Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Mon Oct 13 2025
Day 1
Valid 01Z Tue Oct 14 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...
...California...
As a strong mid-level closed low (-4 standardized height anomaly
at 500 mb) drops southward just off the coast of central California
tonight, an occluded/cold front will follow suit, reaching southern
California's Transverse Ranges between 06-12Z. An anomalous
moisture plume, with PWAT values between 0.9 and 1.2 inches, will
focus along the coast ahead of the cold front and be directed
inland via 20-35 kt of wind at 850 mb.
An inland flash flood threat will remain where weak instability
(generally less than 500 J/kg CAPE) will align with 20-30 kt of
cyclonically curved low level flow to the east of the mid-level low
to support localized hourly rainfall of 0.5 to 1.0 inches.
Along the coast, and especially the Transverse Ranges later
tonight, low level flow of 20-30 kt becomes aligned perpendicular
to the terrain, increasing rainfall via orographic influences.
Recent RAP forecasts have shown pockets of MLCAPE in the 250-750
J/kg range just offshore with inland values up to a few hundred
J/kg which will combine with left-exit region jet divergence to
allow for higher intensity rainfall potential. A frontal
convergence axis of showers and thunderstorms appears likely to
form and steadily translate southward with the cold front. While
the front will be progressive, brief training with the front along
with pre-frontal activity could support localized 2 to 3 inch
rainfall totals through 12Z for the Transverse Ranges.
The main flash flood concern across California for the overnight
period will be urban and burn scar overlap of heavy rain, with the
threat carrying over into the Day 2 ERO period beginning 12Z
Tuesday.
...Southwest to the Central Rockies...
South-central Arizona remained convectively active as of 00Z with
a bubble of 500-1500 J/kg MLCAPE remaining with little to no
inhibition. Strong right entrance region upper level jet divergence
was also present across a good portion of Arizona, helping with
ascent. PWAT standardized anomalies of +2 to +4 above the mean
remained over the eastern half of Arizona into Utah, Colorado and
New Mexico as of 00Z tonight, but drier air will be pushing into
Utah and Arizona through 12Z from the west out ahead of a lead
impulse located ahead of a large closed low along the coast of
California. 25 to 45 kt of 700 mb flow and unidirectional SSW flow
will be supportive of repeating and training rounds of heavy rain
with the best potential for higher rates remaining co-located with
remaining instability pockets. A second area of instability is
expected to support locally higher rain rates later tonight from
near El Paso into portions of southern/central New Mexico, although
this region's flash flood concern is a bit lower than locations to
the west due to drier antecedent conditions and weaker forcing for
ascent.
Very wet antecedent conditions exist across southwestern Colorado/San
Juan Mountains and portions of central Arizona due to 3 to 4 day
rainfall totals of 2 to 6 inches. Additional totals of 2-3 inches
may occur in localized portions of the San Juan Mountains due to
upslope enhancement with 1-2 inch maxima forecast elsewhere.
The main change for the 01Z update include the removal of the
Slight Risk for southeastern Arizona into far southwestern New
Mexico where a lack of instability and better forcing is expected
to suppress convective coverage in the overnight. A portion of the
Marginal Risk was also removed from the northern California coast
where the heavy rainfall threat has ended with the movement of the
closed low.
Otto
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 AM EDT Tue Oct 14 2025
Day 1
Valid 12Z Tue Oct 14 2025 - 12Z Wed Oct 15 2025
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA AND NEW MEXICO...
...Central to Southern California...
Current UA analysis and WV satellite imagery pins the powerful ULL
off the northern CA coast, situated just west of San Francisco Bay.
The forecast trajectory will place the low center off the the
central CA coast by tomorrow morning with a reprieve in lost
latitude as the low center will mostly come to a stop and begin a
slow pivot inland. The proxy of the ULL will create a strong
south-southwest fetch off the Pacific around the base of the ULL
creating a prolonged period of orthogonal upslope flow in-of the
Transverse and Peninsular Ranges of southern CA. The threat will be
ongoing at the very beginning of the forecast cycle as the effects
of heavy rainfall will bleed from the current D1 into this new D1
time period. Consensus in pretty much all guidance maintains a high
confidence posture of locally significant rainfall across those
coastal terrain areas, especially over the eastern Transverse and
northern Peninsular Ranges to the north and northeast of Los
Angeles. The predominant flow is textbook for upslope enhancement
within the confines of those areas which historically can lead to
scattered bouts of flash flooding and localized debris flows. PWATs
will hover around 2 deviations above normal with a small axis of
MUCAPE located right near the coast to just inland over the
terrain. Despite the signal reflected being between 200-500 J/kg of
CAPE, this is likely sufficient to push rates over the terrain to
~1"/hr with perhaps a few stronger cores exhibiting higher rates
for short periods of time.
Considering the multitude of burn scar remnants and just complex
terrain funneling over the Transverse and Peninsular Ranges, this
setup is more than conducive for scattered flash flood concerns, at
a minimum. 00z HREF neighborhood probs for >3" of total precip in
the D1 is between 60-90% over that areas in the eastern Transverse
and northern Peninsular Ranges, overlapping some of the more prone
burn scar areas that remain over these locations. The good news in
the setup is the progression of the heavy rainfall will not be an
all day affair, but still will cover the period between 12-21z
Tuesday with some rainfall encountered just prior to the new D1.
This allowed for a general maintenance of the SLGT risk in place
over the region with a minor adjustment to expand on the southern
flank into the Peninsular Range as models hint at a secondary
maxima situated just northeast of San Diego within the terrain
aligned over western Riverside county.
...New Mexico and West Texas...
Persistence in the general longwave pattern closer to the divide
will create yet another active period for scattered to widespread
convective impacts during the day on Tuesday. As of this time,
elevated moisture on the order of 2-3 deviations above normal,
according to the latest NAEFS signal an environment conducive for
locally heavy rainfall, especially in-of terrain elements that can
provide local enhancement given the persistence of southern flow
overhead. A stout mid-level ridge sits over the Southern Plains
allowing for a channel on the western flank of the ridge to
coincide with the prevailing easterly flank of the broad upper
trough centered over the Western U.S. The latest trend within
guidance is to advect a relatively stout mid-level shortwave out of
Chihuahua late-Tuesday afternoon and evening with a steady south to
north progression as we move through the second half of the
forecast period. The signal is reflected in both the 500mb
vorticity panels, and is defined in the 700mb wind field leading to
higher confidence in the potential of this shortwave advection
scheme over the region.
Recent HREF blended mean QPF has jumped from run to run with a
relative max centered over the Sangre de Cristos of northern NM
with some smaller maxima positioned over the Sacramento Mtns. These
two areas are still fairly sensitive considering multiple burn
scar remnants, including a few very sensitive grounds based within
both terrain features. Totals of 1-2" with locally higher are
forecast within individual deterministic leading to an uptick in
neighborhood probs for >2" now upwards of 60-80% in the
Sacramento's and 35-50% over the Sangre de Cristos. This is more
than enough to spur issues over these two specific areas leading to
an upgrade to a SLGT risk now centered over south-central to
north-central NM. The threat is a little less daunting over areas
outside the terrain, but there could still be issues outside the
higher elevations due to drainage and heavier cells drifting off
the terrain into the adjacent valleys. This is the reasoning for
maintaining the SLGT over the valleys between the two ranges.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 AM EDT Tue Oct 14 2025
Day 2
Valid 12Z Wed Oct 15 2025 - 12Z Thu Oct 16 2025
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS
OF THE CENTRAL AND SOUTHERN ROCKIES...
...Rockies...
The evolution of the longwave pattern across the Western CONUS will
lead to the development of a surface low over the Inter-Mountain
West with a trailing cold front anticipated to move west to east
over the Four Corners into the Continental Divide. The combination
of continued ascent within the RER of a vacating jet streak to the
north and the frontal approach will lead to one last instance of
organized precip within the San Juans down into the Sangre de
Cristos. Considering the nature of the pattern bookending quite an
active stretch within the aforementioned areas, the threat for
flash flooding will lie within the MRGL risk threshold despite the
lack of stronger instability that tends to necessitate a greater
flash flood potential. Antecedent conditions will trend very wet
overall leading to lower FFG's across the terrain of northern NM
into southwestern CO. Totals within guidance are currently running
between 0.5-1" with some locally higher amounts based in the
individual deterministic. The greatest threat for higher totals
lies within the San Juans, an area that has been the benefactor of
significant rainfall totals the past 4 days. This will likely be
the greatest area of focus for the setup, albeit a threat that will
be closer to a 6-8 hr. window as the front approaches. The previous
MRGL risk was generally maintained, but trimmed up the southern
edge as model trends point to limited potential for heavier
rainfall south of the Sangre de Cristos in northern NM.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 AM EDT Tue Oct 14 2025
Day 3
Valid 12Z Thu Oct 16 2025 - 12Z Fri Oct 17 2025
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
334 AM EDT Tue Oct 14 2025
Lingering upper troughing in Atlantic Canada (tied to the current
coastal storm) still supports continued unsettled/windy conditions
over New England through late week with cooler than normal
temperatures to start that will slowly moderate.
The troughing and associated surface frontal boundary in the West
will promote cool and showery (with some snow at higher elevations)
conditions over the Northern/Central Rockies to start, with
broader light rain in advance of the surface low that is poised to
track from the Northern Plains into southern Canada. As the
attendant cold front passes through the Plains, moisture will
increase in advance of it through the Mid-MS Valley where rainfall
could increase by Saturday. The WPC Day 5/Saturday Excessive
Rainfall Outlook (ERO) introduces a Marginal Threat area centered
over the Mid-South given favorable upper support and return flow.
This rainfall will continue eastward next Sunday and Monday/Tuesday
into the Northeast near the track of deepening/consolidating low
pressure (and into the Southeast ahead of the cold front).
Temperatures will be mild ahead of the cold front with values well
into the 70s/80s Friday from the Corn Belt to the Ozarks with 90s
across South Texas.
By late next weekend into early next week, another deepened system
will enter the Pacific Northwest/West Coast with another round of
valley rain and higher mountain snow that will then dig and spread
across the West and downstream to the Rockies. The Day5/Saturday
ERO introduces a coastal Washington Marginal threat area given a
favorable guidance signal.
Schichtel
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
334 AM EDT Tue Oct 14 2025
Lingering upper troughing in Atlantic Canada (tied to the current
coastal storm) still supports continued unsettled/windy conditions
over New England through late week with cooler than normal
temperatures to start that will slowly moderate.
The troughing and associated surface frontal boundary in the West
will promote cool and showery (with some snow at higher elevations)
conditions over the Northern/Central Rockies to start, with
broader light rain in advance of the surface low that is poised to
track from the Northern Plains into southern Canada. As the
attendant cold front passes through the Plains, moisture will
increase in advance of it through the Mid-MS Valley where rainfall
could increase by Saturday. The WPC Day 5/Saturday Excessive
Rainfall Outlook (ERO) introduces a Marginal Threat area centered
over the Mid-South given favorable upper support and return flow.
This rainfall will continue eastward next Sunday and Monday/Tuesday
into the Northeast near the track of deepening/consolidating low
pressure (and into the Southeast ahead of the cold front).
Temperatures will be mild ahead of the cold front with values well
into the 70s/80s Friday from the Corn Belt to the Ozarks with 90s
across South Texas.
By late next weekend into early next week, another deepened system
will enter the Pacific Northwest/West Coast with another round of
valley rain and higher mountain snow that will then dig and spread
across the West and downstream to the Rockies. The Day5/Saturday
ERO introduces a coastal Washington Marginal threat area given a
favorable guidance signal.
Schichtel
