FOUS30 KWBC 070712
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 AM EDT Sun Sep 7 2025

Day 1
Valid 12Z Sun Sep 07 2025 - 12Z Mon Sep 08 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN TEXAS AS WELL AS FOR THE SOUTHEAST COAST AND MUCH OF FLORIDA...

...Southern Texas...

The combination of weak "ridge riding" upper level disturbances, a
strong cold front approaching from the northeast, and abundant Gulf
moisture will all poorly come together across portions of southern
Texas from now through the day today and into tonight. Storms
circulating around a well-defined upper level high over
northeastern Mexico are being driven in part due to upsloping in
the easterly flow off the Gulf. Due to the aforementioned poor
merging of these various ingredients and forcings, expect widely
scattered to scattered convection for the rest of the predawn hours
and into much of the day today. However, these storms will really
struggle to organize to result in any more than an isolated flash
flooding threat. High FFGs, about as high as they get, are
plentiful over much of southern Texas, while soil moisture levels
are about average, which works against flash flooding. Thus, any
isolated flooding in southern Texas today will require slow moving
storms hitting just the right area, likely an urban one, to cause
any impacts today.

...Southeast Coast and Much of Florida...

An area of abundant tropical moisture with embedded weak
troughs/waves will remain around the Florida Peninsula today. The
strongest trough will approach the Gulf Coast side of the state
today from the west. Interactions of this trough, any sea breezes,
and the instability difference in the afternoon between the hotter
Peninsula and the adjacent Gulf and Atlantic will be the primary 
forcing mechanisms for any storms. Thus, while storm coverage will 
likely once again be widely scattered to scattered. The various 
forcings likely competing with each other will work to keep any 
storms and clusters of storms from organizing into a more 
significant flash flooding threat. Further, despite days of 
scattered convection across the Florida Peninsula, soil moisture 
levels across much of the state remain below normal, meaning most 
of the rain that falls from the afternoon and evening storms will 
remain beneficial for most. Thus, the Marginal flash flooding 
threat is primarily focused on urban areas today, as any storms 
will have plenty of moisture to draw from to locally produce
multiple-inch-per-hour rainfall rates.

...New England...

The inherited Marginal Risk across the area was dropped with this
update. All of the rainfall across New England this morning is
stratiform, struggling to produce rates even to a half inch per
hour. This is likely to continue today as the cold front causing
the rain continues to press east towards the coast. While a few
areas of embedded convection from mid-level instability can't be
ruled out, fast motion will also cut into the rainfall amount any
one area picks up today. 

Wegman


Day 2
Valid 12Z Mon Sep 08 2025 - 12Z Tue Sep 09 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS AS WELL AS FOR THE FLORIDA PENINSULA...

...Central Plains...

An area of convection across the central Plains early Monday
morning will be ongoing at the start of the period. The storms will
develop along a warm frontal interface, where a seasonably cold and
dry air mass will be in place across much of the eastern half of
the country, meanwhile warmth and moisture will be moving north on
an LLJ up the Plains to the west. This will result in a somewhat
unusual scenario where the eastward moving storms will be moving
into a very hostile environment. This will greatly impede the
storms ability to make eastward progress. Thus, the greatest flash
flooding threat, albeit isolated, will be in the area where the
storms form. This is most likely Monday morning across central
Kansas. By mid to late morning, typical diurnal heating will
disrupt the warm frontal interface, with the collapse of the
forcing resulting in the typical weakening of the convective
complex across Kansas. Isolated to widely scattered convection will
continue in fits and spurts over eastern Kansas and adjacent areas
of the neighboring states, but the storms are unlikely to regain as
much organization as they'll have in the early morning hours
Monday. Renewed convection may form further east towards the Kansas
City Metro Monday night, but once again are unlikely to be quite as
organized and have as much coverage as Monday morning's convection
further west.

...Florida Peninsula...

Monday will be a repeat scenario of many afternoons of the past
week or more across Florida. A stationary plume of tropical
moisture will remain firmly entrenched across the state. A nearly
stationary low over the western Gulf will draw additional moisture
and storms towards the west coast, while stationary fronts,
boundaries, and old cold pools will locally provide their own
forcing Monday afternoon. Thus, the result will be the
same...multiple clusters of storms popping up at random across much
of the Florida Peninsula, but just as often as not competing with
each other. Once again, any isolated flash flooding threat will be
where cell mergers or slow moving storms happen to stall out over
urban or other flood prone areas. 

Wegman


Day 3
Valid 12Z Tue Sep 09 2025 - 12Z Wed Sep 10 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
SOUTHEASTERN COAST AND THE FLORIDA PENINSULA AS WELL AS FOR
PORTIONS OF NORTHERN IDAHO AND WESTERN MONTANA...

...Southeast...

A trough that has been parked across the Florida Peninsula for well
over a week and associated boundary that has been off the coast of
the Carolinas for the past several days will begin to move
northwest back towards the coast as a warm front on Tuesday. A
mesolow may form along the front which may also help move the heavy
rain associated with the boundary northwestward towards the coast
of the Carolinas. The trough, while it has been in place for an
extended period of time, still will demarcate a strong boundary
between a cool, Fall-like air mass over much of the East Coast, and
a typical summery airmass characterized by hot and very humid
conditions. Thus, the trough will still be plenty able to provide
the forcing for strong thunderstorms capable of heavy rain. As the
heavy rain approaches the coast Tuesday, any flash flooding for
coastal communities will result from that heavy rain,
urbanization, and a coincident high tide. Uncertainty with where
exactly that axis of heaviest rain will set up will preclude any
possible upgrades to the ERO at this time, but too much more of a
westward shift of the heaviest rain into the coast will support
possible upgrades in future updates.

Meanwhile over Florida, the primary forcings will be a slow and
erratic moving low over the western Gulf, and the front/trough
centered off the coast, but moving towards the coast. There is
greater potential for more widespread rains across the Florida
Peninsula on Tuesday due to these two forcings moving in closer
proximity to each other. Regardless, any potential Slight Risk
upgrades will be highly dependent on the rainfall footprint of
prior days' rains.

...Pacific Northwest...

A large cutoff low parked over Washington State and Oregon will
pinwheel several pieces of shortwave energy northward along its
eastern periphery across Idaho and Montana on Tuesday. Abundant
forcing and upslope support will use what moisture is available to
result in scattered convection across the region Tuesday afternoon
during peak heating. Since the cutoff low will be barely moving,
this will allow for an extended period where the region will be in
the most favorable region for widespread lift. Thus, while the
heaviest rainfall rates and most widespread convective converage
will be in the afternoon and evening during peak heating, showers
and storms will continue both before and after this time period,
contributing to an isolated flash flooding threat. Should forecast
moisture availability increase during this time with future
updates, there is some potential a targeted Slight Risk area may be needed.

Wegman
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