649
FOUS30 KWBC 100054
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Thu Oct 9 2025

Day 1
Valid 01Z Fri Oct 10 2025 - 12Z Fri Oct 10 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHWESTERN
US AND THE EAST COAST OF FLORIDA OVERNIGHT...

...01Z Excessive Rainfall Discussion...
Introduced a Slight Risk area over parts of the eastern Florida
peninsula overnight based on the trends seen in latest radar and
satellite imagery of cooling cloud-tops off-shore and confluent
flow helping to channel flow into that part of the peninsula.
Concern is that the high precipitable water (about 2 inches) and
the persistent on-shore flow will result in hourly rates
approaching 2 inches per hour and isolated rainfall totals of 3 to
5 inches. The northern portion of the Marginal risk area was
trimmed a bit but the overall footprint of the Marginal risk area
was similar to that of the 16Z Excessive Rainfall Outlook.

No changes made in the Southwestern United States as moisture
continues to stream into the region with concern remaining for the
potential of flash flooding...especially in slot canyons and other
flashy basins...overnight with the threat persisting beyond the end
of the Day 1 period at 12Z.

Bann


...16Z Update...

...Southwest...

The Slight Risk was expanded eastward a bit in southern Utah to
cover potential for flash flooding across portions of the Slot
Canyons/increased flashy basins where short term RAP forecasts are
showing 500-1000 J/kg MLCAPE later this afternoon which is likely
to translate into increased potential for 0.5 to 1+ inches of rain
in 30-60 minutes as remnant tropical moisture is advected northward
later today. Similarly, an expansion to the Slight Risk across
portions of southwestern Utah and southern Nevada was made where
simulated reflectivity from the 12Z HREF and select University of
Arizona WRF runs showed increased potential for repeating/training
cells later today and tonight.

The Marginal Risk was cut back a bit out of the Colorado Rockies
where instability is expected to be more limited compared to
locations farther to the west/southwest. The 12Z HREF only shows
isolated occurrences of 0.5 in/hr potential (10 percent or less)
and 06Z REFS potential is less than 1 percent through 12Z Friday.


...Florida...

No changes were needed across eastern Florida with similar
potential as described in the discussion from overnight. The 12Z
HREF is highlighting greater potential for 3+ inches for the
central and northern coastline of eastern Florida, perhaps aligned
with the stronger low level flow into the region (20-30 kt) and
better jet support aloft. While the 12Z HREF has cut back on QPF
for southeastern Florida, where weaker onshore flow will be
present, potential for flash flooding will remain where high
moisture and slow cell motions will maintain a concern for
excessive rainfall across the region.

Otto

...previous discussion follows...


...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. As an anomalous
closed low/trough along the West Coast slowly edges eastward, the
mid and upper level reflection of Priscilla will cross the central
Baja Peninsula and shear into Arizona. Some degree of upper level
enhancement to divergence is anticipated within the right-entrance
region of a strengthening sub-tropical jet as the upper trough
moves east. Meanwhile, highly anomalous moisture (+4 to +6
standardized anomalies via the 12Z GFS) will be advancing
downstream from the tropical eastern Pacific into most of the
Desert Southwest. Given steering flow is forecast to be at least 10
to 20 kt from the S to SW, the flash flood concern arises from the
likelihood of repeating and training.

While instability may not rise much above 500-1000 J/kg beneath
areas of cloud cover streaming toward the northeast, the remnant
tropical moisture will probably result in "tall/skinny" CAPE
profiles with potential for efficient rainfall production and
pockets of 1 to 2 in/hr rates (and likely sub-hourly totals of 0.5
to 1 inch). There will be potential for greater CAPE values along
the western edges of the cloud cover (eastern California into
southern Nevada) where better solar heating will overlap with lower
but still anomalous moisture. These flash flood threat for these
regions will likely contain a greater diurnal component, peaking in
the 18-06Z time frame.

The Moderate Risk was nudged northward a bit from the previous
outlook over central/northern Arizona, in line with the latest
model consensus where 2-3 inch storm totals appear most likely,
though localized higher totals remain possible. Some expansion of
the Marginal and Slight Risks was made from Nevada into Utah to
account for potentially higher instability (500-1000 J/kg), able to
support locally higher rates with the likelihood of training and
repeating. In addition, a higher end Slight Risk was extended
eastward into southwestern Colorado where the shearing remnants of
Priscilla should help to extend the heavy rain threat into portions
of western Colorado with upslope enhancement as 700 mb winds
increase into the 20 to 30+ kt range. Scattered 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) through Saturday morning. Low level onshore flow will
direct higher moisture and instability just off of the coast to
inland locations though a lack of instability for locations beyond 20
to 30 miles inland of the coast will likely preclude much in the
way of a flash flood threat for these inland locations. Relatively
weak cell motions will continue a flash flood concern in eastern
Florida, especially the east-central Peninsula which has picked up
heavy rain over the past week. Farther north, the concern will be
more from training/repeating rounds of heavy rain within confluent
and convergence low level wind fields with 24 hour rainfall
potential of 3 to 6 inches, though coverage of these higher totals
should remain isolated. 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). The Marginal was extended northward a bit in
eastern North Carolina to account for possible timing differences
compared to the latest model consensus.

Otto/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...

Tropical moisture will continue to influence an elevated threat of
flash flooding across a large portion of the Desert Southwest into
the eastern Great Basin and central to northern Rockies on Saturday
into Sunday. As remnant tropical moisture from Priscilla continues
to influence the West, moisture will be drawn northward from newly
named Tropical Storm Raymond to the south of Mexico, forecast to
track toward the southern Baja Peninsula through Sunday morning.
Strengthening upper level jet support will increase synoptic scale
lift across the Four Corners region into southwestern Colorado
where upslope enhancement from the Mogollon Rim into the San Juan
Mountains is expected.

Cloud cover will again be somewhat of a concern for generating
higher levels of instability but synoptic scale lift and anomalous
moisture (+4 to +6 standardized PW anomalies from the 12Z GFS at
the start of the period) should act to compensate. Rain rates in
excess of 1 inch per hour (or less time) will remain a threat from
Arizona into western New Mexico and the Four Corners region. A
higher end Slight Risk remains across the Mogollon Rim region of AZ
due in part to expected antecedent rainfall from Friday into
Saturday) and a second 25 percent area was introduced for
southwestern Colorado.

A lower flash flood threat will extend northward through the
central to northern Rockies into Montana and the Canadian border.
It is here where synoptic scale lift will be greatest ahead of an
approaching mid to upper level trough containing jet induced
divergence and diffluence. Moisture will be pushed off toward the
east for the second half of the period with falling snow levels
across the Rockies of southwestern Montana into Wyoming with
any lingering heavy rain continuing a potential for localized flash
flooding for the valley locations.

...Mid-Atlantic Coast...

Uncertainty remains with the forecast of a strengthening coastal
low off the Mid-Atlantic region on Saturday into Sunday. Looking at
trends in the ensembles, there has been a bit of a westward shift
over the past 24-48 hours, but some of the deterministic models
remain farther offshore (UKMET and CMC). Current thinking takes the
surface low near the Outer Banks with slow movement, allowing 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.

Little change was made to the outlook for the Day 3 time frame
given the forecast remain reasonably similar to the prior outlook
with similar uncertainties in low track.

Otto/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