Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Mon Oct 13 2025
Day 1
Valid 16Z Mon Oct 13 2025 - 12Z Tue Oct 14 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEASTERN ARIZONA, SOUTHWESTERN NEW MEXICO, AND SOUTHERN
COLORADO AS WELL AS PORTIONS OF SOUTHERN CALIFORNIA...

16Z Excessive Rainfall Update...
Given the latest HREF blended mean QPF across portions of southern
California...introduced a small Slight Risk area mainly to cover
the potential of run-off problems from burn scars late in the Day 1
period. The overall synoptic pattern still features a closed low
dropping southward along the California coast...accompanied by a
broad shield of light to moderate rainfall with the HREF showing
some localized rainfall amounts approaching an inch during the
06Z-12Z time frame. Decided there was enough of an elevated signal
to support a Slight Risk...which will blend into the previously-
issued Slight Risk for the Day 2 period.

Except for a slight westward expansion of the Marginal in Arizona
to account for the instability shown by the HRRR and ARW during the
afternoon...which may result in convection capable of producing
localized downpours...the on-going Slight Risk areas in the
Southwest and in Colorado remained largely unchanged.

Bann


...Southwest to the Central Rockies...

Elevated moisture running between 2-4 deviations above normal will
remain situated across the Four Corners into the Central Rockies
down through the Southwest CONUS with roots persistent from the
tropical flux associated with now deceased tropical cyclone
Raymond. The persistence in the longwave pattern has allowed for a
continuous onslaught of heavy convective threats across the Desert
Southwest with a large amount of flash flood warnings tied to the
efficient rainfall processes exhibited given the supportive
atmospheric profile. For at least one more period, another round of
heavy precip will focus across portions of AZ/NM up into the San
Juans and adjacent Rockies of CO thanks to the flow actually
becoming even more meridional in-leiu of a buckling upper jet
pattern across the west coast stemming from a digging powerful
shortwave meandering down from the Pacific Northwest (More on this
setup below). As of now, there is a consistent signal in the hi-res
means for scattered heavy convective threats situated along the
eastern half of the Mogollon Rim into NM with a secondary maxima
situated over the San Juans where persistent upslope flow and mid-
level perturbations will enhance rainfall concerns within the
terrain for a third consecutive day.

00z HREF EAS probs for >1" are running high (>60%) for both the
areas mentioned above, plus >2" signals across the San Juans
maintain a strong 50-70% output meaning heavy rain prospects are
increasingly certain in the period. The one caveat to a higher
threat is the general lack of sufficient rates that would promote a
greater flash flood concern, especially one that carries more
significant impacts. Locally impactful outcomes are still plausible
considering some of the antecedent conditions leading into the
setup, so it's not out of the question some considerable flash
flood prospects could materialize in areas outlined in the SLGT
risk. Overall, this setup is conducive for at least scattered flash
flood potential leading to a general continuity of the SLGT from
previous forecasts with some adjustments based on the latest prob
fields.

...California....

Current WV satellite and UA analysis indicates a powerful closed
ULL situated over the Olympics of Washington state with an
anticipated trajectory down the Pacific coast before it finally
loses latitude near Central CA before beginning to pivot inland.
During the forecast upper evolution, a strong south-southwest push
of Pacific moisture will favor an increase in regional PWAT
advection pressing inland across CA through the course of the D1.
The latitudinal push will allow for a persistent onshore component
within the coastal terrain from northwest CA down into the
Transverse Range of Southern CA towards the back end of the
forecast cycle. This will allow for 6-12 hr periods of heavy
rain potential within the confines of those coastal terrain
elements leading to isolated flash flood prospects as the setup
evolves. As of this time, the greatest threats for flash flooding
will be within those coastal terrain proxy's, however the inland
push of slightly anomalous moisture will also produce an element of
heavy rainfall in the Sierra Foothills, especially for the upslope
regions of the central and southern Sierra below 7000ft MSL. These
areas could see impacts from complex drainage within the steep
terrain situated against the higher elevations just east that will
be encountering the first true winter event of the season at the
same juncture. Rates will be relatively capped as MUCAPE fields are
generally forecast to be more benign compared to other instances
we've had in the past. The best MUCAPE signatures are actually
aligned with the Transverse Range and northern Peninsular Range of
southern CA, an area that is likely the most prone for flash flood
potential due to the litany of burn scar remnants present over the
area.

Trends the past 12-24 hrs. now position the flow to be generally
orthogonal to the terrain those locations towards the end of the
forecast cycle, a setup conducive for persistent heavy rainfall
anchored over the same areas for longer periods of time. Despite
the lack of enhanced rates, enough instability and orographic
forcing could lead to rates exceeding 1"/hr at times, more than
sufficient for flash flood concerns to arise right near the final 6
hr. period in the D1. The previous forecast was relatively
unchanged north of the Transverse Range, but with the deeper
meridional inflection in the mean trough, the threat for flash
flooding across the southern CA terrain has sped up in time, thus a
MRGL extension into the Ranges northwest and north of Los Angeles
were implemented to reflect the latest trends. This will be a time
frame to monitor for any targeted upgrades if guidance allows.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt