FOUS30 KWBC 120815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Fri Jul 12 2024

Day 1
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID-ATLANTIC...

...Mid-Atlantic...

No major changes were made with this update, but certainly the
active pattern typical of July continues. A strong front featuring
an impressive moisture gradient is set up along the eastern slopes
of the Appalachians this morning. Meanwhile, a shortwave in the
upper levels will lift up the Ohio Valley and into the Northeast
today through tonight. On the warm side of the surface front, PWATs
are near their maximum for this time of year, with amounts above
2.25 inches over southeast Virginia and the Carolinas. Instability
is somewhat subdued over land, but values quickly spike just off
the coast to over 3,000 J/kg, with a strong southeasterly wind
helping advect it northwestward. Rates with the stronger cells this
morning are routinely exceeding 1 inch per hour.

As the band moves northwestward, it will eventually run into the
front near the I-95 corridor, resulting in a slowing and eventual
stalling of the band. With the upper level shortwave shifting
northeastward, expect multiple rounds of storms training
northeastward along the front today into tonight. The Chesapeake
and Delaware Bay breezes may help to focus that convection by
locally adding forcing, with HREF probabilities over the far 
northern Delmarva of 30% for 8 inches of rain in 24 hours as well 
as a 30% chance of reaching 100 year ARIs in the 40 km neighborhood
probabilities. For the ERO, the area remains in a high-end Slight,
but should those 8 inch probabilities realize, expect flash
flooding. The guidance suggests overnight tonight is the best time
for the heaviest rain for northeastern MD, northern DE, far
southeastern PA, and southwest NJ where the highest threat remains.
It's possible a targeted MDT may be needed for this region, as it's
the one part of MD not in a drought as noted in the latest drought
monitor. The DC-Baltimore I-95 corridor's forecast rainfall totals
have also increased with a small northwestward adjustment to the
axis of heaviest rainfall in the forecast, so urban and small
stream concerns have also increased if the band over eastern MD
stalls there later today into tonight. There remains a high
likelihood of a sharp gradient of rainfall totals on the north and
western side of the front due to the rapid dropoff of atmospheric 
moisture on the dry side of the front. Thus, much of western MD, 
eastern WV, and western VA will see very little if any rainfall 
from this event.

For the Carolinas, many areas can expect 2 separate rounds of rain
as the front pushes west today, then retreats back to the east
overnight. This should overall reduce the flooding threat, though
some areas along the Pamlico and Albemarle Sounds have seen some
heavy rain in recent days which may locally increase the flash
flooding threat.

...Southern Missouri...

A Marginal Risk area was introduced with this update for continued
convection through the morning associated with the current
convection over southeastern Kansas. The storms are very slow-
moving, but some of the guidance does speed it up through the
morning with increasing westerly flow. Much of southeastern
Missouri was recently hard hit with Beryl's remnants, and therefore
have lower FFGs and therefore somewhat more favorable antecedent
conditions.

...Southeast Texas...

Also due to the remnants of Beryl, the potential for typical daily
afternoon convection over this hard hit area may cause isolated
flash flooding in the possible event any stronger cells remain
nearly stationary. A small Marginal Risk was introduced for this
reason.

Wegman

Day 2
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024

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

...Mid-Atlantic Coast into New England...

In coordination with BOX/Taunton, MA and OKX/Upton, NY forecast
offices, a Slight Risk area was introduced with this update. A
strong front draped across New England will be invigorated by the
passage of a shortwave trough in the upper levels late Friday night
through Saturday morning. While the shortwave will eventually help
the front to slide eastward, this will be after it adds lift to the
ongoing convection from NYC north and east. Light and variable
Corfidi Vectors will become SW or parallel to the front at 10-15
kt. This will support training convection across southern New
England. NASA Sport soil moisture imagery shows much of southern
New England is about average for soil moisture conditions. However,
abnormal moisture with PWATs over 2 inches are expected Saturday
morning. With expected instability also being advected northward
with the front, expect locally intense rainfall rates with the
stronger cells.

Lighter and less organized convection is likely to continue
Saturday morning along much of the Mid-Atlantic coastline as the
front that brought the heavier rain into the I-95 corridor on
Friday pushes east. The Marginal risk was extended to include all
of the Mid-Atlantic coast assuming that rainfall from Friday will
lower FFGs.

...Marginal Risks...

Upper Midwest: The inherited Marginal Risk was shifted east a fair
bit based on the latest guidance. A cold front crossing over
Minnesota, northern Wisconsin, and the western U.P. should lead to
mostly progressive storms across this region, but given favorable
antecedent conditions from recent heavy rainfall and the potential
for training cells on the southern end of any segments that
develop, the Marginal risk still looks good with no plans for any
upgrades.

...Southeast Texas...

Continued soil sensitivity in this region post-Beryl and another 
round of afternoon convection may once again cause isolated flash
flooding. A Marginal Risk was introduced with this update but is a
lower-end Marginal.

...Arizona/New Mexico...

Increasing moisture associated with the monsoon should result in
some increase in convective coverage Saturday afternoon.
Nonetheless much of the activity should be confined to northwestern
Arizona and around the Sangre de Cristo Mountains of north-central
New Mexico, where burn scars may increase the potential for flash
flooding.

Wegman

Day 3
Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
GREAT LAKES AS WELL AS FOR ARIZONA AND PORTIONS OF ADJACENT UTAH,
NEW MEXICO, AND COLORADO...

...Great Lakes...

A series of weak shortwave troughs will move across the Great Lakes
and southwesterly flow of Gulf moisture advects into the region.
Expect widely scattered showers to impact the area, resulting in
isolated flash flooding.

...Four Corners...

Increasing moisture with the monsoon will advect northward into the
Mogollon Rim of Arizona. Meanwhile storms that develop over the 
mountains of Mexico may advect northwestward across the border near
Nogales, where there's good agreement on over an inch of new 
rainfall. While coordination with the associated offices resulted 
in agreement to remain a Marginal, it's likely a Slight Risk will 
be needed for portions of Arizona with future updates for Sunday 
afternoon.

Wegman
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