Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Sun Oct 18 2026

Day 1
Valid 01Z Mon Oct 19 2026 - 12Z Mon Oct 19 2026

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

Chenard
 

Day 2
Valid 12Z Mon Oct 19 2026 - 12Z Tue Oct 20 2026

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

Introduced a Marginal Risk for portions of the southwest (SE AZ/SW
NM). Remnant energy from a lingering upper-trough just to the west
will spread eastward over the region later tonight and into the
day Monday helping to trigger scattered storms. An influx of
modestly anomalous moisture from the Pacific will lead to
sufficient instability (1000 J/KG CAPE) by Monday afternoon to
support locally heavy downpours. The now available hi-res
guidance, particularly the HRRR, is rather bullish on the potential
for rain rates of 1-2" per hour, more than enough to lead to
isolated flash flooding. Both the HREF and RRFS ensembles also have
moderate to high probabilities of rainfall exceeding 1" and low-
end probabilities for 2".

Putnam
 

Day 3
Valid 12Z Tue Oct 20 2026 - 12Z Wed Oct 21 2026

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

...Southwest...

Modestly anomalous moisture will remain in place Tuesday ahead of
the more potent upper-trough beginning to dig southward along the
West Coast. However, overall instability remains low and with the
bulk of upper-energy still remaining to the west of the region, the
risk for heavy rainfall and flash flooding should remain very low.

Putnam
 

Day 4 and Day 5

Valid 12Z Wed Oct 21 2026 - 12Z Fri Oct 23 2026

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

20z Update: Introduced a broad Slight risk across portions of the
Southwest on both days 4 and 5. This is a favorable setup for heavy
rainfall and at least scattered flash flooding with anomalous
moisture and a deep mid/upper level low pushing across the region.
Deep southerly flow ahead of the approaching trough will advect
tropical moisture northward, with a PW plume of near record values
for mid/late October. As is typical with these events, instability
will be the key factor in determining the magnitude and extent of
flash flooding...as these broad moisture plumes can often bring
abundant cloud cover limiting destabilization. However, both the
ECMWF and AIFS depict CAPE over 1000 J/kg on both Wednesday and
Thursday across portions of AZ/NM and over 500 J/kg even into
portions of NV/UT/CO. There is an initial moisture/instability
surge on Wednesday over AZ well ahead of the upper low, and then
the steepening lapse rates with the upper low on Thursday should
also produce some destabilization.

So while 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 Wednesday into
Thursday. The ingredients in place also 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.

Chenard

...Previous Discussion...

...Southwest...
On Wednesday/Wednesday night, a deep layer cyclone approaches
SoCal and scoops up moisture from a dying Trudy and flings it into
the Southwest. Near the cold low itself, MU CAPE of 250-500 J/kg
and precipitable water values of 1-1.3" with up to 30 kts of
onshore flow at 850 hPa is forecast. There's enough of a heavy rain
signal for a Marginal Risk for the Transverse and Peninsular
Ranges, which is there to help cover burn scar concerns. In and
near the Four Corners, MU CAPE is forecast to rise to 500-750
J/kg, precipitable water values of 0.75-1.25" are anticipated. The
ingredients available along with deterministic guidance QPF output remain
wet enough for a Marginal Risk, though the higher totals seen
24 hours ago have come down a bit, possibly due to a ~6 hour slow
down/westward shift of the overall system when compared to this
time yesterday. Hourly rain amounts to 2" would be possible across
portions of AZ, NM, and the Four Corners in isolated to widely
scattered spots.

For Thursday/Thursday night, the system shifts eastward across
Southwest and Great Basin/Intermountain West into portions of the
Southern High Plains, with similar moisture and instability in
tow. The guidance advertises local totals in the 2-3" range, and
hourly rain amounts to 2" remain possible. The mountains of SoCal
appear to have enough moisture/instability left and guidance QPF
signal to introduce a Marginal Risk there with the Marginal Risk
across the interior West shifting eastward. It is possible that
the risk levels could increase as we move closer to this weather
setup -- particularly if instability trends reverse/increase
-- but the slowing trend in the guidance with this system
introduces enough uncertainty not to do go with a risk level
beyond Marginal either day at the present time.

Roth
 

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt