Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Tue Jan 14 2025

Valid 12Z Fri Jan 17 2025 - 12Z Tue Jan 21 2025

...Strong Arctic surge pushing into the central U.S. by the weekend
and gradually expanding eastward...

...Overview...

By this weekend the guidance continues to show the mean pattern
evolving into a very amplified regime consisting of a deep trough
extending from Canada through the lower 48 and a ridge whose axis
extends over and well northward from the eastern Pacific. As this
occurs, very cold air will drop into the central U.S. with the most
extreme anomalies likely to be during Sunday-Monday. The trough
axis should nudge far enough east by next Tuesday to offer some
moderation over the northern-central Plains while coldest anomalies
become aligned more over the East and South. Within this highly
confident large scale pattern, embedded shortwave uncertainties
continue to pose issues for specifics of potentially enhanced
rainfall over the South around the end of the week into the weekend
as well as for East Coast into western Atlantic frontal waviness
that could produce lighter precipitation of various types.
Meanwhile expect an area of snow to push south through the
Rockies/High Plains with the cold surge before tapering off next
week.

...Guidance/Predictability Assessment...

At the start of the period, a system forecast to track just north
of Minnesota and Lake Superior will be the main feature of note.
Among 12Z/18Z runs, recent trends and continuity (plus machine
learning guidance) favored ECMWF runs and the 12Z UKMET which were
in the middle of the spread. New 00Z runs seem to be narrowing
the spread. Meanwhile guidance has come into better agreement for
shearing shortwave energy that originates from a short-term upper
low to the southwest of California, but a lot of spread has
persisted for more significant shortwave energy digging into the
West during Friday-Saturday. ML models showed a fair amount of
spread as well but leaned away from the more extreme sides of the
dynamical envelope (mostly fast GFS runs and slow/amplified CMC
and CMCens runs). This led to a preference toward an average of the
12Z and 00Z/13 ECMWF runs along with the 12Z UKMET as well as the
12Z GFS that was closer than most other GFS runs. The intermediate
solution for the shortwave led to weak waviness from the East Coast
into western Atlantic during Sunday-Monday. ML models also depicted
this wave potential but with a fair amount of spread for timing,
but all keeping the wave fairly weak. New 00Z guidance offers some
hints at convergence as the previously slow ICON trended toward the
ECMWF runs and the CMC is not quite as extreme before (but still
extreme versus other guidance). The UKMET has trended faster
though. By late in the period there are various ides for shortwave
details over the West, though not much support exists for the
degree to which the 00Z GFS brings shortwave energy through the
Pacific ridge. Also some GFS runs have tended to bring the ridge
axis somewhat farther east than most other solutions. Finally, by
next Tuesday the ML models cluster fairly well in the direction of
ECMWF runs and 00Z ICON in bringing surface high pressure into the
southern tier versus some other solutions that keep it somewhat
farther north at that time.

Combined considerations for the above features led to a preference
consisting of relatively greater emphasis on the past two ECMWF
runs, with minority input from the 12Z GFS/UKMET. The forecast
introduced some ensemble mean input (18Z GEFS/12Z ECens) late in
the period, reaching a total of 40 percent by next Tuesday.

...Weather/Hazards Highlights...

The forecast is fairly consistent with respect to the Northwest
seeing a moderate cooling trend on Friday followed by a pronounced
surge of Arctic air dropping south over the eastern Rockies into
the Plains by this weekend. Expect increasing coverage of
temperatures that are 15-35F below normal, with expansion of the
cold air bringing highs down to 15-25F below normal as far east as
locations between the Mississippi River and Appalachians by Monday
and continuing toward the East Coast by Tuesday. The most extreme
anomalies are likely to be over the central High Plains and
vicinity during Sunday-Monday, while coldest temperatures on an
absolute basis should be over far northern areas where some subzero
highs are possible in that same time frame. The northern-central
Rockies/Plains should begin to moderate on Tuesday. The West will
see a period of cool but not extreme temperatures from the weekend
into the start of next week, with near normal readings eventually
beginning to push eastward from the West Coast. Ahead of the cold
front progressing south/east from the Plains Friday onward, an area
of above normal temperatures will progress from the central U.S. on
Friday to the East Coast by early Sunday. The Upper Midwest may see
lows that are 20F or more above normal early Friday, with plus
5-15F anomalies more common elsewhere within this area of relative
warmth.

By Friday night, approaching shortwave energy and a combination of
fronts may start to generate some enhanced rainfall across the
southern tier, over or east of the Lower Mississippi Valley. This
activity should continue into the Southeast during Saturday and
Saturday night with the supporting wavy surface front possibly
supporting the potential for some training. As the event starts,
guidance is clustering better for the shearing shortwave but
generally does not suggest high enough rain rates to cause runoff
issues. Then on Saturday there is potential for locally heavier
rainfall but with a majority of it over areas that have dry
antecedent conditions, plus with some model spread for specifics.
Therefore the Days 4-5 Excessive Rainfall Outlooks currently do
not show any risk areas, but will continue to monitor for any
trends toward a combination of better clustering and higher
rainfall totals. Some rainfall may persist over parts of the
Southeast beyond Saturday.

Much lighter precipitation of varying types will be possible
farther north over the East during the weekend and possibly into
early next week. There are still significant differences for
potential frontal wave development, keeping confidence low for
details of precipitation coverage and amounts. However what signals
exist suggest that any waves should be fairly weak with relatively
light precipitation (which could be of the wintry variety from the
Mid-Atlantic/Appalachians northward). Probabilities of 0.25 inch
liquid in the form of snow reflect this, with low coverage of 10
percent or greater values. Some locally heavier snow will be
possible to the lee of the Great Lakes from the weekend into early
next week. Farther west, expect snow to move southward across the
Rockies/High Plains late this week into the weekend as part of the
Arctic surge, with upslope flow helping to enhance accumulations
over some areas.

Rausch


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