661
FXUS21 KWNC 171900
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT October 17 2025

SYNOPSIS: Strong low pressure at the surface and mid-levels is favored to shift
eastward from the northeastern Pacific and bring an elevated risk of hazardous
winds, heavy precipitation, and high elevation heavy snowfall across many parts
of the West Coast and Interior West throughout the forecast period. The
persistence of above-normal and potentially heavy precipitation may lead to
flooding across parts of the Northwest.  Tied to the evolution of the mid-level
troughing, surface low development is favored in the lee of Rockies, increasing
chances for heavy precipitation across the central contiguous U.S. (CONUS)
during the middle of the period. Additional mid-level low pressure favored over
the Great Lakes late next week looks to promote enhanced northwesterly flow and
cooler minimum temperatures where frost and/or first autumn freezes may
adversely impact vulnerable vegetation east of the Mississippi.

HAZARDS

High risk of heavy precipitation for parts of northern and central California
and southern Oregon, Sat-Sun, Oct 25-26.

High risk of high elevation heavy snow over the Sierra Nevada and Klamath
mountains, Sat-Sun, Oct 25-26.

Moderate risk of heavy precipitation from the Bay Area of California northward
to southwestern Oregon, Sat-Mon, Oct 25-27.

Moderate risk of high elevation heavy snow over portions of the Sierra Nevada,
Klamath, and southern Cascade Mountains, Sat-Mon, Oct 25-27.

Moderate risk of heavy precipitation over the Pacific Northwest, Sat-Wed, Oct
25-29.

Moderate risk of high elevation heavy snow over the Cascades, Sat-Wed, Oct
25-29.

Moderate risk of high winds from the Bay Area of California northward to the
Pacific Northwest, Sat-Wed, Oct 25-29.

Moderate risk of high elevation heavy snow for portions of the northern Rockies
and northern Intermountain, Sat-Mon, Oct 25-27.

Moderate risk of high winds over parts of the Great Basin, Rockies, Southwest,
and the Great Plains, Sat-Tue, Oct 25-28.

Slight risk of heavy precipitation for portions of California and the Pacific
Northwest, Sat-Fri, Oct 25-31.

Slight risk of heavy snow for the Sierra Nevada, Klamath, and Cascade
Mountains, Sat-Fri, Oct 25-31.

Slight risk of heavy snow for portions of Northern Intermountain, Northern and
Central Rockies, Sat-Wed, Oct 25-29.

Slight risk of high winds for much of the western half of the CONUS, Sat-Fri,
Oct 25-31.

Slight risk of much below normal temperatures for portions of the Great Lakes,
Ohio and Tennessee Valleys, Appalachians, and Mid-Atlantic, Sat, Oct 25.

Slight risk of heavy precipitation for portions of the Plains and the
Mississippi, Tennessee and Ohio Valleys, Mon-Wed, Oct 27-29.

Slight risk of high winds for the southern Mainland and southeast Alaska,
Sat-Mon, Oct 25-27.

Flooding possible across parts of the Pacific Northwest and northern
California.

DETAILED SUMMARY

FOR MONDAY OCTOBER 20 - FRIDAY OCTOBER 24:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY OCTOBER 25 - FRIDAY OCTOBER 31: The week-2 hazards perspective
remains on track today with much of the outlook focusing on potential
Atmospheric River (AR) activity producing multiple weather related hazards of
elevated risk throughout the western CONUS. Early in week-2, good agreement
exists among the dynamical model ensemble mean 500-hPa height forecasts
featuring a potent 500-hPa trough encroaching the West Coast, with a 850-hPa
flow pattern favorable for ample incoming Pacific moisture to supply locally
heavy precipitation. Some differences between the 0z GEFS and 0z ECMWF
ensembles are in the details of the upper-level features, with the former
favoring stronger and deeper negatively tilted trough axis, along with a
stronger jet protruding into California. As a result, there is the potential
for more AR activity further south in California in the ECMWF compared to the
GEFS, however both models continue to well agree in regards to the timing of
this potential event with the highest AR related risks occurring on days 8 and
9 (Oct 25-26). Thereafter, these risks are anticipated to retreat northward
into the Pacific Northwest and ease by days 10 and 11 over California as the
trough axis shifts further inland and mid-level heights begin to rise over the
southwestern CONUS. During this time, however, ensembles continue to point to
additional troughing offshore in the eastern Pacific, which looks to maintain
above-normal precipitation for the northwestern CONUS and potentially bring a
renewed threat of heavy precipitation later in October.



Based on Integrated Vapor Transport (IVT) tools, as well as the latest raw and
calibrated precipitation guidance, high risks for heavy precipitation and high
elevation heavy snow remain issued for parts of northern and central California
and southern Oregon for Oct 25-26. Within these highlighted areas, both the
GEFS and ECMWF Probabilistic Extreme Tools (PETs) indicate at least a 50%
chance of 3-day amounts exceeding the 85th climatological percentile, with the
uncalibrated ECMWF favoring similar chances for amounts exceeding an inch (two
inches) on day 8 (days 8-10). In deference to the ECMWF and model trends, these
high risk areas are expanded in coverage southward to approximately the Bay
Area of central California. Across the Pacific Northwest, there are comparably
lower probabilities in percentile space in the PETs, though raw tools show
30-40% for 3-day amounts exceeding two inches persisting through the middle of
week-2, which is also consistent with IVT guidance. To better account for the
evolution of the heavy precipitation risk across the West Coast, moderate risk
areas of heavy precipitation and high elevation heavy snow remain issued for
portions of California and Oregon for Oct 25-27, with a separate pair of
moderate risk areas for heavy precipitation and high elevation heavy snow
focused over the Pacific Northwest, both valid through Oct 29. Although soils
are not currently oversaturated over parts of the West Coast, a flooding
possible hazard remains posted as this area may be more responsive to flooding
given the elevated precipitation risks, and with increased precipitation
amounts favored during week-1 to help prime conditions. At the base of the
amplified troughing, a moderate risk for high winds also remains issued (Oct
25-29) where PETs depict higher chances (30-40%) for wind speeds exceeding the
85th percentile through the middle of week-2. Broader slight risk areas for
heavy precipitation and heavy snow are still posted for the West Coast, and
remain valid for the entirety of week-2 due to the potential for anomalous
troughing reloading later in October.



With much of the anomalous mid-level troughing shifting eastward next weekend,
enhanced Pacific moisture being advected over high elevation areas of the
Interior West supports the potential for heavy snowfall over portions of the
Rockies and the Northern Intermountain region. Based on raw snow tools and the
Snow Water Equivalent (SWE) PET, a moderate risk of heavy snow remains issued,
and is now valid through Oct 27, with a broader slight risk area extending
further southward into the Four Corners, valid through Oct 29.  A separate
moderate risk area for high winds is also posted (Oct 25-28) over the higher
elevations of the Interior West, with its coverage also expanded southward into
the Southwest and Southern Rockies in the updated outlook where PETs and raw
tools favor higher signals for wind speeds exceeding the 85th percentile with
gusts of over 34 knots. A broad slight risk area of high winds is posted for
nearly the western half of the CONUS to capture the wind potential associated
with the amplified mid-level pattern favored during week-2.



Early in week-2, anomalous troughing favored over the Great Lakes looks to
bring more northwesterly flow and a round of anomalously cold temperatures into
portions of the eastern U.S. While both GEFS and ECMWF PETs are weak in
percentile space (less than 20% chances for minimum temperatures falling below
the 15th percentile) associated with this cooler air, there continues to be
support for sub 40 degree F minimum temperatures as far south as the Tennessee
Valley. Because many areas across the eastern CONUS have not yet experienced
their first autumn freezes and/or frost, infringing cold temperatures could
impact vulnerable vegetation, and a slight risk of much below normal
temperatures is highlighted for Oct 25 before temperatures are expected to
moderate later in the period.



Towards the middle of week-2, models are showing stronger signals for shortwave
energy being pinched off over the Great Plains associated with the building
heights over the southwestern CONUS, leading to surface low development in the
lee of the Rockies. The ECMWF is more pronounced with this realization, and
given the mean surface high pressure (associated with the aforementioned cold
air early in week-2) that is favored to the east to bring moist return flow
from the Gulf, the precipitation response is becoming increasingly reflected in
the raw and calibrated tools. Therefore, a slight risk of heavy precipitation
is posted over portions of the eastern Plains, and Mississippi, Tennessee, and
Ohio Valleys for Oct 27-29.



Over Alaska, a slight risk of high winds is posted (Oct 25-27) with its
coverage remaining focused over the southern Mainland and the Southeast. While
much of the stronger mid-level troughing has shifted southward to affect the
western CONUS, PETs still maintain increased signals for wind speeds exceeding
the 85th percentile over the region.

FORECASTER: Nick Novella

$$