334
FXUS21 KWNC 081914
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 08 2024

SYNOPSIS: By late next week, a departing frontal system is expected to increase
the potential for high winds over the northeastern contiguous U.S (CONUS).
Upstream, strong mid-level low pressure shifting into the Interior West brings
increased risk of high elevation heavy snow and high winds through the middle
of the period.  Additional surface low formation in the lee of the Rockies may
also bring another round of heavy precipitation across the central U.S. Surface
low pressure shifting eastward from the Bering Sea may lead to episodes of
strong winds for much of coastal Alaska.

HAZARDS

Slight risk of heavy precipitation for parts of the Great Plains, Mississippi.
Tennessee. and Ohio Valleys, Sun-Tue, Nov 17-19.

Slight risk of heavy snow across many portions of the Rockies, Sat-Tue, Nov
16-19.

Slight risk of heavy snow for the higher elevations of the Sierra Nevada range
of California, Sat, Nov 16.

Slight risk of high winds across many portions of the northeastern CONUS, Sat,
Nov 16.

Slight risk of high winds for the entire western CONUS and Central and Northern
High Plains, Fri-Tue, Nov 16-19.

Slight risk of high winds across coastal portions of southwestern Mainland
Alaska, and the Aleutians, Sat-Mon, Nov 16-18.

DETAILED SUMMARY

FOR MONDAY NOVEMBER 11 - FRIDAY NOVEMBER 15:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY NOVEMBER 16 - FRIDAY NOVEMBER 22: The week-2 hazards perspective
remains mostly on track based on model guidance over the past few days. By late
next week, models remain in good agreement featuring a shortwave trough lifting
out over the northeastern CONUS, with a frontal system bringing increased
precipitation to many parts of the Great Lakes, Mid-Atlantic, and moisture
starved areas of the Northeast. By day 8 (Nov 16), much of the precipitation is
expected to be offshore with the mean surface low favored to be off of Nova
Scotia, however, ensembles feature a fairly strong surface pressure gradient
over the Northeast with surface high pressure filling in over the Southeast and
the Mid-Atlantic its wake of the system. This supports the continuation of a
slight risk of high winds for the northeastern CONUS, and remains valid through
Nov 16. While there continues to be not much support for ample cold air
advection and a significant drop in temperatures in the wake of the system, it
is worth noting that the Great Lakes water temperatures have been substantially
above normal (and in some cases record breaking) during the past month or so
according to GLSEA/NOAA. As such, parts of the Great Lakes look to be primed
for lake-effect precipitation/snow should any anomalously cold air work its way
into the region.



Upstream, models have been consistent with an amplified 500-hPa trough
encroaching the West Coast late next week. While much of the enhanced onshore
flow looks to have timed off for the Pacific Northwest as the trough axis moves
inland, the ECMWF favors a redevelopment of negative height departures on days
9 and 10 (Nov 17-18), suggestive of a secondary shortwave disturbance possibly
bringing a renewed risk of heavy precipitation to the region early in the
period. This is reflected in the ECWMF Probabilistic Extremes Tools (PETs) with
increased chances of 3-day amounts exceeding the 85th percentile and 1 inch
over western Oregon and Northern California early in week-2. However, both the
raw and calibrated GEFS are less supportive of this potential, resulting in no
heavy precipitation and high elevation heavy snow hazards at this time.
Further south, the position of the mean trough axis still appears favorable for
enhanced moisture flow into parts of central and southern California by the
outset of the period. While generally weaker compared to yesterday, the GEFS
Snow Water Equivalent (SWE) PET shows increased signals over the Sierra Nevada
range, and a slight risk of high elevation heavy snow remains issued for Nov
16. Accompanied by cooler temperatures forecast, the favored increase in
moisture over California is expected to quell fire weather potential and
provide relief to areas affected by wildfire incidents in recent weeks.
However, as California heads into the wet season, any heavy precipitation
potential bears monitoring as this may trigger debris flows over any burn
scarred areas of the state.



As the anomalous troughing moves further inland through the middle of the
period, the combination of continued Pacific moisture with cyclonic flow aloft
over the higher elevations is expected to remain conducive for periods of
potentially heavy snow over the Rockies. A notable development concerns the
amplitude of the mean trough favored in the 0z GEFS over the Interior West,
which is considerably weaker compared to yesterday`s solutions, as well as the
latest 0z ECMWF.  Given this, and larger differences between the GEFS and ECWMF
PETs, this limits confidence in the designation of any moderate risks, and
prompts the continuation of a slight risk area of heavy snow across the higher
elevations of the West, as well as a broad slight risk area of high winds for
much of the western CONUS, both valid for Nov 16-19.



With mean troughing over the Interior West, surface low development in the lee
of the Rockies continues to be favored in the ensembles.  The aforementioned
weaker troughing aloft in the GEFS also leads to a weaker surface reflection of
the mean low, as well as a lowered heavy precipitation potential in both the
raw and calibrated guidance compared to the ECWMF.  Notwithstanding, there is
sufficient support in the GEFS and ECMWF PETs for the continuation of a slight
risk of heavy precipitation over portions of the Plains, Mississippi, Ohio and
Tennessee Valleys, and is now valid for Nov 17-19 based on the daily ensemble
mean precipitation totals.  On the backside of the surface low, accumulating
snowfall is possible over portions of the High Plains where the ECMWF ensemble
continues to feature increased daily snowfall amounts in the daily means during
the middle of week-2. However, there is high uncertainty in regards to the
strength, timing, and eventual track of the surface low, with little support
for snowfall amounts exceeding hazard criteria to warrant a corresponding heavy
snow hazard at this time.



Over Alaska, anomalous 500-hPa troughing (ridging) is favored to develop over
the Bering Sea (south of the Aleutians), where ensembles continue to show
additional low pressure development to increase the risk of high winds.
Although a secondary mean low is favored to form in the Gulf of Alaska,
guidance suggests this feature is not particularly anomalous. Therefore, a
slight risk of high winds remains for the Aleutians and the southwestern
Mainland, with coverage removed over the Southeast. It is also now valid
through Nov 18 as the high wind potential is expected to ease by the middle of
the period based on PET guidance.

FORECASTER: Nick Novella

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