590
FXUS21 KWNC 041901
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 04 2026

SYNOPSIS: Dynamical models continue to advertise highly amplified mid-level low
pressure encroaching the West Coast during week-2, promoting periods of
enhanced onshore flow consistent with atmospheric river activity. Although
models vary in regards to the timing and coverage of activity, there continues
to be elevated chances for hazardous impacts including heavy precipitation,
high winds, and high elevation heavy snowfall, with possible flooding over the
Pacific Northwest and northern California during the period. Meanwhile, an area
of departing mid-level low pressure may bring periods of high winds over
portions of the northeastern U.S. early in the period. Surface low pressure
development is also possible in the lee of the Rockies, resulting in enhanced,
and possibly heavy precipitation over many parts of the Great Plains,
Mississippi, Tennessee and Ohio Valleys towards the middle of the period.

HAZARDS

Moderate risk of heavy precipitation for parts of the Pacific Northwest, and
northern and central California, Thu-Sun, Nov 12-15.

Moderate risk of heavy high elevation snow across the Klamath and the Sierra
Nevada Mountains, Thu-Sun, Nov 12-15.

Moderate risk of high winds across coastal portions of Washington, Oregon, and
much of California, Thu-Sun, Nov 12-15.

Slight risk of heavy precipitation across the West Coast and into parts of the
Desert Southwest, Great Basin, and Southern Rockies, Thu-Wed, Nov 12-18.

Slight risk of heavy high elevation snow across the Cascades, Klamath, and
Sierra Nevada Mountains, Thu-Wed, Nov 12-18.

Slight risk of heavy high elevation snow for parts of the Northern
Intermountain, Great Basin, and Central and Northern Rockies, Thu-Wed, Nov
12-18.

Slight risk of high winds for much of the West Coast and Interior West,
Thu-Wed, Nov 12-18.

Slight risk of high winds for parts of Northern and Central Appalachians, Lower
Great Lakes, Upper Mid-Atlantic, and the Northeast, Thu, Nov 12.

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

DETAILED SUMMARY

FOR SATURDAY NOVEMBER 07 - WEDNESDAY NOVEMBER 11:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY NOVEMBER 12 - WEDNESDAY NOVEMBER 18: The hazards perspective
mostly remains on track since yesterday. Highly amplified 500-hPa mean
troughing is favored to encroach the West Coast next week consistent with
Atmospheric River (AR) activity where there are elevated chances for periods of
heavy precipitation, high winds and high elevation heavy snowfall for portions
of the Pacific Northwest and California.  Based on the evolution of the 500-hPa
pattern and Integrated Vapor Transport (IVT) tools, there is a better
identification of at least two periods of enhanced moisture intrusions and
resultant increases in precipitation, with one beginning late in week-1, and
another occurring during the early portion of week-2. However, there continues
to be variations among the models in regards to the timing, strength, and
spatial extent of these intrusions, which is again contributing to some
uncertainty in the updated outlook. Regardless, ensembles continue to paint an
overall rather wet picture over the West Coast, where the enhanced onshore flow
also looks to maintain an increased risk of additional weather related hazards
further east with anomalous moisture overspreading much of the Interior West
during the period. The favored persistence of strong troughing for the entirety
of week-2, also continues to support the potential for AR activity being of
longer duration through the middle of November.



Early in week-2, the GEFS and ECMWF ensemble differ in regards to the shape and
strength of the anomalous mean troughing, resulting in different daily
precipitation reflections along the West Coast. With the GEFS featuring an
extension of the trough axis into southern California on day 8 (Nov 12), there
is a pretty far southerly stretch of enhanced precipitation that is absent in
the ECMWF and Canadian by comparison at the outset of week-2. As the GEFS shows
rising heights and a relaxation of precipitation amounts over the lower half of
the West Coast late next week, the ECMWF and Canadian concurrently feature a
strong uptick in precipitation mainly focused over the Pacific Northwest and
northern California into the following weekend. Taken together for the 3-day
period, the GEFS and ECWMF Probabilistic Extreme Tools (PETs) remain in fair
agreement indicating between 30-50% chances of amounts exceeding the 85th
climatological percentile, with the highest probabilities for precipitation
exceeding an inch and two inches focused from central California northward to
western Washington. Although higher probabilities of precipitation exceedance
are featured in the raw tools (>50% chances for exceeding an inch over northern
California in both the GEFS and ECMWF), the day-to-day volatility of the raw
tools and aforementioned uncertainties with the evolving height pattern
precludes any high risk hazard designations at this time. Therefore, moderate
risks of heavy precipitation and high elevation heavy snow remain posted and
continue through day 11 (Nov 15), before much of the troughing aloft is favored
to become less amplified during the later portion of the period. A possible
flooding hazard also remains for the lower elevations of northern California
and the Pacific Northwest. The increased precipitation favored leads to basin
responses during week-2 in some models, as any excess precipitation over burn
scarred areas may also trigger localized flooding.



Surrounding the moderate risk areas, a slight risk of heavy snow remains posted
for the higher elevations of the West Coast, as well as a slight risk of heavy
precipitation issued for the entirety of the West Coast and extending eastward
into parts of the Desert Southwest, Great Basin and the Rockies. Within the
slight risk areas, there are increased chances for 3-day amounts exceeding the
85th percentile in the PETs and enhanced tropospheric moisture is favored to
advect eastward in the GEFS and ECMWF precipitation water anomaly forecasts.
Based on continued support in the GEFS Snow Water Equivalent (SWE) PET and raw
snow tools, a slight risk of heavy, high elevation snow remains posted for
parts of the Great Basin, Northern Intermountain, and Rockies, valid for all of
week-2. In regards to hazardous winds, a moderate risk area remains posted over
much of coastal California and the Pacific Northwest (Nov 12-15) near and ahead
of the base of the amplified troughing favored, as well as a broader area of
slight risk of high winds covering the western third of the CONUS (Nov 12-18)
to capture any shortwave energy propagating across the Interior West during the
period.



Downstream across the eastern U.S., a reamplification of 500-hPa troughing and
an accompanying strong surface low pressure system remain favored over the
Canadian Maritimes late in week-1. This surface low still appears to be slow to
lift out and weaken early in the period where mean surface high pressure
settled over the Great Lakes looks to induce periods of potentially strong
winds over the northeastern CONUS. Similar to yesterday, any enhanced
precipitation is expected to remain offshore ahead of the trough axis, however,
a slight risk of high winds remains posted and valid through Nov 12 across this
region.



Towards the middle of the period, lee side cyclogenesis over the Great Plains
is possible with shortwave energy favored to propagate across the Interior
West. With the mean surface high established over the Southeast, more return
flow from the Gulf and increased precipitation amounts are favored over parts
of the Great Plains, Mississippi, Ohio and Tennessee Valleys. However, PETs and
raw tools are not quite supportive of a corresponding precipitation risk at
this time, but this potential will continue to be monitored in upcoming
outlooks.



While much of the strong troughing over the northeastern Pacific is likely to
result in periods of strong winds and heavy precipitation along the southern
and southeastern coast of Alaska as well, hazardous thresholds are not expected
to be exceeded so no hazards are posted for Alaska at this time.

FORECASTER: Nick Novella

$$