Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Sat Jan 11 2025

Valid 12Z Tue Jan 14 2025 - 12Z Sat Jan 18 2025

...Arctic airmass to reinforce east of the Rockies next week...


...Overview...

Into midweek the guidance shows a ridge building into the Northwest
while elongated energy to the south most likely forms a closed low
somewhat southwest of southern California, while deeper mean
troughing extends over the northeastern U.S. into the Plains. Then
shortwave energy reaching British Columbia by early Thursday will
amplify south/east with time while eastern Pacific/Alaska ridging
builds, by Saturday yielding a northern Canada through
southwestern U.S./Rockies trough that could also include the
sheared remains of the upper low previously off California. Eastern
U.S. mean troughing should pass into the Atlantic by the end of
the week. This pattern evolution will lead to cold temperatures
over the East Tuesday-Thursday followed by another push of cold air
dropping down the High Plains and vicinity by next Saturday. The
Great Lakes will see a combination of lake effect and synoptic-
system snow during the first half of the period. Then by next
Saturday organized precipitation should develop over the east-
central U.S. along a cold front, while an area of snow pushes down
the northern half of the Rockies.

...Guidance/Predictability Assessment...

Based on guidance available through the 18Z cycle, a blend of the
last two 6-hourly GFS runs and last two 12-hourly ECMWF runs
(slightly more weight to the more recent runs) provided the best
representation of dynamical and machine learning (ML) guidance
ideas through the period while downplaying lower-predictability
details in any one model run. This solution fit within the theme of
the ensemble means while offering better detail, while embedded
detail differences suggest a lingering potential for adjustments
with some small-medium aspects of the forecast. The 12Z CMC/UKMET
differed from the remainder of guidance in one or more ways,
favoring their exclusion from the forecast.

As for some specifics, exactly how southwestern U.S. into Pacific
energy splits through midweek is still up for debate, with perhaps
even more spread today regarding how the offshore closed low may
form and what energy could linger inland and then progress
eastward (possibly leading to a Gulf wave and some precipitation as
per the new 00Z ECMWF). ML models show somewhat cleaner upper low
formation than some dynamical guidance but still with a lot of
spread for specifics. These issues will also affect how quickly the
upper low energy opens up as troughing amplifies to the north.

Regarding that troughing, most guidance is consolidating for the
shortwave energy reaching western Canada by early Thursday. Latest
GEFS mean runs and 00Z CMC are somewhat slow though. By Friday the
ML models support a moderately deep (a compromise closer to recent
GFS/ECMWF runs versus the ensemble means) surface low reaching
near/north of Minnesota/Lake Superior. Then by Saturday the ML
models show a fair amount of spread for the exact character of the
amplifying northern stream trough energy, so a multi-model/run
approach provides a reasonable starting point with the expectation
that some details will likely change.

...Weather/Hazards Highlights...

The forecast pattern will support another surge of Arctic air that
should reach the eastern half of the country by Tuesday-Thursday.
Of particular note, expect areas of hazardous cold to extend from
the Upper Midwest into the southern half to two-thirds of the East
for one or more days during that time frame. Temperatures should
be 10-25F below normal with even colder wind chills. A few
locations around the upper Ohio Valley/central Appalachians could
challenge daily records for cold highs on Tuesday. Cold cyclonic
flow will promote periods of lake effect snow, some of it heavy,
through Thursday while a frontal system may produce some light
synoptic-scale snow on Thursday as well. Eastern U.S. temperatures
will moderate by Friday-Saturday as a front pushes into the central
U.S. and eventually Great Lakes. This front may also begin to
generate precipitation over the east-central U.S. by Saturday with
some heavier totals possible somewhere between the Ohio River and
Gulf Coast.

Elsewhere, a wavy front may bring lingering rainfall to the Florida
Peninsula into Tuesday while some mostly light rain may extend
into southern Texas for a time. Upper ridging that builds over
Northwest through Thursday should keep most of the western U.S.
dry. However, strong Interior West and Rockies high pressure
through Tuesday-Wednesday will lead to a threat of high winds over
parts of southern California. Some light and scattered
precipitation may reach the Northwest by late week with the
arrival of a front. Then some upslope-enhanced snow may move down
the eastern slopes of the Rockies Friday-Saturday behind the front.
Slightly below normal temperatures over the West will moderate to
near/slightly above normal into Thursday, followed by a cooler
trend in the Northwest on Friday and much colder readings dropping
through the northern-central Rockies on Saturday (highs 10-20F or
so below normal). Warmest temperature anomalies next week should be
over the northern Plains/Upper Midwest during Wednesday-Friday
with some locations 10-20F above normal for highs with anomalies
perhaps a few degrees higher for morning lows. The southern Plains
into the South may see some above normal temperatures Friday-
Saturday.

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