Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Wed Oct 21 2026
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...

...2030Z Update...

...Southwest...

Once again no significant changes were made as far as the broad
scale forecast. However, a higher-end Moderate Risk was introduced
for most of the Moderate Risk area in southwest Colorado, including
the San Juan Mountains. In addition to a nominal increase in the
forecast rainfall for that area, analysis of the latest 12Z runs of
the guidance show there are numerous concerning thresholds that
will be threatened with this upcoming event. These thresholds
include:
1)an over 90% chance of 5 inches or more of rain in 24 hours to
occur 2) similar chances of seeing 100 year Annual-Exceedence-
Intervals met 3) 1-2 day rainfall records in areas such as Durango
and Telluride broken 4) PWATs of near 1 inch amounting to 4-5 sigma
above normal 5) Instability between 500-1,000 J/kg, which for most
areas is not a lot, but for this area of the Southwest, amounts to
enough to support persistent convection 6)A snowmelt component with
much of the early season snowpack on the summits of the San Juans
likely melting as snow levels rise to 12,000 to 13,000 ft and 7) A
climatological component such that the last time the Durango area
saw 4-5 inches of rain in 24 hours, major river flooding resulted,
and there was no snowmelt to assist in the last event.

All of that is to say that the flooding concerns continue to
increase, and there is some potential that a High Risk upgrade may
be needed, especially if rainfall forecasts continue to increase
and CAMs support continued rounds of convection.

Elsewhere across the Southwest, significant rains will also impact
western New Mexico, the Mogollon Rim area of Arizona, and much of
the National Parks and slot canyons from southern Utah north and
east to the Uinta Mountains in the north. A large Slight Risk area
remains for much of the rest of the 4 Corners region, with a higher
end Slight for the southern tier of Utah as well as the Mogollon
Rim of east-central Arizona.

Lingering rainfall as the "bowling ball" low in the West continues
progressing east into southern California could continue any
flooding that occurs this afternoon into tonight. Nearly all of the
rain with this event for SoCal will be over by the start of the
period, so a Marginal Risk was maintained.

...Southeast Florida...

Onshore flow into the urban corridor from West Palm Beach down to
Miami may allow robust convection occurring off the coast over the
Gulf Stream to try to move into coastal communities. These
occurrences will be few and far between. The Marginal Risk was
maintained due to the urban influence potentially allowing for an
isolated instance of flash flooding.

Wegman

...Previous Discussion...

Southwest/Four Corners...
The Slight Risk area remains for parts 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. A new
Moderate Risk was raised 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. The ingredients in place still suggest that at
least locally significant impacts are a possibility. Hourly rain
amounts to 2.5" are possible given the ingredients available. This
is a higher end Slight Risk for portions of southeast AZ and
southwest CO.  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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt