Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Mon Nov 17 2025
Day 2
Valid 12Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST CENTRAL ARIZONA...

20z Update:

Upgraded to a Slight risk across portions of west central AZ with
this update. Strong forcing just ahead of the upper low will
support repeated rounds of showers and embedded heavier convective
elements within the deep layered southerly flow. Not a lot of
instability forecast, but values upwards of 500-1000 j/kg should
be enough to support locally heavier rainfall rates...over 0.5"/hr
but generally under 1". Nonetheless the repeated rounds and some
upslope component should result in rainfall locally exceeding
2-3" over the day. HREF neighborhood probabilities of exceeding 2"
are 50-70% and 3" are 40-50%. This rainfall should be enough to
result in isolated to scattered flash flood impacts, especially
over any more sensitive areas.

We also added a small Marginal risk centered over KY. A low moving
across the OH valley will place KY in the warm sector, with
instability forecast to increase over 1000 j/kg by Tuesday
afternoon. An initial batch of showers should move across during
the morning with the warm front, with another more robust
convective round expected during the afternoon and evening as
instability increases ahead of the cold front. Both the HREF and
REFS indicate 1"/hr neighborhood probabilities of 20-40% and the
HREF neighborhood probabilities of exceeding 3" during the period
are locally over 15%. It's a pretty marginal situation, but given
the degree of moisture transport and forecast uptick in
instability it does seem like an event that could locally exceed
model QPF forecasts...thus can not rule out some isolated, mainly
urban, flash flood concerns.

Chenard

...Previous Discussion...

Maintained the previous;y issued Marginal risk area over portions
of Arizona with little need for more than cosmetic changes in
deference to the model guidance. A closed low within a broader
trough digs southward along the the west coast of the US and then
starts to move inland late. The overall synoptic pattern seems to
be handled comparably by the ECMWF/GFS and UK models for much of
Day 2 but some of the subtle differences in the details result in
non-negligible differences in QPF and the attendant risk of
excessive rainfall.

Precipitable water values get near 2 standard deviations above normal
for this time of year in southern-western AZ and most-unstable
CAPE values climb above 500 J/Kg south of the Mogollon Rim and into
the deserts and Phoenix Metro. This will maintain the risk of
elevated (0.50"+) hourly rainfall rates Tue-Tue night as noted from
the RRFS exceedance probabilities (through 00Z Wed), while also
implied from the 16/12Z RRFS 3-hourly QPFs. Much like the models
from Sunday afternoon...the 24hr QPF guidance from the 17/00Z model
cycle has come up. At this point there is still not enough
confidence from a duration perspective to upgrade to a Slight Risk
over the lower terrain south of the Mogollon Rim, which would
conceivably include the Phoenix area, but the potential to do so
will exist over the next day or so. The machine-learning first
guess fields also highlight part of the Marginal risk area with
some spotty 5 to 10 percent of exceeding flash flood guidance. At
this point...the machine-learning covers only the first half of Day
2...so the upcoming day shift will have a complete look at the
objective guidance for the Day 2 period.

Bann


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