893
FOUS30 KWBC 090745
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Thu Oct 9 2025

Day 1
Valid 12Z Thu Oct 09 2025 - 12Z Fri Oct 10 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEASTERN
CALIFORNIA, SOUTHERN NEVADA, SOUTHWESTERN UTAH, AND WESTERN ARIZONA...

...Southwest...

Models/observations all continue to suggest that a dramatic
increase in low- and mid-level moisture content should occur across
the Southwestern U.S. in tandem with eastward movement of a broad
mid-level trough just west of California and moist mid-level
trajectories emanating from Priscilla. At least a few pockets of
surface heating should occur beneath the moisture plume, resulting
in sufficient surface-based instability for deeper convection
during the afternoon and evening. Efficient rainfall processes
should occur with any storms that form due to the aforementioned
moisture content (with PW values reaching nearly 2 inches across
southwestern Arizona/far southeastern California). Mid-level wind
fields suggest potential for appreciable (20-30 knot) storm
motions, although the tendency for convection to train and occur
on/near sensitive ground conditions (i.e., burn scars, slot
canyons) are all suggestive that at least a few instances of flash
flooding can be expected during the forecast period. Models are
also suggestive of continued precipitation potential overnight
despite a loss of surface heating - likely due to ascent from both
Priscilla and the mid-level trough off the West Coast. Several
areas of 1 inch rainfall totals are possible, and isolated 2-2.5
inch totals could also occur.

...Florida...

Models continue to produce deep moist convection across the east
coast of Florida during the afternoon and evening hours. Widespread
1-3 inch rainfall amounts are expected. As typical for the region,
abundantly moist thermodynamic profiles will combine with weak
kinematics for slow-moving cells with very high rainfall rates
(exceeding 2 inches/hr at times). These cells will pose an
urbanized flash flood threat from Jacksonville metro southward
through Miami/Dade. Weak onshore flow has also contributed to a
coastal flood risk while hindering runoff processes in a few
locales. This could exacerbate flood/flash flood concerns on a
localized basis.

Cook

Day 2
Valid 12Z Fri Oct 10 2025 - 12Z Sat Oct 11 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF ARIZONA...

...Southwest...

A synoptically evident heavy rain threat is expected to unfold
across the region Friday into early Saturday. Models continue to
depict a large-scale trough just west of the California coastline
that should slowly move eastward and spread geopotential height
falls across much of the Southwest and Great Basin. 00Z models have
sped up the timing of the absorption of Priscilla (perhaps
remnants of Priscilla by this forecast timeframe) within the
broader-scale trough and now depict them directly impacting
portions of Arizona by as early as 18Z Friday. Ascent from
Priscilla, ascent from the larger-scale trough, and abundant
moisture streaming into much of the Great Basin (with attendant
terrain influences) all point to potentially widespread flash
flooding during the forecast period. Of note - the overall synoptic
pattern has notable parallels to early October 2018, when remnants
of Hurricane Rosa moved into the Great Basin and prompted
extensive flash flood impacts across central Arizona.

The Moderate Risk has been introduced across portions of central
into northeastern Arizona where models (particularly the GFS and
ECMWF) depict multiple hours of repeating thunderstorm potential
especially focused near terrain-favored areas of the Mogollon Rim.
Widespread 1-2 inch rainfall amounts (and locally higher totals)
can be expected. Significant impacts are possible given the
prolonged heavy rainfall potential. While specific timing of
convective axes are still unclear at this timeframe, some eastward
progression of the heaviest rainfall is generally depicted by most
model guidance, with precipitation areas potentially reaching the
AZ/NM border region through 12Z Saturday.

Outside of the Moderate Risk area, models depict potential for
scattered thunderstorms to occur throughout the forecast period
from southeastern California northeastward through eastern Utah and
southwestern Colorado. A few instances of flash flooding will
remain possible especially near urban areas, burn scars, and slot
canyons.

...Southeastern U.S. Coastal Areas...

Models continue in agreement that an organizing area of low
pressure will strengthen over the southern Gulf Stream (east of
Florida) during the forecast period. Models are also in agreement
that at least a few spots along the coast will experience heavy
rainfall at times through 12Z Saturday - especially in eastern
Florida. This heavy rainfall potential will occur in conjunction
with dramatically strengthening onshore flow across coastal areas
that should promote flooding while hindering runoff. This "combo-
flood threat" (both freshwater and saltwater flooding) will exist
across coastal areas throughout the forecast period while peaking
fairly early across the east coast of Florida (between 12Z Friday
and 00Z Saturday) and peaking across coastal Carolinas areas from
00Z Saturday through the end of the forecast period (12Z Saturday).

Cook

Day 3
Valid 12Z Sat Oct 11 2025 - 12Z Sun Oct 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
FOUR CORNERS STATES AS WELL AS FOR PORTIONS OF COASTAL NORTH
CAROLINA AND VIRGINIA...

...Southwest...

Predominant southwesterly flow ahead of a deep longwave trough
progressing east from the West Coast to over the Intermountain West
will draw plentiful tropical moisture from the remnants of Priscilla
into the Four Corners Region. This will spike atmospheric moisture
levels over portions of Arizona to as high as 5 sigma above normal.
Thus, moisture availability for rainfall will not be a limiting
factor in the slightest (pun intended). However, what will limit the
potential for more significant flooding will be a lack of
instability. While the remnants of Priscilla will bring some
instability northward, for most areas MUCAPE values are expected to
remain under 1,000 J/kg. Thus, the areas of heaviest rainfall will
be focused on south and west facing slopes, as upslope becomes a
primary localized forcing mechanism. With a Moderate Risk upgrade on
Day 2, a higher end Slight is in effect for the portions of the
Mogollon Rim expected to be hardest hit from the aforementioned
upslope continuing into Day 3.

The Slight up to the 4 Corners/southwest Colorado remains unchanged
with this update. Similar to further south and west, upslope will be
a primary forcing mechanism, in addition to the upper divergence
ahead of a strong shortwave which will push east from California to
over Utah by Sunday morning. This too will locally increase the
forcing needed to squeeze out additional rainfall into the San Juans
throughout the period.

Finally, the inherited Marginal Risk was expanded north to cover
much of central Montana with this update. Additional shortwave
energy, perhaps moving even faster than the energy further south,
will allow for a localized negative tilt to the shortwave, which
should greatly increase the forcing needed for widespread light to
moderate rainfall into central Montana. A potent cold front
associated with the approach of the upper level trough will also
locally increase rainfall rates ahead of the front. See the Heavy
Snow Discussion (HSD) for additional details on the snowfall
potential in the higher elevations of western Montana with this same
disturbance.


...Mid-Atlantic Coast...

The center of a large and slow-moving Nor`easter will make its way
up the coast from off Jacksonville, FL Saturday morning to near
Wilmington, NC by Sunday morning. The expansive precipitation shield
associated with this large low will be impacting the coast of the
Carolinas already at the start of the period, with its northernmost
extent moving into southern New England by the end of the period
Sunday morning. The biggest rainfall threat with this storm will be
across coastal North Carolina and Virginia. The guidance is in
reasonable agreement that once the center reaches near the border of
the Carolinas, it`s forward speed will slow to a crawl through
Saturday night. This will allow the comma-head region of the storm
to sit in place across coastal North Carolina and Virginia for a
long-duration, likely lasting into the D4/Sunday period. Meanwhile
strong and gusty northeast winds will pile up tidal/ocean water at
the coast. While coastal flooding concerns are likely to be a much
more significant concern as far as magnitude of impacts, ocean water
backing up the mouths of bays, rivers, and inlets is likely to
significantly impede draining the fresh water that falls from
rainfall, resulting in more widespread flooding concerns from the
combination of the two sources of water.

The Slight Risk remains in effect for all of coastal North Carolina,
and has been expanded north to include the Hampton Roads area, for
similar combination of tidal and fresh water flooding likely to
impact the urban area there as well. The primary piece of
uncertainty, which is the biggest motivator to keep the flooding
risk at a Slight, is how much instability can work into the system`s
precipitation north of the low center. The expected lack of
instability, albeit increasing and with convection moving from
higher instability areas to the south into lower instability areas
to the north, supporting persistence, should limit precip rates to
usually more manageable levels, which therefore will limit the
instances of flash flooding, but that is highly dependent on the
low`s track and definitely could change.

Wegman


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt