076
FXUS21 KWNC 141928
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 14 2024

SYNOPSIS: Multiple model ensembles depict deep low pressure at the surface and
aloft over the Great Lakes region at the outset of week-2, which has the
potential to bring heavy rain, prolonged lake-effect snow and strong winds to
portions of the central and eastern Contiguous U.S. (CONUS) as well as a shot
of cold air to portions of the central and southern CONUS. Parts of the
Southern Plains could experience the first freeze of the season.

HAZARDS

Moderate risk of much below-normal temperatures for portions of the Southern
Plains and Lower Mississippi Valley, Fri, Nov 22.

Slight risk of much below-normal temperatures for much of the Southern Plains,
Lower Mississippi Valley, and Southeast U.S., Fri-Sun, Nov 22-24.

Moderate risk of heavy snow for portions of the Great Lakes prone to
lake-effect snow, Fri-Sat, Nov 22-23.

Slight risk of heavy snow for much of the Great Lakes, as well as portions of
the Ohio Valley, Central Appalachians, and Northeast U.S., Fri-Tue, Nov 22-26.

Slight risk of heavy precipitation for portions of the Northeast, Fri, Nov 22.

Moderate risk of high winds for much of the Great Lakes, Ohio and Tennessee
Valleys, as well as portions of the Middle and Upper Mississippi Valley,
Central Appalachians, and Northeast U.S., Fri, Nov 22.

Slight risk of high winds for much of the eastern CONUS, Fri-Sat, 22-23.

DETAILED SUMMARY

FOR SUNDAY NOVEMBER 17 - THURSDAY NOVEMBER 21:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY NOVEMBER 22 - THURSDAY NOVEMBER 28: Ensemble solutions from the GEFS
and ECENS both depict a deep mid-level trough with an associated surface low
over the Great Lakes region at the outset of week-2. This system is the product
of strong lee cyclogenesis which is expected to bring hazardous weather to the
Great Plains during week-1. Model consensus favors the surface low to become
occluded and stall early in week-2, lingering over the Great Lakes for several
days. Model guidance has consistently indicated this to be a potent storm
system with significant potential to generate a variety of hazardous weather
conditions spread out over much of the U.S. east of the Rockies.



Ensemble and deterministic model solutions continue to favor an occluded low
pressure system to settle over Ontario during the week-2 period, resulting in a
prolonged period of westerly to northwesterly surface flow over the Great
Lakes, which are at or near record high surface temperatures. Cold air aloft
results in very steep lapse rates, further contributing to favorable
lake-effect snow conditions. Consensus among models is for this favorable setup
to be at its strongest for Nov 22-23, therefore a moderate risk for heavy snow
is posted for portions of the Great Lakes most prone to lake-effect snow, while
a broader slight risk of heavy snow is posted for the Great Lakes, much of the
Ohio Valley, and portions of the Central Appalachians and Northeast U.S. for
Nov 22-26. Surface temperatures are forecast to be near freezing, this
increases uncertainty regarding snow totals with rain likely mixing with snow
at times, and may make for sloppy conditions across the lake effect snow belts.
With a concurrent enhanced potential for high winds the potential for excessive
snow accumulations might be further reduced, but instead is replaced with a
potential for blizzard conditions.



Further ahead of the low pressure, models depict enhanced precipitation over
portions of the Northeast U.S., with ECMWF raw probabilistic guidance
indicating at least a 20% chance of 3-day precipitation accumulations exceeding
1 inch north and east of New York City. While the likeliest potential of heavy
precipitation is now in week-1, there remains enough uncertainty regarding the
progression of this storm system that a slight risk of heavy precipitation
remains posted for portions of the Northeast U.S. for Nov. 22.



The GFS deterministic runs described above also depict strong surface pressure
gradients over much of the eastern CONUS over the first half of the forecast
period. This is better-supported today by the Probabilistic Extremes Tools
(PETs) for the ECMWF and GEFS, which both indicate at least a 20% probability
of maximum wind speeds exceeding the 85th percentile for much of the eastern
CONUS. Deterministic model solutions also consistently depict strong surface
pressure gradients associated with this storm system. Thus, a slight risk
(20-40 percent chance) for high winds remains for much of the eastern CONUS for
Nov 22-23. Furthermore, a moderate risk (40-60 percent chance) for high winds
is posted for much of the Great Lakes, Ohio and Tennessee Valleys, as well as
portions of the Middle and Upper Mississippi Valley, Central Appalachians, and
Northeast U.S., for Nov 22, where model guidance has consistently placed very
tight surface pressure gradients.



Anomalously cold air is favored to be associated with the upper level trough
over the central CONUS, bringing potentially impactful minimum temperatures to
portions of the Southern Plains. The GEFS and ECENS PETs both indicate at least
20% probabilities of minimum temperatures falling below the 15th percentile
over much of the Southern Plains, and at least 40% probability of sub-freezing
temperatures for North Texas, Oklahoma, and Arkansas, warranting a moderate
risk for much below-normal temperatures for the area described to cover Nov 22.
 Additionally, both PETs indicate the potential for frost conditions all the
way to the U.S.-Mexico border and extending eastward to cover most of the
Southeast U.S., therefore a slight risk for much below-normal temperatures is
posted for the remainder of the Southern Plains and Lower Mississippi Valley as
well as portions of the Tennessee Valley and Southeast U.S. for Nov 22-24.

FORECASTER: Danny Barandiaran

$$