FOUS30 KWBC 200806
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Tue Oct 20 2026

Day 1
Valid 12Z Tue Oct 20 2026 - 12Z Wed Oct 21 2026

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR
PORTIONS OF ARIZONA...

...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
sunset. The 03z RAP indicates that MU CAPE could broach 1000 J/kg
across southern AZ, migrating westward from the AZ/NM border
towards Phoenix. When the moisture surge comes in from the Gulf of
CA/tropical eastern Pacific, bringing precipitable water values up
to 0.75-1.25". There is some concern that shortwaves rounding the
incoming upper level low/trough could lead to some convection
Tuesday night into very early Wednesday. Raised a Marginal Risk for
portions of southern and central AZ given the parameters that come
in after sunset, of which there is some, albeit disagreeable,
signal for in the 00z REFS/HREF guidance.

Roth
 

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 3
Valid 12Z Thu Oct 22 2026 - 12Z Fri Oct 23 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST AND FOUR CORNERS REGION THURSDAY/THURSDAY NIGHT...

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. 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+
J/kg in eastern AZ early on. 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. Trends will continue to be monitored and can not
rule out the need for embedded Moderate risk area(s) as we get
closer to the event.  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 Fri Oct 23 2026 - 12Z Sun Oct 25 2026

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

Southwest US...

The potential remains for locally heavy rainfall to linger on
Thursday into Thursday night as the axis a an upper level trough
remains west of the region according to the 20/00Z GFS and ECMWF.
That leaves much of eastern and central New Mexico with deep layer
southerly fetch of moisture. The potential for excessive rain is in
a relative minimum on Thursday before arrival of additional
middle- and upper-level shortwave energy reinvigorates the chances
of heavy rainfall on Friday across parts of the High Plains to the
lee of the southern Rockies. The adjustments made to the Marginal
Risk area of Thursday was minimal while a Slight Risk area was
introduced in the period ending from Friday and into the early
morning hours of Saturday.

South Florida...

The numerical guidance has been showing increasing moisture
with embedded rain approaching the southern Florida peninsula from
the southeast. Latest numerical guidance suggests locally heavy
rainfall could begin as early as Friday but the signals for greater
coverage and higher rainfall amounts are forecast to occur on
Saturday as an inverted trough approaches. Given the amount of
urbanization in the area combined with locally heavy rainfall in
the area during the past week...opted for a Slight Risk at this
point on Saturday.

Bann
$$
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