridor and the central Appalachians.
Like previous days, the storms will be only loosely organized, but
given the stagnant weather pattern with a negatively-tilted
longwave trough off to the west, expect storms to once again move
over many of the same areas again Saturday afternoon as on Friday.
The Marginal Risk remains largely unchanged, though a bit expanded
to the east. Despite the repeating days of storms, antecedent
conditions remain largely dry over the area, which should preclude
most flash flooding. With that said, should Friday's storms become
more numerous than expected, it's possible an upgrade may be
needed for portions of this area, particularly around eastern PA,
which should be in the axis of heaviest rainfall both Friday and
again on Saturday.

Wegman




Day 3
Valid 12Z Sun Sep 10 2023 - 12Z Mon Sep 11 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS...

A Slight Risk upgrade was introduced for portions of the central
Plains with this update. An MCS will develop Sunday evening and
persist through the night. Storms will slowly move southward
through KS and into the OK and TX Panhandles Sunday night. A cold
front tracking south down the Plains will clash with a northward
moving LLJ and abundant Gulf moisture tracking north across TX and
into the central Plains. This will be the focus for storms and the
developing MCS. Once the storms develop Sunday afternoon, the
opposing air masses will keep the front separating the two from
moving very much, which will allow for training/repeating storms
over the same areas into Sunday night across the Slight Risk area.
Antecedent conditions in this area have been bone dry, so the
soils in the area are likely to be hard and unlikely to absorb
much heavy rainfall. This in turn should result in enhanced runoff
from any stronger storms likely to develop in the area. The best
chance of flash flooding will be in urban and flood-sensitive
areas where strong and repeating storms produce prolonged heavy
rainfall.

Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6Ywh7grmyJju4zvGaCam1_1u-sfIBPCzsV7CvYL4Dvxz= bO9lCMiUbePtCMr-8y6joSpUFTIXo8qYi72RQcDmvnmZpds$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Ywh7grmyJju4zvGaCam1_1u-sfIBPCzsV7CvYL4Dvxz= bO9lCMiUbePtCMr-8y6joSpUFTIXo8qYi72RQcDmMPq_ybE$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Ywh7grmyJju4zvGaCam1_1u-sfIBPCzsV7CvYL4Dvxz= bO9lCMiUbePtCMr-8y6joSpUFTIXo8qYi72RQcDm1jps1pk$=20


$$



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