FOUS30 KWBC 060953
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
553 AM EDT Sat Sep 6 2025

Day 1
Valid 12Z Sat Sep 06 2025 - 12Z Sun Sep 07 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PHILADELPHIA
THROUGH THE NEW YORK CITY METRO...

...Northeast...

The combination of several ingredients all at the same time in an
urban corridor has increased the potential for flash flooding. A
healthy LLJ will advect abundant Gulf/Atlantic moisture north, up
the Eastern Seaboard on Saturday. Expect PWATs to exceed 1.75
inches from Florida into New England as a result of the 25-35 kt
850 mb LLJ moving that moisture northward. Meanwhile, a strong but
rather slow-moving cold front is approaching the Northeast from the
Great Lakes. This cold front will act as a catalyst/forcing
mechanism for numerous showers and thunderstorms to develop this
afternoon in the warm, muggy air mass. Meanwhile, in the upper
levels, a 130 kt SSW to NNE oriented jet will be in place from
western Pennsylvania north to near the southern tip of Greenland.
This puts the urban corridor in the favorable right entrance region
of that jet streak, which will support lift and additional
thunderstorm development. Finally, the storms and strongest forcing
will be moving through the area at peak heating in the mid-
afternoon. This will allow the storms to be at their strongest with
the maximum amount of heating having occurred prior the convective
initiation. A subtle upper level shortwave will also move through
the jet and enter the jet streak this afternoon. This shortwave
will support additional convective development to occur behind the
initial line of storms. This additional convective development
expected is depicted in several of the CAMs model runs, most
notably the 00Z HRRR. While never a perfect prog, the HRRR's
solution if even close to correct will result in multiple inches of
rain from Philly north through NYC. Similar solutions are depicted
in the ARW2, RRFS, and NAMnest.

Expect an initial line of storms to move across northern New Jersey
and into the NYC Metro during the mid to late afternoon, between
3-6pm, with additional lines of progressively weaker convection
moving across the region well into the evening. While most areas
have been very dry of late, with the initial stages of drought
setting in, the consensus of most of this overlapping/training
convection is for it to follow I-95 from Philly up into the NYC
metro, where heavy urbanization has locally lowered FFGs. Thus,
despite empty streams and creeks, multiple rounds of heavy rain in
either continuous or rapidly repeating fashion is still likely to
cause widely scattered instances of flash flooding where the storms
are most persistent over urban areas. In coordination with
OKX/Upton, NY and PHI/Mount Holly, NJ forecast offices, a Slight
Risk upgrade was introduced with this update.

Further north into New England, the forcing is not as likely to
align as areas further south, and therefore only one or two lines
of storms are expected in this area, which has also been very dry
in recent weeks. This should limit flash flooding to isolated
instances. Further south into the DMV, once again expect a line of
storms to move through during the afternoon, but with the forcing
further north, little if any backbuilding or additional convective
development is expected behind the initial line of storms.
Nevertheless with an abundance of moisture available for the storms
to tap as they rumble across the Mid-Atlantic, as well as other
urban concerns, the Marginal Risk remains in place through much of Virginia.

...Tennessee Valley...

The same cold front moving across the Mid-Atlantic and Northeast
will also slowly push south across the Tennessee Valley today.
Being closer to the primary moisture source (the Gulf), storms in
this area will have even more moisture to feed from. However, the
front pushing south will strongly limit any potential for training
or backbuilding. Thus, for most areas, just one round of storms is
all that can be expected. Cell mergers and pre-linear convective
development could allow for isolated instances of flash flooding,
so a Marginal Risk was expanded from the Mid-Alantic down through
Tennessee and northern portions of the Southeast.

...Florida...

A stalled out front draped across the state, as well as an
abundance of deep tropical moisture in place will support yet
another round of afternoon and evening thunderstorms across much of
the Peninsula today into tonight. Since the front, while present,
will be quite diffuse, the guidance is in poor agreement as to
where any storms may organize over Florida, so a larger Marginal
Risk is all there is the confidence for, especially considering the
larger amounts of rain needed to get flash flooding in much of
Florida. The inherited Marginal Risk is largely unchanged.

...West...

Very few changes were needed out west for expected continued
monsoonal convection across much of the Southwest/Four Corners
region. The Marginal risk area was expanded a bit down the Rio
Grande in west-central Texas, but otherwise the area remains
largely the same with few changes. As is typical with the monsoon,
there will likely be feast or famine with many of the storms across
the Southwest today. Most areas will be dry or only see some light
rain, but where the storms are able to organize and concentrate,
some areas may see multiple inches of rain. Given the area's
proclivity to flash flood, the Marginal remains in place for most
of the West.

Wegman


Day 2
Valid 12Z Sun Sep 07 2025 - 12Z Mon Sep 08 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE SOUTHERN
HALVES OF TEXAS AND FLORIDA...

...Texas...

Gulf moisture in place across Texas will interact with some of the
remnant moisture from former Hurricane Lorena on Sunday. PWATs
rising to between 1.75 and 2 inches will be enough moisture to
support widely scattered convection across much of the southern
half of the state. There's considerable disagreement on the
guidance as to where the most convection will organize. Since the
overall forecast is little changed from the inherited forecast, the
Marginal Risk remains largely in lace with few changes.

...Florida...

The stalled out front that remains across Florida will extend
northeastward off the coast of the Carolinas on Sunday. Thus,
expect yet another round of afternoon and evening convection over
much of the Florida Peninsula. There remains some indication that
the convection will favor the eastern side of the state, but since
the area remains in a pattern where much of the Peninsula sees
numerous showers and thunderstorms developing each afternoon, but
with little in the way of organization to them, the inherited
Marginal remains roughly the same, depicting all of the southern
half of the state in an isolated risk for flash flooding. This is
most likely should any storms develop and sit over urban areas, or
in an area that has seen particularly heavy rain in recent days.

Wegman


Day 3
Valid 12Z Mon Sep 08 2025 - 12Z Tue Sep 09 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FLORIDA PENINSULA AND THE GEORGIA COAST...

...Southeast...

As on previous days, much of the Florida Peninsula will remain
under the influence of a stalled out front that has been sitting
over the state for much of the past week. This will result in yet
another afternoon of numerous convection, which will take advantage
of abundant tropical moisture in the area to support additional
convective development from prior storms' cold pools. Meanwhile,
the front as a whole will very slowly drift northwestward with
time, which will spread the rain chances along the beaches from
Georgia through the Carolinas. There's high uncertainty as to
whether most of the rainfall will even make it to the coast.
Outside of Florida, the coastal areas most likely to see heavy rain
will be the Georgia coastal communities. Otherwise, as in previous
days, there will be little to support organized convective growth
across Florida, so the storms behave erratically in the guidance,
with no particularly strong signals for heavy rain anywhere in
Florida. Therefore, once again, the Marginal Risk remains in place.

Elsewhere, there are a few signals for potential heavy rain into
portions of the upper Midwest, as well as coastal Oregon. In both
areas, the signals are diffuse enough as to preclude any Marginal
Risk issuance for this update. They will continue to be watched, as
any increase in the signal for heavy rain may require a Marginal
Risk issuance.

Wegman
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