923
FXUS21 KWNC 041914
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 04 2024

SYNOPSIS: Surface low pressure near the northeastern contiguous U.S. (CONUS)
may bring rain or inland snow to the region around the beginning of week-2 or
just prior. A short burst of anomalously cold temperatures is possible across
portions of the south-central CONUS in the wake of a cold front. Mid-level low
pressure is forecast to develop across the West Coast of the CONUS favoring a
more active pattern in the West, with increased chances for coastal rain and
high elevation snow.

HAZARDS

Slight risk of much below normal temperatures across portions of the Southern
Plains and Lower Mississippi Valley, Thu, Dec 12.

Slight risk of heavy precipitation across portions of the Northeast, Thu, Dec
12.

Slight risk of heavy precipitation across portions of the Pacific Northwest and
the northern half of California, Thu-Mon, Dec 12-16.

Slight risk of heavy snow across portions of the Interior Northeast, Thu, Dec
12.

Slight risk of heavy snow across portions of the Cascades, Klamath, and Sierra
Nevadas, Thu-Mon, Dec 12-16.

Slight risk of heavy snow across portions of the Northern and Central Rockies,
Thu-Mon, Dec 12-16.

Slight risk of high winds across portions of the Great Lakes and Northeast,
Thu, Dec 12.

Slight risk of high winds across much of the northwestern quadrant of the CONUS
extending into the Northern and Central Plains, Thu-Mon, Dec 12-16.

DETAILED SUMMARY

FOR SATURDAY DECEMBER 07 - WEDNESDAY DECEMBER 11:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY DECEMBER 12 - WEDNESDAY DECEMBER 18: Model solutions continue to
diverge related to the phasing of northern and southern stream energy over the
eastern CONUS late in week-1 into the outset of week-2. The 0z deterministic
ECMWF maintains a stronger northern stream system tracking quickly into the
Great Lakes, with warmer temperatures and rain over parts of the Northeast, but
tapering by the beginning of week-2. Recent runs from the deterministic GFS
have depicted a more active southern stream system. Although the 0z run kept
most of the moisture offshore, the 6z and 12z runs have more northern stream
interaction, with the 12z especially, depicting a snowstorm over parts of the
Interior Northeast ongoing on day-8 (Dec 12). The ensemble solutions generally
lean toward the more progressive deterministic outcomes, with most of the
precipitation occurring before the start of week-2, and the Probabilistic
Extremes Tools (PETs) for both the ECENS and GEFS do not indicate any
significant precipitation signals. However, given continuity with yesterdays
forecast, slight risks for heavy rain and snow are kept across portions of the
coastal and Interior Northeast respectively for day-8 (Dec 12) to account for
continued expected changes in model runs. It is also worth noting that the
North Atlantic Oscillation is forecast to transition from negative to positive
around this time, which can sometimes lead to a more amplified storm in the
East. A warmer solution could raise flooding concerns along parts of the Great
Lakes, particularly Erie and Ontario, due to snowmelt. A slight risk of high
winds is also highlighted across portions of the Great Lakes and Northeast, Dec
12, to account for the potential of enhanced northwesterly flow on the backside
of the system, which could also lead to additional Lake Effect Snow.
Deterministic solutions from the ECMWF and GFS continue to indicate an active
northern stream through the end of week-2. However, ensembles depict a trend
toward more ridging and warmer temperatures in the East by mid-December
reducing confidence in the snowfall potential.



A cold front is forecast to drop southward through the Southern Plains and
Lower Mississippi Valley late in week-1. Models and PETs generally depict
elevated probabilities for minimum temperatures dipping into the 30s deg F as
far south as parts of coastal Texas and Louisiana, with higher chances of
subfreezing temperatures across northern and western Texas. As a result, a
slight risk for much below normal temperatures is highlighted across the area,
Dec 12, with some locations having yet to experience a first freeze. This is
forecast to be a quick shot of cold air with moderation forecast by day-9 (Dec
13). While agricultural impacts should be minimal, increased concern is related
to elevated heating demands anticipated during this timeframe. A hard freeze
(temperatures less than 28 deg F) is possible across portions of the Tennessee
Valley and Southeast, but impacts are forecast to be minimal with the growing
season over in these areas. Therefore, the northern extent of this hazard is
not expanded.



The 0z ECENS, GEFS, and CMCE are in good agreement regarding troughing
developing along the West Coast of the CONUS at the start of week-2. The trend
toward lower heights is supportive of increasing onshore flow across the West
Coast and this is reflected in the Integrated Vapor Transport tool derived from
both models. The GEFS PET depicts parts of the Pacific Northwest having at
least a 20 percent chance 3-day precipitation totals exceed the 85th
climatological percentile and 1-inch throughout the period, with the ECENS
focusing these enhanced probabilities further south into portions of northern
and central California. Given the continued improving agreement, a slight risk
of heavy precipitation is now posted across portions of the Pacific Northwest
extending southward into the northern half of California, Dec 12-16. An
accompanying slight risk of heavy snow is also posted across the Cascades,
Klamath, Sierra Nevadas, and the Northern and Central Rockies, supported by the
GEFS Snow Water Equivalent (SWE) PET which depicts at least a 20 percent chance
SWE exceeds the 85th climatological percentile and 1-inch, and the uncalibrated
ECMWF probabilities indicating elevated chances for daily snowfall totals
greater than 6 inches over parts of the highlighted areas. A broad slight risk
of high winds is also highlighted Dec 12-16 across much of the northwestern
quadrant of the CONUS extending into the Great Plains, with the strongest
signals for wind speeds greater than 25-mph being along the immediate West
Coast and also across the High Plains tied to increased downslope flow,
although probabilities for wind speeds exceeding the 85th climatological
percentile are elevated in the GEFS and ECENS PETs across the Intermountain
suggestive of an increased high wind risk in these areas as well.



Ridging to the south of Alaska during week-2 favors the axis of strongest
onshore flow shifting away from Southeast Alaska decreasing the heavy
precipitation potential relative to the week-1 period. Although some enhanced
precipitation is still possible across Southeast Alaska, hazards thresholds are
not forecast to be reached. High probabilities for above-normal temperatures
are predicted across most of Alaska for week-2.

FORECASTER: Thomas Collow

$$