484
FXUS21 KWNC 021947
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 02 2026

SYNOPSIS: Dynamical models remain in good agreement in regards to timing and
strength of an area of mid-level low pressure encroaching the West Coast,
promoting enhanced onshore flow and elevating chances for atmospheric river
activity during the period. Confidence continues to increase for hazardous
impacts from heavy precipitation, high winds, and high elevation heavy snowfall
over the Pacific Northwest and California, stretching southward along the West
Coast and expanding eastward over the Interior West with time. 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, enhanced, and possibly
heavy precipitation may occur over many parts of the Great Plains, Mississippi,
Tennessee and Ohio Valleys towards the middle of the period.

HAZARDS

High risk of heavy precipitation for parts of the Pacific Northwest and
northern California, Tue-Thu, Nov 10-12.

High risk of heavy high elevation snow across the Cascades of western
Washington, Tue-Thu, Nov 10-12

Moderate risk of heavy precipitation for parts of the Pacific Northwest,
northern and central California, Tue-Sat, Nov 10-14.

Moderate risk of heavy high elevation snow across the Cascades, Klamath, and
the Sierra Nevada Mountains, Tue-Sat, Nov 10-14.

Moderate risk of high winds across coastal portions of Washington, Oregon, and
much of California, Tue-Sat, Nov 10-14.

Slight risk of heavy precipitation across much of the West Coast Tue-Mon, Nov
10-16.

Slight risk of heavy high elevation snow across the Cascades, Klamath, and
northern Sierra Nevada Mountains, Tue-Mon, Nov 10-16.

Slight risk of heavy precipitation across portions of southern California into
the lower Four Corners, Wed-Sun, Nov 11-15.

Slight risk of heavy high elevation snow for parts of the Northern
Intermountain, Great Basin, Central and Northern Rockies, Wed-Sun, Nov 11-15.

Slight risk of high winds for much of the West Coast and Interior West,
Tue-Mon, Nov 10-16.

Slight risk of high winds for parts of Southeast, Appalachians, Ohio Valley,
eastern Great Lakes, Mid-Atlantic, and the Northeast, Tue-Wed, Nov 10-11.

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

DETAILED SUMMARY

FOR THURSDAY NOVEMBER 05 - MONDAY NOVEMBER 09:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY NOVEMBER 10 - MONDAY NOVEMBER 16: By early next week, dynamical
models remain in good agreement featuring a highly amplified 500-hPa trough
over the northwestern Pacific, where accompanying Atmospheric River (AR)
activity potentially impacting the West Coast continues to be the primary
hazards threat during week-2. Over the past week or so, a key area of outlook
uncertainty has been with the models difficulty to progress the mean Pacific
trough eastward, resulting in a period of enhanced onshore flow that has been
fixed in forecast lead space in subsequent ensemble run cycles. However, this
appears to no longer be the case, as ensemble mean 500-hPa height anomaly
forecasts now favor an encroachment of the trough over the West Coast occurring
within the early week-2 timeframe, which also becomes more negatively tilted by
day 9 (Nov 11) in the mean solutions. A more coherent evolution of the elevated
AR potential is also well supported in the Integrated Vapor Transport (IVT)
tools, where both the GEFS and ECMWF now show the strongest probabilities for
IVT amounts exceeding 250 and 500 kg/ms shifting from days 9 and 10 to day 8
(Nov 10) compared to yesterday. In addition, stronger precipitation responses
are indicated in the latest precipitation tools, and although Probabilistic
Extreme Tools (PETs) generally maintain 30-40% chances for 3-day amounts
exceeding the 85th percentile supportive of a moderate risk, the uncalibrated
ECMWF guidance remains quite robust, indicating at least 80% (60%) chances for
3-day amounts exceeding 1 inch (2 inches) over parts of the Pacific Northwest
and northern California.



As a result of the better evolution of the mean trough, and stronger signals in
the precipitation guidance to increase outlook confidence, a high risk of heavy
precipitation and an accompanying high risk of heavy, high elevation snowfall
are posted for Nov 10-12. The high risk of heavy precipitation extends
southward to just north of the Sacramento area of California based on raw
precipitation tools. Snow levels are expected to be limited to the highest
elevations, and the high risk of heavy snow is posted over the Cascades of
western Washington where IVT signals are strongest early in the period. A
flooding possible hazard is also introduced for the lower elevations of
northern California and the Pacific Northwest. The substantial increase in
precipitation favored has led to responses during week-2 in some models, as any
excess precipitation over burn scarred areas may also trigger localized
flooding.



The elevated AR risk is favored to shift southward with time, potentially
reaching as far south as 30N by the middle of the period. This is best
reflected in the ECMWF PET, which depicts higher chances (>50%) for 3-day
amounts exceeding the 85th percentile focused over southern California and the
Desert Southwest by later next week. While this signal uptick in percentile
space is attributed at least in part to a drier climatology over this part of
the country, increasing and potentially heavy precipitation amounts are
indicated in the raw tools further south. Therefore, the moderate risk of heavy
precipitation and moderate risk of heavy snow is expanded southward to cover
more of central California and the Sierra Nevada Mountains, and extended
through day 12 (Nov 14), and the enveloping slight risk areas remain posted and
valid for all of week-2. The elevated AR potential also supports the
continuation of a moderate risk of high winds and is likewise expanded
southward based on increased signals in the PETs, valid for the same period.
Based on the uncalibrated precipitable water anomaly forecasts featuring
enhanced tropospheric moisture overspreading much of the Interior West with
time, wind PETs, as well as raw and calibrated precipitation snow tools, a
separate slight risk of heavy precipitation remains posted over southern
California and into the lower Four Corners (Nov 11-15), and a slight risk of
high elevation snow has been introduced over parts of the Northern
Intermountain, Great Basin, and Rockies for Nov 11-15. In addition, the
potential for shortwave energy propagating over the Interior West also supports
periods of elevated winds, and a slight risk of high winds is expanded to cover
the western third of the CONUS for the entirety of week-2.



Downstream across the eastern U.S., ensemble guidance continues to feature a
deamplifying shortwave trough lifting out over the western Atlantic early in
the period. Although the GEFS is considerably stronger with this mid-level
feature compared to the ECMWF, the heaviest precipitation amounts are expected
to remain ahead of the trough axis which is positioned offshore by the start of
the period. Given this, and precipitation tools favoring drier amounts across
the eastern U.S., the slight risk of heavy precipitation is discontinued in the
updated outlook. However, the corresponding surface low located over New
England and the Canadian Maritimes is favored to be relatively deeper in the
updated guidance where an ensuing mean surface high developing over the
Mississippi Valley may induce periods of gusty winds. Therefore, a slight risk
of high winds is posted over parts of the eastern U.S. for Nov 10-11.



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 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.
Another area that bears monitoring for heavy precipitation concerns the Florida
Panhandle and the Southeast as tools favor more tropical moisture being
advected into the region towards the middle of the period. Several ECMWF
ensemble members point to a potential tropical low drifting northward from the
southern Caribbean, however there is little to support for this potential in
the GEFS or the Google Deepmind guidance at this time.



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

$$