FOUS30 KWBC 210803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Wed Oct 21 2026

Day 1
Valid 12Z Wed Oct 21 2026 - 12Z Thu Oct 22 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST & FOUR CORNERS WEDNESDAY/WEDNESDAY NIGHT...

...Southwest...
On Wednesday/Wednesday night, a deep layer cyclone moves to Point
Conception and scoops up moisture from the tropics, flinging it
into the Southwest. A broadening zone of 300-700 kg/ms of
Integrated Vapor Transport envelops the Southwest. Near the cold
low itself, MU CAPE of ~500 J/kg and precipitable water values of
1-1.3" with up to 30 kts of onshore flow at 850 hPa is forecast,
which would support hourly amounts to 2" where cells manage to
train or backbuild within the terrain. The elevated Slight Risk was
maintained for portions of SoCal, including the coastal ranges
north of Point Conception and the Transverse Ranges.

Across portions of the Southwest, MU/ML CAPE is forecast to rise to
1000-3000 J/kg with lesser amounts to the north across the Great
Basin/Rockies, with precipitable water values rising into 0.75-1.5"
range. The ingredients available along with deterministic guidance
QPF output appear sufficient to maintain the existing Slight Risk.
Hourly rain amounts to 2.5" would be possible across portions of
AZ, NM, and the Four Corners in widely scattered to scattered
spots due to cell training or where cells backbuild or merge.
Urban areas, burn scars, slot/box canyons, and dry washes/arroyos
would be the main areas of concern with the heavy rainfall anticipated.

...Lower TX Coast...
A low-level feature retrograding through the northwest Gulf is
expected to move inland and potentially lead to heavy rainfall
very near the Padre Island coast. The guidance isn't sure whether
or not heavy rain will be onshore or offshore, so this is a
conditional threat. Precipitable water values rise to 1.5-1.75". ML
CAPE of 1000+ J/kg is expected to slosh ashore. Raised a Marginal
Risk as a precaution.

Roth
 

Day 2
Valid 12Z Thu Oct 22 2026 - 12Z Fri Oct 23 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHWEST COLORADO THURSDAY/THURSDAY NIGHT...

Southwest/Four Corners...
The Slight Risk area remains for much of the Southwest/Four 
Corners on Thursday as a deep mid/upper level low and accompanying 
amplified upper trough moves from CA into the Great Basin, with a
new Moderate Risk for southwest CO, which was coordinated in
conjunction with the Grand Junction forecast office. Heavy to 
potentially excessive rainfall remains a concern given the 
magnitude of the tropical moisture being advected into the region, 
with Integrated Vapor Transport of up to 600 kg/ms. Deep southerly 
flow ahead of the approaching trough will advect an airmass with 
precipitable water values of 1-1.5" into the region, very high by 
October standards. Freezing levels will be quite high in southwest 
CO, close to the mountain peaks. MU CAPE is forecast to be 
1000-2000 J/kg near the AZ/NM border during daytime heating, with
500 J/kg values not far upstream of southwest CO. The ingredients 
in place still suggest that at least locally significant impacts 
are possible. Hourly rain amounts up to 2.5" are possible given 
the ingredients available. This is a higher end Slight Risk for 
portions of AZ, NM, and UT. Urban areas, burn scars, slot/box 
canyons, and dry washes/arroyos would be the main areas of concern 
with the heavy rainfall anticipated.

South Florida...
The 00z HREF/REFS are either very close, or just ashore, with a
heavy rain signal across portions of the Gold Coast as moisture
returns from the Atlantic/Bahamas with instability from the Gulf
Stream. The concern would be any convective banding that tries to
develop in the antiparallel flow with height seen in that area. The
threat is conditional, but hourly amounts to 2" are possible should
any bands set up in this environment, which would be problematic in
urban areas.

Roth
 

Day 3
Valid 12Z Fri Oct 23 2026 - 12Z Sat Oct 24 2026

...THERE IS A MARGINAl RISK OF EXCESSIVE RAINFALL IN AND NEAR THE
SOUTHERN HIGH PLAINS...

Southern High Plains...
The guidance shows a modest heavy rain signal in and around the
Southern High Plains and far west TX as Gulf inflow moves upslope
into the area and Pacific moisture flows in aloft ahead of an
incoming upper level low/trough and Integrated Vapor Transport has
values up to 600 kg/ms. Precipitable water values of 1-1.25" are 
forecast there, which are high for the region and for October. MU 
CAPE rises to ~1000 J/kg. The flow should be fairly unidirectional 
from the south, so cell training is possible should enough 
convective coverage develop. The guidance QPF remains modest, so 
left the Marginal Risk as is.

South Florida...
The global guidance is either very close, or just ashore, with a
heavy rain signal across portions of the Gold Coast as moisture
returns from the Atlantic/Bahamas with instability from the Gulf
Stream. The concern would be any convective banding that tries to
develop in the antiparallel flow with height seen in that area. The
threat is conditional, but hourly amounts to 2" are possible
should any bands set up in this environment, which would be
problematic in urban areas.

Roth
 

Day 4 and Day 5
Valid 12Z Sat Oct 24 2026 - 12Z Mon Oct 26 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE WESTERN/SOUTHWEST U.S. AND SOUTH FLORIDA...

The current synoptic pattern for both the Western CONUS and for 
southeast FL coast maintain signals for heavy to potentially 
excessive rainfall this weekend. In each outlook area...only small 
adjustments were made based on latest deterministic and ensemble 
guidance on Saturday.

Guidance for Florida remained focused on immediate coastal areas
as the primary point of emphasis, correlating with the urban
corridor that tends to be more prone to flash flooding given the
amount of impervious surfaces.

The potential for excessive rainfall farther west takes shape in
the Southwest U.S. generally to the lee of the southern Rockies on
Saturday that propagates north and east-ward on Sunday. The threat
will be tied to the anomalous moisture advection into the area
with the best ascent located within the downstream diffluent flow
aloft. Spaghetti plots of QPF from the EC Ensembles and the GEFS
on Saturday still primarily depict areas along and east of the
Continental Divide as the main areas of concern with some signals
for locally 2+ inches. Conditions supporting heavy to excessive
rainfall persist over portions of the western High Plains but the
northern extent of the precipitation shield will be fighting a feed
of cooler and more stable airmass being fed into the region by
building surface high pressure.

Bann
$$
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