Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1133 AM EDT Tue Oct 20 2026
Day 2
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 300-600 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. Per coordination with
LOX/the Oxnard CA forecast office, went ahead and upgraded to a
Slight Risk for portions of SoCal, including the coastal ranges
north of Point Conception and the western Transverse Ranges to its
east, into the Los Angeles metropolitan areas.

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.

Roth


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt