FOUS30 KWBC 191556
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Mon Oct 19 2026

Day 1
Valid 16Z Mon Oct 19 2026 - 12Z Tue Oct 20 2026

..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST...

Afternoon heating combined with unseasonably high moisture for
mid-October (PWATs 0.75-1.0", 2-2.5 standard deviations above the
mean) will lead to sufficient instability (~1000 J/KG CAPE) over
southeastern Arizona/southwestern New Mexico today to support some
more robust thunderstorms capable of locally heavy downpours. The
hi-res guidance shows the potential for hourly rates of 1"+ which
could lead to some isolated instances of flash flooding, especially
for urban areas as well as terrain sensitive areas such as burn
scars, box/slot canyons, and dry washes/arroyos.

Putnam
 

Day 2
Valid 12Z Tue Oct 20 2026 - 12Z Wed Oct 21 2026

...Southwest...
Anomalous moisture will remain in place Tuesday ahead of the deep
layer cyclone digging southward offshore CA. Integrated Vapor
Transport broaches 500 kg/ms by early Wednesday morning, with the
main increase after daytime heating. However, overall instability
remains fairly low with splotches of 500 J/kg of MU CAPE available.
The risk for heavy rainfall and flash flooding is non-zero, but
appears to be below the 5 percent threshold at the present time.

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Roth
 

Day 3
Valid 12Z Wed Oct 21 2026 - 12Z Thu Oct 22 2026

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

...Southwest...
On Wednesday/Wednesday night, a deep layer cyclone moves into
southwest CA and scoops up moisture from a dying Trudy, flinging
it into the Southwest. A broadening zone of 300-600 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. There are enough
ingredients available/heavy rain signal for the existing Marginal
Risk for the Transverse and Peninsular Ranges, which is there to
help cover burn scar concerns.

Across portions of the Southwest, MU/ML CAPE is forecast to rise
to 1000-1500 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. Urban areas, burn scars, slot/box canyons, and dry
washes/arroyos would be the main areas of concern with the heavy
rainfall anticipated.

Roth
 

Day 4 and Day 5
Valid 12Z Thu Oct 22 2026 - 12Z Sat Oct 24 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST THURSDAY...

Maintained the Slight Risk area in parts of the Southwest on
Thursday as a deep mid/upper level low moves across the region.
Heavy to potentially excessive rainfall remains a concern given the
magnitude of the tropical moisture being advected into the region. Deep
southerly flow ahead of the approaching trough will advect
an airmass with precipitable water values that approach record
values for the latter part of October. There is still spread in the
amount of instability available by then but the overall uptick in
model QPF over the past couple of runs is indicative of the models
are doing better at capturing the steepening lapse rates with the
upper low on Thursday should also produce some destabilization.

By Friday, the trough axis has have made its way across the
southern Rockies and emerged out over the adjacent High Plains.
Some of the moisture will be lost in the complex terrain as the
system moves eastward...but additional low level moisture should
start being drawn out of the Gulf to support a lower-end risk of
excessive rainfall lingering over portions of New Mexico and
portions of adjoining states.

Even though confidence on the exact details remain low given we
are 4 to 5 days out, there is growing confidence in a heavy rain
and flash flood threat over the Southwest region beginning on Day 3
which continues into Thursday. The ingredients in place still
suggest that at least locally significant impacts are a
possibility. Trends will continue to be monitored and can not rule
out the need for embedded MDT risk area(s) if/where enough
instability materializes.

Bann
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