480
FXUS21 KWNC 031926
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 03 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
outlook confidence has decreased due to some drying trends in the guidance
since yesterday, there continues to be elevated chances for hazardous impacts
including heavy precipitation, high winds, and high elevation snowfall, with
possible flooding over the Pacific Northwest and California during the period.
Meanwhile, an area of departing mid-level low pressure may bring periods of
high winds over portions of the eastern U.S. early in the period. With surface
low pressure development 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,
northern and central California, Wed-Sun, Nov 11-15.

Moderate risk of heavy high elevation snow across the Klamath and the Sierra
Nevada Mountains, Wed-Sun, Nov 11-15.

Moderate risk of high winds across coastal portions of Washington, Oregon, and
much of California, Wed-Sun, Nov 11-15.

Slight risk of heavy precipitation across the West Coast and into parts of the
Desert Southwest, Great Basin, and Southern Rockies, Wed-Tue, Nov 11-17.

Slight risk of heavy high elevation snow across the Cascades, Klamath, and
northern Sierra Nevada Mountains, Wed-Tue, Nov 11-17.

Slight risk of heavy high elevation snow for parts of the Northern
Intermountain, Great Basin, Central and Northern Rockies, Wed-Tue, Nov 11-17.

Slight risk of high winds for much of the West Coast and Interior West,
Wed-Tue, Nov 11-17.

Slight risk of high winds for parts of Northern and Central Appalachians, Lower
Great Lakes, Upper Mid-Atlantic, and the Northeast, Wed-Thu, Nov 11-12.

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

DETAILED SUMMARY

FOR FRIDAY NOVEMBER 06 - TUESDAY NOVEMBER 10:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY NOVEMBER 11 - TUESDAY NOVEMBER 17: A highly amplified 500-hPa
mean trough is expected to encroach the West Coast next week, and bring an
elevated risk of Atmospheric River (AR) activity that could lead to heavy
precipitation, high winds, and high elevation heavy snow for portions of the
Pacific Northwest and California. However, the latest 500-hPa height anomalies,
precipitation, and snow forecasts from the dynamical models have generally
backed off somewhat on the likelihood of these potential hazards relative to
yesterday. Daily analysis of mid-level heights forecasts from both the GEFS and
the ECMWF ensemble runs during the past few days feature a rising height trend
over the eastern Pacific and the West Coast, which could contribute to either a
delayed onset of AR activity or weaker troughing affecting the West Coast, at
least initially during the period. In response to this trend, precipitation
guidance continues to paint a rather wet picture, but the uncalibrated tools in
particular, favor an overall reduction in probabilities in exceeding various
hazardous precipitation thresholds compared to yesterday. On the other hand,
Integrated Vapor Transport (IVT) tools still remain robust with the AR
potential, while continuing to indicate a coherent evolution of these AR
related risks entering the late week-1 timeframe. In addition, these tools are
beginning to better resolve a secondary surge of enhanced onshore flow
affecting the West Coast tied to a reloading mean mid-level trough in the
ensembles towards the middle of the period. Taken all together, there is a
lesser degree of forecast confidence in the updated hazards outlook over the
western U.S., though the favored persistence of strong troughing for the
entirety of week-2, continues to support the potential for a series of storms
and AR activity being of longer duration through mid-November.



In light of these uncertainties with the evolving height pattern and the
associated precipitation responses over the West Coast, the high risks of heavy
precipitation and heavy high elevation snow are discontinued in the updated
outlook. The greatest loss in support is most evident in the uncalibrated
ECMWF, which dropped from 60-80% (50-60%) to 40-60% (20-50%) chances for 3-day
amounts exceeding an 1 inch (2 inches) over the Pacific Northwest and northern
California early in the period. Based on diminished signals in both the GEFS
Snow Water Equivalent (SWE) PET and raw snow tools, the moderate risk of heavy
snow is also removed over the Cascades of western Washington and parts of
western Oregon. The most coherent signal for heavy snow has become more focused
over parts of Klamath and Sierra Nevada mountains, where any heavy snowfall
remains favored over the highest elevations. Despite the reduced support for
the high risk hazard designations, 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.



Probabilistic Extreme Tools (PETs) from the GEFS and ECMWF continue to show
elevated chances (30-40%) of 3-day precipitation amounts exceeding the 85th
percentile, with little changes in these signals in percentile space compared
to yesterday.  Given very good support in the IVT tools indicating high (>80%)
chances for values exceeding 250 kg/ms focused approximately between 40N and
50N, a moderate risk of heavy precipitation remains posted and an accompanying
heavy snow hazard is issued over the higher elevations over California and
southern Oregon. Similar to yesterday, much of the enhanced precipitation is
expected to shift southward with time, with the highest daily amounts favored
from the central California Valley northward along the West Coast. To capture
this shift, and any renewal of enhanced onshore flow later in the period as
reflected in the IVT and daily precipitation tools, the moderate risk areas are
extended through day 12 (Nov 15). Surrounding these moderate risk areas over
the West Coast, a corresponding slight risk of heavy precipitation remains
issued for the entirety parts of the Desert Southwest, Great Basin and the
Rockies, where there are elevated chances for 3-day amounts exceeding the 85th
percentile in the PETs and enhanced tropospheric moisture is favored to spread
eastward in the GEFS and ECMWF precipitation water anomaly forecasts. 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 wind potential, a moderate risk of high winds remains posted over
much of coastal California and the Pacific Northwest (Nov 11-15) 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 11-17)
to capture any shortwave energy propagating across the Interior West during the
period.



Downstream across the eastern U.S., much of the troughing over the Southeast
that was previously favored to lift out and weaken late in week-1, is now
expected to reamplify over the Northeast and Canadian Maritimes by the outset
of the period. Similar to yesterday, any enhanced precipitation is expected to
remain offshore ahead of the trough axis, however, a deep surface low
reflection remains favored to induce periods of potentially strong winds over
the northeastern CONUS. Therefore, a slight risk of high winds remains posted
and valid through Nov 12 across this region, with coverage omitted over parts
of the lower Mid-Atlantic and Southeast in the updated outlook based on raw
ECMWF wind guidance.



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

$$