Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Sun Jan 12 2025
Valid 12Z Wed Jan 15 2025 - 12Z Sun Jan 19 2025
...Cold over the East to moderate after midweek followed by a
strong Arctic surge into the Rockies/Plains next weekend...
...Overview...
Latest guidance continues to show a rapid transition to a very
amplified upper pattern by next weekend, consisting of an eastern
Pacific through (and perhaps beyond) Alaska ridge and a deepening
trough most likely aligned from over/north of Hudson Bay through
the Plains and Southwest U.S. This evolution should bring much
colder air and some snow south into the Rockies and Plains by next
weekend, while a leading cold front spreads precipitation over the
eastern third to half of the country around the end of the week.
This may include heavy rainfall over parts of the South. Ahead of
this transition as of midweek, guidance is still trying to figure
out details of energy that should consolidate into an upper low
southwest of California (beneath an upper high briefly extending
into the Northwest) and then open up/eject inland as flow to the
north amplifies. Meanwhile the last in a series of central-eastern
U.S. shortwaves (during a chilly period over the East) should
cross the East Coast by early Friday. Most of the country should be
dry Wednesday-Thursday aside from some snow in the Great
Lakes/central Appalachians and a little snow starting to reach the
far northern Rockies.
...Guidance/Predictability Assessment...
During the first half of the period, primary forecast
considerations involve the still- challenging details of
southwestern U.S. into Pacific energy that should consolidate into
an upper low, shortwave energy progressing into southern Canada and
northern tier U.S. (leading to opening/ejection of the Pacific
upper low), and another shortwave over the central U.S. Overall the
latest machine learning (ML) models favor a more coherent upper
low to the southwest of California than what some operational model
runs have been showing lately, and not as far offshore as seen in
the western half of the dynamical model spread. These themes
recommend somewhat more ensemble mean weight in the forecast than
may typically be the case. Meanwhile 12Z ECMWF (deep) and recent
GFS runs (flat/weak) were on opposite extremes for the character of
the central U.S. shortwave that progresses quickly eastward after
early Wednesday. ML models favor an intermediate solution initially
and faster progression than in the 12Z ECMWF--leading to a drier
pattern from southern Texas through Florida versus that model run.
The new 00Z ECMWF has adjusted favorably toward other guidance in
this respect. For the shortwave entering western Canada and the
northwestern U.S. by Thursday, clustering has improved overall but
CMC runs have been persistently slow versus the majority. ML models
side with the non- CMC solutions with the shortwave, and over the
past two days have averaged in the lower half of the 990s mb
(comparable to a compromise among latest dynamical models) for the
surface low tracking near northern Minnesota/Lake Superior as of
Friday. Above considerations led to an early-period blend
consisting of 60 percent ensemble means (18Z GEFS and 12Z
ECens/CMCens) with remaining input from 12Z/18Z GFS runs and the
12Z ECMWF. The blend shifted to 60 percent total ECMWF/ECens weight
by early Friday to provide a better starting point for the
northern tier system at that time.
After early Friday, the overall pattern increases in predictability
given the large scale of mean features, though various lower-
predictability shortwave issues exist. Guidance distribution
appears most random in specifics of ejecting Pacific low energy,
which will likely have some influence on possible scattered light
precipitation over the Southwest and more so heavier activity
farther eastward, and northern tier into Great Lakes energy. Both
may have an effect on frontal waviness. Meanwhile, the ML models
show a similar distribution as dynamical guidance with respect to
the western/southwestern amplitude of digging western energy,
generally between the flat GFS/GEFS mean and most amplified CMC--
with the ECMWF runs tending to be the most consistent and leaning
toward the amplified half of the spread. The combined array of
ML/dynamical guidance favored tilting the late-period forecast
blend about 70 percent toward the ECMWF/ECens/CMCens relative to 30
percent GFS/GEFS.
...Weather/Hazards Highlights...
Portions of the East will see cold temperatures continue through
at least Wednesday. The core of most extreme anomalies should be
over and near the Ohio Valley with readings up to 15-20F or so
below normal. Expect some moderation on Thursday, on the way to
near or somewhat above normal temperatures over the East by the
weekend. The northern Plains and Upper Midwest will see a period of
above normal temperatures (perhaps 20-25F above normal for morning
lows, a little less extreme for daytime highs) mid-late week with
the southern tier also on the warm side around the end of the week.
The Northwest will start to see a cooling trend on Friday, but
then during the weekend the Rockies and Plains will see a
pronounced surge of Arctic air. By Sunday expect a broad area of
highs 15-35F below normal, with the greatest anomalies likely to be
over the central High Plains and vicinity.
Wednesday-Thursday will feature fairly low coverage of meaningful
precipitation across the lower 48. The best organized activity
should be across parts of the Great Lakes into the central
Appalachians with periods of snow from a combination of lake
effect and a weakening frontal system passing through the region.
Meanwhile strong Interior West and Rockies high pressure through
Wednesday will lead to a threat of high winds over parts of
southern California, enhancing the fire weather conditions.
Some light and scattered precipitation may reach the Northwest by
late week with the arrival of a front. Then expect upslope-
enhanced snow to move down the eastern slopes of the Rockies
Friday into the weekend behind the front. Farther east,
the leading portion of the front should eventually begin to
interact with increasing Gulf moisture to produce a broadening area
of precipitation over the eastern U.S. from Friday into the
weekend. Some heavy rainfall may be possible from the lower half of
the Mississippi Valley eastward across the South. Wintry
precipitation should be confined more to northern areas given the
warming trend over the East, though some areas in the middle
latitudes could see a changeover depending on the timing of
moisture relative to temperature trends.
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
