835
FOUS30 KWBC 172006
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Mon Nov 17 2025

Day 1
Valid 16Z Mon Nov 17 2025 - 12Z Tue Nov 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF
SOUTHERN CALIFORNIA...

16z update:
Current observational trends and 12z CAM guidance continue to
solidify the current thinking and placement of ERO areas and
magnitudes moving forward through 18.12z.

Core of the warm conveyor belt/atmospheric river is starting to
reach southwest California and rounding Point Conception. Weakly
confluent 850-700mb 35-45kt southerly flow along the front and 1 to
1.25" total Pwats (around .5" in the same layer) is resulting in
400-500 kg/m/s IVT values. Though the moisture plume is narrowing
and starting to advance with upstream height-falls, the onshore
flow will intersect the Transverse Range more ideally to maintain
solid rainfall rate potential over .5"/hr (HREF probs of 70-90% on
the Transverse and 30-40% on the Peninsular Ranges). The forward
progress is likely to limit overall totals along the spine of the
terrain to 1.5-2" with an isolated 2.5" not out of the realm of
possibility. Combine this with higher soil saturation in the 55-80%
range per NASA SPoRT 0-40cm relative soil moisture values (which
are well over the 95-98th percentiles), suggestive of enhanced
runoff capability and continued flooding potential. Even urban
locations of Southern California will remain with .5-.75"
potential total in short duration maintaining solid Slight Risk
level coverage for urban flooding concerns.

Downstream, early morning (18.06z-18.12z) south to north moving
thunderstorms given modest moisture and 500+ J/kg of MUCAPE through
the Lower Colorado Valley will have some potential for enhanced
rates capable of an isolated incident of flash flooding and so
little change in placement/thinking with the downstream Marginal
Risk into S NV and W AZ as well.

Gallina


~~~~Prior Discussion~~~~
A vigorous mid-level wave (initially centered near 39.9N, 125.5W
at the start of the forecast period) will migrate south-
southeastward along the California coast today while cutting off
from westerlies farther north. As this occurs, strong ascent
(associated with areas of 0.3-0.6 inch/hr rain rates in northern
California this morning) will spread southeastward along a cold
front and also within a robust upslope regime on the western sides
of the Sierra. 1-1.2 inch PW values near the coast and aforementioned
synoptic factors will foster several areas of 1-2 inch rainfall
totals, with locally higher amounts possible across the Transverse
Ranges of southern California.

The aforementioned front will result in a focused axis of heavy
rain that will translate from north to south along the southern
California coast. Across the Slight Risk area, this heavy rainfall
will occur over both sensitive locales (from recent burn scars and
debris flows) and urban/hydrophobic surfaces. The greatest
concentration of flash flood risk will likely occur across the
Transverse Ranges and adjacent areas of Los Angeles Metropolitan
area through tonight.

Outside of this area, models indicate very isolated, but slow-
moving convective activity beneath the upper low center across
northern and central California, with heavy rainfall potentially
falling over soaked ground conditions and burn scars. Isolated
flash flood potential cannot be ruled out here.

Lastly, the cutting off upper low will spread strong mid/upper
difluence across the Lower Colorado River Valley especially in the
03-12Z timeframe. Models are consistent in developing at least one
or two bands of convective structures that drift northward across
the area fairly quickly, but also result in localized training. One
or two instances of flash flooding cannot be ruled out in this
regime.

Cook

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 3
Valid 12Z Wed Nov 19 2025 - 12Z Thu Nov 20 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH CENTRAL TEXAS TO A PORTION OF THE ARKANSAS RIVER VALLEY AND
OVER THE SOUTHWEST...

20z Update:

Added a Marginal risk across portions of NM and AZ with this
update. With the closed low trending slower, scattered showers and
embedded thunderstorms are likely to continue into Wednesday
across these areas. Overall it looks like less of a focus than day
2, but nonetheless still enough of a convective signal to suggest
localized flash flooding could continue to be a concern.

Chenard

...Previous Discussion...

a mid- and upper-level trough will be making its way eastward
across portions of the southern Great Basin on Wednesday and into
early Thursday. East of that system...low level moisture will start
to be drawn northward with dewpoints in the 60s by Wednesday afternoon
which should help feed showers and thunderstorms from the afternoon
into Wednesday night/early Thursday morning. Most unstable CAPE
pushes 1000 J per kg and precipitable water values in excess of
1.5 inches becomes established from parts of south-centrsl Texas
into eastern Oklahoma and western Arkansas. Based on the
GEFS...the precipitable water axis is roughly between 2 and 2.5
standardized anomalies above climatology for this time of year.
Based on that combination...suspect locally heavy rainfall rates
are possible. There has been a tendency for the upper trough to
slow its forward speed with a similar delay in convection
initiation. With the event spanning into Thursday with broader
areal coverage and an increasing risk of excessive rainfall (see
WPC Medium Range graphics and discussions for further
details)...opted to maintain the Marginal risk on the uncertainty
of timing.

Bann


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