Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
752 PM EST Sat Nov 15 2025
Day 1
Valid 01Z Sun Nov 16 2025 - 12Z Sun Nov 16 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...
...16Z update...
No significant changes were made to the outlook given 18Z HREF
guidance, including exceedance probabilities, which were fairly
similar to the 12Z ensemble suite.
Hurley
...previous discussion follows...
A large and energetic upper level low centered off the coast of
southern California will approach and make its way inland during
this period. As the low approaches this morning, it will draw a
very moisture-rich moisture plume into southern California, due to
its connection with the tropics. This will be in the form of an
atmospheric river. While it has been raining in light to moderate
intensity over much of the overnight period, when this atmospheric
river approaches, it will cause a notable increase in rainfall
intensity. This is due not only to the moisture content of the
atmosphere increasing to nearly 1.5 inches PWAT, but also the
advection of instability. 400 J/kg of MUCAPE is expected at the
peak of the event late this morning. This will support scattered
elevated convection. Thunderstorms with maximum rainfall rates to 1
inch per hour are likely across much of coastal southern
California today.
Rainfall totals yesterday broadly ranged from between 1/2 and 1
inch across the L.A. basin to 1.5 to 2 inches from Santa Barbara
west to Point Conception. Today with multiple rounds of showers and
thunderstorms capable of heavy rain, expect those amounts to at
least double from San Diego north to Point Conception. As usual in
these strong southerly flow setups, the heaviest rains will be in
the Transverse Ranges north of L.A., with likely runoff impacting
the adjacent urban areas. Given the area's sensitivity to heavy
rain, the Moderate Risk was largely left the same with this update,
albeit with a small southward expansion over all of Orange County
and far northern San Diego County. Otherwise there were no major
changes as the event remains on track with good agreement in the
guidance.
Around midday, the back edge of the heaviest rains will move
across the coast, reducing further rainfall to scattered off-and-on
showers for much of the rest of the forecast period. The vigorous
shortwave forcing the heaviest rain will push north into the
interior of the Southwest, ending the heavy rain threat at the
coast. Thus, the heavy rain threat will continue further inland
across the southeastern California deserts, as well as into
southern Nevada and western Arizona. The greatest forcing inland
will be across the southern Sierra Nevada from Bakersfield north
and east to southwestern Nevada. It's in this region that the
greatest threat for inland flooding will be present, and where a
higher end Slight Risk remains in effect. Given it's November, the
higher elevations of the southern Sierra Nevada will see multiple
feet of snow from this event.
For Las Vegas, expect light rain to impact the city and points west
for much of the morning, but the heaviest rains will move through
from midday into the evening. Being further inland with a bit less
moisture to work with, a Slight Risk remains in effect for that
area with few changes to the overall forecast.
Finally, the upper low itself will move ashore in the late
afternoon, then track northeast to near the Lake Tahoe region by
12Z. Wraparound rainfall, while much lighter, will persist from the
Bay Area south to Santa Barbara, which could worsen any ongoing
flooding occurring there late tonight.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
805 PM EST Sun Nov 16 2025
Day 1
Valid 01Z Mon Nov 17 2025 - 12Z Mon Nov 17 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SANTA LUCIA
RANGE OF CENTRAL CALIFORNIA...
...16Z update...
Upper level flow across central-northern CA is becoming more
difluent this evening as the amplifying mid-upper level trough
pivots towards the coast. Deepening QG forcing and moistening
within the renewed Warm Conveyor Belt (WCB) is underway across the
Marginal Risk area, which was trimmed a bit for the 01Z Day 1 ERO
update based on the 12-18Z guidance and most recent HREF and RRFS
probabilistic suite. Within the Marginal Risk area, the 18Z HREF
indicates pockets of 50-70% probabilities of hourly rainfall rates
exceeding 0.50", overlapping the same areas in consecutive hours
around the Bay Area into the northern Central Valley, which would
renew a localized/isolated flash flood threat across urban areas
and/or burn scars.
Hurley
...Previous discussion...
A pair of deep shortwaves will impact much of California through
the period today. The one that has been causing heavy rain across
much of southern California is pressing inland this morning. Rates
have come down a bit overnight due to waning instability, but much
of central California and portions of southern California remain in
more-or-less steady light to occasionally moderate rainfall. As
the upper low departs to the east, the steady rain across much of
inland California should diminish through this afternoon. Following
this first shortwave, another vigorous, deepening longwave trough
will track SSE out of the North Pacific and approach the northern
California coast late tonight/early Monday morning. This second
shortwave will have much more upper level energy with it by tonight
than the first does now. This longwave trough will also have some
instability with it. The result will be steady rain associated with
the approaching low plowing into the northern California coast
this evening, then spreading south in the form of a line of
moderate rain with embedded thunderstorms capable of brief periods
of heavy rain. It will cross the Bay area late this evening, then
push south almost to Point Conception by 12Z Mon. The heaviest rain
will push inland, reaching the central Sierra around the same
time.
Much of the Santa Lucia Range in central California have picked up
between 5 and 8 inches of rain over the last week. Continued light
to moderate rain may add another inch through this morning, but the
heavy rains tonight with the cold front will likely drop another
1-2 inches. Rock and landslides have been reported from today's
rain. Thus with shifting winds and heavy rain tonight, it's
probable there may be additional impacts from the storms. In
coordination with MTR/Monterey, CA forecast office, a Slight Risk
upgrade was introduced with this morning's update. Elsewhere the
surrounding Marginal remains unchanged.
...Southwest Utah...
The Marginal Risk area in southwest Utah also remains unchanged.
Upslope flow into the Pine Valley Mountains may cause isolated
instances of flash flooding as the area of rain currently over
southern Nevada pushes east into the region, where enhanced upslope
will cause the rain to intensify. The highest elevations above
8,000 ft or so may see snow, which will limit the flooding
potential at lower elevations.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
805 PM EST Sun Nov 16 2025
Day 2
Valid 12Z Mon Nov 17 2025 - 12Z Tue Nov 18 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOS
ANGELES BASIN OF SOUTHERN CALIFORNIA...
...20Z update...
Some modifications were made to the outlook areas in CA.
Specifically, extended the Slight Risk slightly westward to the
Central CA Coast based on the latest (12Z) CAM guidance as well as
HREF and RRFS 0.50"+/hr and 1.00"+/hr rainfall rate probabilities
on Monday. Meanwhile, also largely based on those HREF and RRFS
exceedance probabilities, have also hoisted a Marginal Risk across
southwest and central portions of AZ.
Hurley
...Previous discussion...
The deep and energetic trough that will be centered off the
northern California coast at the start of the period will track
southeast down the coast to over the San Joaquin Valley by early
Tuesday morning. The trough will cut off into a second upper level
low to impact the state by Monday afternoon. The heaviest rains
will be out ahead of this cutoff low, where the overall flow is
onshore out of the southwest. A line of heavy showers with embedded
thunderstorms will move down the coast starting around Point
Conception early Monday morning, reaching the L.A. Basin around
midday. It will take between 2-4 hours for the heavy rain band to
sweep through. During that time, rainfall rates peaking between a
half and 3/4 inch per hour in the thunderstorms may cause widely
scattered instances of flash flooding, especially on the south
facing slopes of the Transverse Ranges down to the L.A. Basin. For
this reason, the inherited Slight Risk remains in place with very
few changes. At least some of the heavy rain will also penetrate
inland into some of the California Deserts, including around Death
Valley N.P. Given how recently the area saw heavy rains from
yesterday, this additional heavy rain may also cause isolated
instances of flash flooding. Thus, the Marginal Risk was expanded
north and east to include a small portion of southern Nevada.
Wegman
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
805 PM EST Sun Nov 16 2025
Day 3
Valid 12Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND LOWER GREAT BASIN...
...20Z update...
Based on the guidance trends, have expanded the Marginal Risk area
to include more areas off the Mogollon Rim and into the deserts in
southern and western AZ, southeast CA, and southeast NV. PW values
get near 2 standard deviations above normal in southern-western AZ
(0.75-1.00"), while MUCAPEs climb between 500-750 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 12Z RRFS and RGEM hourly and
3-hourly QPFs. 24hr guidance QPFs have come up from 12Z Tue-12Z
Wed, especially from the global models, which is a bit concerning,
yet understandable considering how the upper trough begins to pivot
more slowly as it amplifies from the shortwave energy diving
southward on its backside. At this point there's still not enough
confidence from a duration perspective to upgrade to a Slight Risk
over the lower terrain (all rain) areas south of the Mogollon Rim,
which would include the Phoenix area, but the potential to do so
will exist over the next day or so.
Hurley
...Previous discussion...
A Marginal Risk area across portions of Arizona and southern Utah
was introduced with this update. The cutoff low that will spread
heavy rain across the L.A. Basin on Monday will slowly push east
across southern California through Tuesday. Once again the heaviest
rains associated therewith will be out ahead/east of the low
center, putting Arizona and Utah in the area most likely to see
heavy rain. Instability will once again be non-zero, peaking
between 200 and 400 J/kg, which combined with above normal moisture
will result in an area of training thunderstorms, likely along the
Mogollon north and east of Phoenix. The southerly flow will
continue across northwest Arizona and into southern Utah, where wet
antecedent conditions from today's rains will support additional
flash flooding developing with heavy rains in the Pine Valley
Mountains east to Lake Powell.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
235 PM EST Sun Nov 16 2025
Ample upper trough energy and organized precipitation set to dig
through the West Coast within an emerging southern stream flow over
the next few days will spread across the Great basin/Southwest
midweek to include some enhanced mountain snows. Mid-later week
downstream system progression and lead moisture and instability
return flow genesis into wavy fronts over the south-central to
eastern U.S. and meso-scale features should lead to emergence of a
growing area of moderate to heavy rains and strong thunderstorms
over the south-central U.S., along with eastward progression later
week into the weekend. Accordingly, there is a WPC Day 4/Wednesday
Excessive Rainfall Outlook Marginal Risk area for the south-
central U.S. and a Day 5/Thursday Slight Risk area there given
cumulative heavy convective/repeat rain potential.
Another digging east Pacific system may offer some enhanced
precipitation chances to the West Coast states Thursday/Friday
along with closed low potential offshore and to the Southwest next
weekend along with uncertain organized precipitation to monitor.
Pre-frontal temperatures will be much warmer than average for much
of the south-central/southern U.S. into this period to include
some record temperatures upwards to 10-20 degrees above normal.
This translates to highs commonly into the 80s, with some warming
shifting to the East late week/weekend. Meanwhile, rounds of
troughing in California to the West/Southwest will lead to below
average highs this week about 5-10+ degrees below climatology.
Putnam/Schichtel
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
235 PM EST Sun Nov 16 2025
Ample upper trough energy and organized precipitation set to dig
through the West Coast within an emerging southern stream flow over
the next few days will spread across the Great basin/Southwest
midweek to include some enhanced mountain snows. Mid-later week
downstream system progression and lead moisture and instability
return flow genesis into wavy fronts over the south-central to
eastern U.S. and meso-scale features should lead to emergence of a
growing area of moderate to heavy rains and strong thunderstorms
over the south-central U.S., along with eastward progression later
week into the weekend. Accordingly, there is a WPC Day 4/Wednesday
Excessive Rainfall Outlook Marginal Risk area for the south-
central U.S. and a Day 5/Thursday Slight Risk area there given
cumulative heavy convective/repeat rain potential.
Another digging east Pacific system may offer some enhanced
precipitation chances to the West Coast states Thursday/Friday
along with closed low potential offshore and to the Southwest next
weekend along with uncertain organized precipitation to monitor.
Pre-frontal temperatures will be much warmer than average for much
of the south-central/southern U.S. into this period to include
some record temperatures upwards to 10-20 degrees above normal.
This translates to highs commonly into the 80s, with some warming
shifting to the East late week/weekend. Meanwhile, rounds of
troughing in California to the West/Southwest will lead to below
average highs this week about 5-10+ degrees below climatology.
Putnam/Schichtel
