441
FXUS21 KWNC 271852
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT October 27 2026

SYNOPSIS: An area of mid-level low pressure approaching the West Coast is
predicted to bring periods of heavy precipitation, high elevation snow, and
high winds across parts of Northern California through the Pacific Northwest.
Another area of low pressure is forecast to move across the Eastern Contiguous
U.S. (CONUS) early in week-2, bringing increased chances for heavy
precipitation and high winds to much of this region.

HAZARDS

Moderate risk of heavy precipitation for portions of southwestern Oregon and
northwestern California, Fri-Sun, Nov 6-8.

Moderate risk of heavy snow for portions of the northern Sierra Nevada and
Klamath Ranges, Fri-Sun, Nov 6-8.

Slight risk of heavy precipitation for portions of the Pacific Northwest and
Northern California, Fri-Tue, Nov 6-10.

Slight risk of heavy snow for the Cascades, Klamath and Sierra Nevada ranges,
Fri-Tue, Nov 6-10.

Slight risk of high winds for coastal portions of the Pacific Northwest, and
the northern half of the California coast, Fri-Tue, Nov 6-10.

Slight risk of heavy precipitation for most of the East Coast states, Tennessee
Valley, and the eastern Gulf Coast region, Wed-Thu, Nov 4-5.

Slight risk of high winds for most of the Great Lakes region, Ohio Valley,
central and northern Appalachians, Northeast, and Upper Mid-Atlantic, Wed-Thu,
Nov 4-5.

DETAILED SUMMARY

FOR FRIDAY OCTOBER 30 - TUESDAY NOVEMBER 03:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY NOVEMBER 04 - TUESDAY NOVEMBER 10: A mid-level trough is forecast
to progress from the Bering Sea into the Gulf of Alaska during the first half
of week-2. Negative height departures in the ensemble mean runs surpass 180
meters for most of the week-2 period. A corresponding area of surface low
pressure is predicted beneath the mid-level trough. This sets up the potential
for a long duration atmospheric river event for the Pacific Northwest and
Northern California.



Probabilistic Extremes Tools (PETs) from the ensemble means indicate a greater
than 20-30% chance of 3-day precipitation amounts exceeding the 85th
climatological percentile over Northern California and southwestern Oregon by
the middle of week-2. Uncalibrated ensemble guidance from the GEFS and ECENS is
forecasting a greater than 50% chance of 3-day amounts exceeding 1 inch across
this region and extending northwards through the Pacific Northwest. The GEFS
also forecasts a greater than 50% chance of 3-day amounts exceeding 2 inches
over portions of Northern California and southwestern Oregon. The ECENS is
significantly more subdued in terms of both predicted amounts and spatial
coverage of heavy precipitation. The Integrated Water Vapor Transport (IVT)
tools also highlight strong probabilities of IVT exceeding 250 kg/m-s, the
preferred value for determining an impactful atmospheric river event.



Therefore, a moderate risk of heavy precipitation is posted for Nov 6-8 for
portions of northwestern California and southwestern Oregon where localized
flooding is possible. A corresponding moderate risk of heavy snow is posted for
portions of the northern Sierra Nevada range, valid for the same period. A
slight risk of heavy precipitation and a slight risk of high winds are posted
for coastal portions of the Pacific Northwest and northern California as far
south as Monterey Bay, Nov 6-10. A slight risk of heavy snow is posted for the
Cascades, Klamath and Sierra Nevadas, Nov 6-10.



A surface low pressure system is forecast to move across the Eastern CONUS on
Nov 4-5. A slight chance of heavy precipitation is posted in advance of the
associated cold front, which includes most of the East Coast states, Tennessee
Valley, and the eastern Gulf Coast region. The ECENS and GEFS PETs both support
this slight risk area where precipitation is expected to exceed the 85th
percentile and 3-day amounts are forecast to exceed 1-inch. The main difference
between these two model ensembles is the ECENS extends the heavy precipitation
risk farther southwestward into the Lower Mississippi Valley, compared to the
GEFS which extends the risk only to the eastern Gulf Coast region. A slight
risk of high winds is delineated across most of the Great Lakes region, Ohio
Valley, central and northern Appalachians, Northeast coastal plain,  and the
Upper Mid-Atlantic, Nov 4-5. This is broadly associated with the mid-level
dynamics and area of surface low pressure associated with this storm system and
the expected areas of enhanced pressure gradients.

FORECASTER: Anthony Artusa

$$