Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
128 AM EST Sat Jan 18 2025

Valid 12Z Tue Jan 21 2025 - 12Z Sat Jan 25 2025

...Hazardous cold temperatures early week across the South and
East should gradually moderate with time...

...Confidence increasing for a southern tier winter storm Monday-
Tuesday...


...Overview...

Models and ensembles continue to show a very amplified mean
pattern early next week consisting of a deep upper trough over much
of the lower 48 and an eastern Pacific ridge. Confidence remains
high in a surge of Arctic air which should be in place across the
eastern half of the nation by early week due to this trough aloft
and cold high pressure at the surface. This trough should lift
north and east as the week progresses for gradually moderating
temperatures, but troughing is forecast to reload in the Central
U.S. for later week. Systems on the southern periphery of this
pattern will cause precipitation from the Southern Plains to
Southeast. There is increasing confidence in a winter storm across
the southern tier, which could produce snow/sleet close to the
western/central Gulf Coast across the Southeast and Carolinas, with
notable freezing rain possible from the Florida Panhandle and just
north. There is still uncertainty with the wintry precipitation
amounts and axis though. A strong shortwave into the Northwest late
next week should bring a round of generally light to moderate
precipitation to that region.


...Guidance/Predictability Assessment...

The models and ensembles show more than typical model
disagreement, even on the large scale. There is some agreement on a
general pattern but the mid to larger scale differences are very
apparent by late period and would have some notable impacts on
sensible weather, especially across the South. There is increasing
agreement on a shortwave shifting through the Deep South Tuesday-
Wednesday, and increasing confidence in significant wintry impacts
as far south as the Gulf Coast/northern Florida. Plenty of
uncertainty in the details and exact precip types, much of which
will take until the short range to resolve fully. A multi-model
blend seemed to serve as a good starting point for the WPC
forecast.

Behind this, the next potent shortwave will dig into the northern
Rockies and drop south and eastward with time. There are
considerable differences as early as Thursday both with timing and
amplitude of this shortwave. Through the 12z/18z model runs, the
UKMET was the fastest with this shortwave and the ECMWF was the
slowest/deepest. Preferred a solution closer to the GFS/CMC which
were better clustered with the ensemble means as well. This results
in a slightly flatter wave and thus less precipitation over
southern locations for late week.

The next shortwave enters the Northwest around Friday and there
are also notable differences in the timing of this as well. The CMC
is the quickest with this, while the ECMWF and GFS show reasonable
timing consistent with the ensemble means. The late period WPC
blend heavily favored the means, given increasing uncertainty not
only with this system, but also the evolution of the central U.S.
trough as it shifts eastward next weekend.


...Weather/Hazards Highlights...

By Tuesday, an expansive area of hazardously cold Arctic air will
be in place from the Southern Plains to the Midwest and South/East
with daytime highs at least 20F below normal for most. The coldest
anomalies are likely to be over the Southern tier and Great
Lakes/Midwest where highs could be 20-30F below normal. Coldest
temperatures though should be over the Upper Midwest where highs
near zero and lows in the -20s are likely on Tuesday. Wind chills
should be even colder and more dangerous. Compared to the short
range period, the northern- central Rockies/Plains should moderate
considerably by Tuesday- Wednesday (though with a potential for
gusty winds) while anomalously cold weather may persist into
Wednesday and possibly to some extent Thursday across the South and
East. The West on the other hand can expect near normal
temperatures gradually warming to slightly above normal before
trending colder again by next weekend.

A shortwave moving through the southern tier Tuesday-Wednesday
will provide support for precipitation across the South as moisture
overruns fronts in the Gulf of Mexico. Given the cold airmass in
place, the majority of the precipitation will be wintry, perhaps
all the way to the Gulf Coast. Southern tier snow potential
continues to increase in the winter weather probabilities for 0.25
inch liquid in the form of snow. Additionally, freezing rain/ice
remains a concern given the possibility of a warm nose atop Arctic
cold air, which is most likely in the Florida Panhandle into
southern Georgia and coastal South Carolina. WPC Key Messages are
in effect for this South winter storm. Just to the south of the
winter concerns, moderate to heavy rain is possible across central
Florida on Tuesday. A Marginal Risk may be needed in future cycles
for Tuesday's ERO, but at this point, there is still uncertainty on
QPF totals, instability, and rain rates.

Farther north, the pattern will be favorable for lake effect snow
across/downwind of the Great Lakes into Tuesday, with the highest
amounts off of Lake Ontario. Depending on pattern evolution, by
mid- late next week, precipitation may increase across the South
again including some winter weather, but models have shown
considerable variations, and generally have trended lighter/away
from the potential, but will still need to be monitored.

The mean upper ridge in the West Coast will keep precipitation
amounts low and limited to the Pacific Northwest. Farther inland
the northern/central Rockies could see light snow at times with
uncertain snow showers also possible farther east into the High
Plains. Additional rounds of precipitation may move into the
Northwest late week. Also of note, a pattern favorable for stronger
offshore winds is possible across southern California and vicinity
around early week which will increase fire weather concerns.
Continue to monitor Storm Prediction Center fire weather forecasts
for further information.


Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw