512
FXUS21 KWNC 262042
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 26 2024

SYNOPSIS: One or more surface high pressure systems are expected to advance
southeastward from the Canadian Prairies into the central and eastern
contiguous U.S. (CONUS), increasing the chances for much below normal
temperatures over a large part of this area. This includes chances for first
freezes across the southern tier of the eastern CONUS, possibly as far south as
central Florida. The surface pressure gradient between these high pressure
systems and lower pressures in southeastern Canada creates a risk of periodic
high winds ushering the colder air into the central and eastern CONUS. The cold
air overrunning unusually warm Great Lakes should set the stage for
lake-enhanced snow showers and squalls over climatologically favored areas
downwind of the Lakes. There is significant uncertainty with respect to the
location, timing, and track of individual low pressure systems, if any, over
the central and eastern CONUS. This makes it very difficult to specify details
of weather-related hazards, but there is enough agreement to introduce an
enhanced risk of heavy snow in western parts of the mid-Atlantic and lower
Northeast Piedmont.

HAZARDS

Moderate risk of much below normal temperatures in the Southeast and lower
mid-Atlantic Region, Wed, Dec 4.

Slight risk of much below normal temperatures in the Ohio and Tennessee
Valleys, Southeast, mid-Atlantic Region, and lower Northeast, Wed-Sun, Dec 4-8.

Slight risk of heavy snow in the climatologically-favored regions for
lake-enhanced snowfall in the central and western Great Lakes, Wed-Sat, Dec 4-7.

Slight risk of heavy snow in climatologically-favored regions for lake-enhanced
snowfall in the eastern Great Lakes and the central and northern Appalachians,
and along western parts of the Piedmont in the mid-Atlantic and Northeast,
Wed-Sun, Dec 4-8.

Slight risk for periodic high winds over central and northern portions of the
CONUS from the Great Plains to the East Coast, Wed-Tue, Dec 4-10.

Slight risk of heavy precipitation in south-central and southeastern Alaska,
Wed-Tue, Dec 4-10.

DETAILED SUMMARY

FOR FRIDAY NOVEMBER 29 - TUESDAY DECEMBER 03:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY DECEMBER 04 - TUESDAY DECEMBER 10: The GEFS and ECENS ensemble
means indicate a broad trough stretching across the eastern one-third to
one-half of the CONUS during week-2. There continues to be significant
uncertainty among the various deterministic and ensemble mean models regarding
the development and expected evolution of low pressure systems and the
potential for impactful weather across the central and eastern CONUS, but
guidance is consistent in showing one or more high pressure systems moving from
southwestern or south-central Canada into the central and eastern parts of the
CONUS, toward the axis of a mid-level trough. There is a slight risk (20-40%
chance) of much below normal temperatures (15th historical percentile or lower)
posted from the Ohio and Tennessee Valleys eastward to the Atlantic coast from
southern New England to central Florida, Dec 4-9. Tools are more robust with
the cold air outbreak on day 8 (Dec 4), when there is a moderate risk of much
below normal temperatures for a smaller part of the Southeast and the lower
mid-Atlantic Region. The first freeze of the season may be observed late week-1
or early week-2 along the immediate central Gulf Coast and as far south as the
interior central Florida Peninsula, though most of the Florida citrus growing
region should not be affected by a hard freeze. These crops are highly
vulnerable to a damaging first freeze if it comes during late autumn or early
winter. During the last half of the week, a re-inforcing shot of cold air is
expected to push into the eastern CONUS. The Probabilistic Extremes Tool
derived from the European ensemble is more aggressive with the chances for
significantly colder than normal temperatures than other tools, keeping chances
for temperatures in the lowest 15th climatological percentile over 30 percent
in parts of the Northeast as late as day 12 (Dec 9). Beyond that, the PET tools
and deterministic models show temperatures to remain colder than normal, but
staying above hazardously cold levels.



To the south of this series of cold high pressure systems, some tools show
enhanced chances for heavy precipitation early week-2 in part of the southern
CONUS. The best model agreement for enhanced precipitation is across eastern
Texas, with at least some guidance pointing toward heavy precipitation anywhere
from Deep South Texas into the Lower Mississippi Valley. There is considerable
uncertainty about this potential, and even the most aggressive tools are not
remarkably wetter than hazards thresholds, so for now, no hazard is posted.



Cold air streaming across the unusually warm waters of the Great Lakes is
likely to promote lake-enhanced snowfall for the climatological downwind
snowbelt areas. Periods of locally heavy lake-enhanced snowfall warrant posting
a slight risk of heavy snow for climatologically-favored areas near the Great
Lakes valid Dec 4-7, and continuing through Dec 8 along the west slopes of the
Appalachians and downwind of Lakes Erie and Ontario. Meanwhile, tools remain
inconsistent regarding the potential development of low pressure somewhere over
the East during the early to middle part of week-2. But despite the continuing
uncertainty, tools are a little more bullish today in showing a possible heavy
snow event that could affect western parts of the mid-Atlantic and Northeast
CONUS Piedmont, plus the higher elevations of northeastern Pennsylvania and
southeastern New York. The slight risk for heavy snow has been extended farther
east than yesterday to reflect this change, but it still appears as though
heavy snow will spare most of all of the East Coast Megalopolis.



While one or more cold high pressure systems sinks from Canada into the central
and eastern CONUS, lower surface pressures should prevail to the north or
northeast of the Great Lakes. The pressure gradient between these two features
should be strong enough at times to pose a slight threat for hazardously high
winds over the eastern half of the CONUS anywhere from the central and northern
Great Plains eastward to the Atlantic Coast, or roughly the northeastern
one-quarter to one-third of the CONUS. High winds are not expected in any one
location for the entirety of the week-2 period, but due to the uncertainty in
the timing and detailed evolution of surface high and low pressure systems in
eastern North America, a set-up favorable for high winds could become
established for relatively short periods at any point during week-2.



Guidance is a bit more robust with the potential for a series of strong low
pressure systems to track across or near the Gulf of Alaska. The timing,
strength, and track of individual storm systems is highly uncertain, but a
large-scale pattern favorable for periods of stormy weather is expected along
parts of the southern tier of Alaska throughout week-2. For this reason, a
slight risk of heavy precipitation is posted for south-central and southeastern
Alaska for the entirety of the period. In addition, strong winds will be
possible at times across the entire southern tier of the state, including parts
of the Aleutians. Right now, it appears that dangerously high winds will
primarily affect offshore areas, so no high wind hazard is posted, but this
situation will need to be closely monitored for signs that stronger winds may
occur inland.

FORECASTER: Rich Tinker

$$