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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Mon Jan 13 2025

Valid 12Z Thu Jan 16 2025 - 12Z Mon Jan 20 2025

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

...Overview...

It generally remains the case that the models and ensembles still
show the mean pattern evolving toward a very amplified regime
consisting of a ridge extending from the eastern Pacific well
northward and a Hudson Bay into northern Mexico mean trough by the
weekend. As a result expect very cold air to drop into the central
U.S. then, with below normal temperatures expanding into the
eastern half of the country by next Sunday- Monday. Although the
large scale pattern has well above average
confidence/predictability, there are embedded shortwave details
that guidance has been showing significant difficulty in resolving.
As a result confidence is below average for any associated surface
waves that could have a significant influence on sensible weather
over the East (with rain south and potential wintry weather farther
north) from the weekend into early next week. There are also
uncertainties with a leading shortwave crossing the southeastern
quadrant of the lower 48 around Thursday and a lower latitude
Pacific upper low whose energy should open up and eject across the
lower 48.

...Guidance/Predictability Assessment...

For early in the period on Thursday, there are still notable
differences with a shortwave forecast to cross the southeastern
quadrant of the country. In particular, the 00Z GFS is a deep and
slow extreme relative to other new guidance and this results in the
model run spreading much more rainfall across Florida. While the
majority of guidance is signaling a weaker/faster shortwave with a
corresponding drier forecast over Florida, there have been previous
random solutions with a GFS-like scenario. Preferences side with
the majority scenario but with the understanding that a low-
probability risk still exists for the alternative. The 06/12 UTC
GFS has trended from the outlier 00 UTC GFS run toward concensus.

Guidance has recently nudged a little stronger with the system
forecast to track near northern Minnesota/Lake Superior around
Friday and continue into eastern Canada thereafter. Amplifying
dynamics will push the trailing cold front south and east, with
notable spread/continuity changes for shortwave energy forecast to
drop through the West. Recent ECMWF/GFS have been somewhat more
progressive than CMC/CMCens runs in showing that show the greatest
amplitude/sharpness of western energy. Early on some of these
differences influence the details of shearing Pacific upper low
energy (with some influence on surface waviness/southern tier
rainfall farther east). 12 UTC models generally maintain these
still pluasible guidance camps without clear resolve.

Then, shortwave differences have a more significant influence on
potential surface development toward next Sunday-Monday. Latest
ECMWF runs and some machine learning (ML) models show potential for
some combination of shortwave energy to support a Mid-Atlantic
wave that tracks just offshore New England and into or south of
the Canadian Maritimes. Yesterday's 18Z GFS fit into this cluster
as well, but yesterday's 12Z and new 00Z runs show a more inland
surface low track that seems to have minimal support from other
guidance. The newer 12 UTC GFS now develops a organized low, but
just offshore. CMC/ICON runs show a flatter evolution due to
holding the sharpest upper troughing farther west. Finally, there
is a fair amount of spread among dynamical/ML models for the
strength and location of high pressure over the central U.S. by
next Monday, with no dominant clustering evident at this time.

Accordingly, the WPC medium range product suite was primarily
derived from a broad composite model and ensmeble mean blend
through medium range time scales. This remains broadly aligned
with WPC product continuity and the National Blend of Models.

...Weather/Hazards Highlights...

After a chilly start to Thursday over parts of the East, expect
temperatures to moderate from late week into the weekend as above
normal temperatures over the Plains/Mississippi Valley on
Thursday-Friday (lows up to 15-25F above normal over the Upper
Midwest) then reach the East with less pronounced anomalies ahead
of a cold front. Behind the front, expect the Northwest to see a
cooling trend on Friday, but then during the weekend the eastern
Rockies into the Plains will see a pronounced surge of Arctic air.
Expect increasing coverage of temperatures that are 15-35F below
normal, with expansion of the cold air bringing highs down to
10-20F below normal over the East by next Monday. The most extreme
anomalies are likely to be over the central High Plains and
vicinity, while coldest temperatures on an absolute basis should be
over far northern areas where some subzero highs are possible.

Much of the lower 48 should be fairly dry on Thursday. 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. By Friday or Friday night, approaching shortwave energy and
a combination of fronts may start to generate some enhanced
rainfall across the southern tier. However differences in shortwave
details lead to a fair amount of spread for location and magnitude
of amounts. Therefore the Day 5 Excessive Rainfall Outlook does not
reflect any risk areas at this time. Potential exists for much of
the East to see at least some precipitation in the Saturday into
Sunday or Monday period, but guidance differences for potential
surface development keep confidence low for details of
precipitation coverage and amounts. Best potential for wintry
weather will be from the Mid-Atlantic northward, with rain farther
south and a possible transition zone in-between. 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 enhanced accumulations over some areas.

Rausch/Schichtel


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
220 PM 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 the eastern Pacific and well northward. 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...

While models agree regarding upper troughing over the
central/eastern CONUS and an Arctic high coming in at the surface,
systems on the periphery of these features are more uncertain and
could have sensible weather impacts. At the start of the period, a
low pressure system forecast to track just north of Minnesota and
Lake Superior and its trailing front will be the main feature of
note. A multi-model blend seemed reasonable for this system as it
pushes through the East and forms relatively weak (until perhaps
strengthening once offshore) low pressure waves along it.

Meanwhile there are likely to be multiple shortwaves digging south
through the western side of the trough, creating some differences
in the upper pattern from the eastern Pacific to the Rockies during
much of the period. The 00Z CMC was aggressive enough with
shortwave energy to reorient the trough axis to be more positively
tilted over the weekend, unlike consensus. The newer 12Z CMC run
trended more favorably toward the other models. Differences with an
initially eastern Pacific upper low continue to cause model
differences downstream as it pivots eastward. Deterministic and AI
models and ensemble members vary quite a bit on whether/how much
energy is held back into the Southwest by next Tuesday. The general
trend over the past day seems to be for more energy to separate
into the southern stream at least somewhat from the main trough.
Trends will continue to be monitored because this will have
considerable effects on forecast aspects like precipitation that
could be wintry next week.

The WPC forecast used a blend of the 00Z ECMWF/06Z GFS/00Z CMC/00Z
UKMET in decreasing proportional order early in the period, but
phased out the CMC and UKMET in favor of the GEFS and EC ensemble
means, and increased the proportion of means to half Days 6-7 while
maintaining some EC and GFS inclusion.


...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 and lows in the -20s are expected. Wind chills should be
even colder and more dangerous. Key Messages are set to begin from
WPC in part due to these cold temperatures. The northern-central
Rockies/Plains should begin to moderate on Tuesday while cold
weather is expected to linger farther south. 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 over the Lower
Mississippi Valley and Southeast. The reasonably fast frontal
movement and moderate rain rates should keep the flash flooding
potential pretty minimal for the Day 4 ERO time frame. This
activity should continue across the Southeast during Saturday and
Saturday night with the supporting wavy surface front lying down a
bit to become more east-west oriented, possibly supporting some
training. Thus there is some potential for locally heavier rainfall
into the Day 5/Saturday ERO period in the Southeast and Florida
Panhandle. As rain occurred in that general vicinity yesterday,
soil moisture percentiles are actually showing some above average
anomalies there currently, but soils should dry out by late week.
Continued to hold off on introducing any ERO areas as there is
uncertainty in whether the forecast rain amounts would cause
flooding issues, but there is likely a nonzero threat especially if
training or repeating showers and storms with heavy rates occur.
Some rainfall may persist over parts of the Southeast, especially
northern Florida, 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, though higher elevations like the
Adirondacks have higher probabilities. Some locally heavier snow
will be possible to the lee of the Great Lakes from the weekend
into early next week, as well as across the Appalachians with
upslope flow.

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. Snow squalls are of concern Friday with the cold
frontal passage. Into Monday or Tuesday of next week, it is
uncertain how much precipitation emerges over the southern tier
from Texas eastward on the periphery of the upper trough and
surface high pressure. However, the southern extent of the cold air
at the surface means that if there is precipitation it would
mainly be in the form of wintry precip, perhaps all the way to the
Gulf Coast, and freezing rain/ice is an increasing concern for
parts of the South given the possibility of a warm nose atop Arctic
cold air.

Uncertainties in the setup aloft over the Southwest into next week
make the pattern at the surface somewhat uncertain as well.
However, there is a general trend for weaker winds over southern
California Friday into the weekend, but offshore flow may
strengthen again by Monday as 850mb winds increase (dramatically in
some models). Keep an eye on SPC fire weather forecasts for
further information.


Tate/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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
202 PM EST Wed Jan 15 2025

Valid 12Z Sat Jan 18 2025 - 12Z Wed Jan 22 2025

...Strong Arctic surge will push into the central U.S. this
weekend and expand east early next week...

...Overview...

A very amplified pattern is expected through the medium range
period, with an eastern Pacific ridge and a deep trough extending
through the lower 48. This pattern will allow Arctic air to surge
south into the central U.S. this weekend, and the anomalously cold
air mass will expand east and southeast early next week. Within the
highly confident overall trough, guidance is coming closer to
converging for details of a couple lower-predictability leading
shortwaves--one aiding in potentially heavy rain over parts of the
Southeast on Saturday and another that may produce some snow over
the Rockies on Saturday and then a progressive wave with mostly
light precipitation of various types near the East Coast on Sunday.
Guidance is also starting to pick up on a late period wave that
could create winter weather concerns in the southern U.S. mid-next
week.


...Guidance/Predictability Assessment...

Model guidance has come into good agreement on the overall pattern
and shortwave details through the weekend. On Saturday, a sheared
shortwave will push east-northeast across the southern tier while a
second shortwave moves south across the Rockies. This secondary
shortwave will move quickly east on Sunday, swinging across the
South to the Mid-Atlantic. There are still minor differences in the
timing of the second shortwave, but overall model agreement on
these features is good.

Model spread increases significantly next week as they try to
resolve details of shortwaves in the West. Another shortwave will
likely drop south into the western U.S. by Monday, but the
subsequent evolution of this feature is uncertain. The evolutions
will be influenced by timing and strength of another shortwave that
will likely move into the Northwest mid-next week, and there is a
lot of spread for how this feature may pass through the mean ridge
to the west. However the pattern evolves, the character of this
energy will play an important role in the surface pattern across
lower latitudes along with the coverage and amounts of what could
be meaningful wintry precipitation over parts of the South.

For WPC's afternoon forecast, a blend of the GFS/ECMWF/CMC/UKMET
were used through the weekend with heavier weight placed in the GFS
and ECMWF which are the best clustered and are demonstrating the
most skill with this weather pattern. For Monday-Wednesday,
increasing amounts of the GEFS and ECENS ensemble means were added
to the blend while the UKMET and CMC amounts were gradually
decreased.


...Weather/Hazards Highlights...

The well-advertised surge of Arctic air will bring hazardous cold
to much of the lower 48 this weekend into early next week. Cold
will spread from the Rockies and Plains on Saturday to the South
and Midwest on Sunday and to the East and Southeast on Monday.
Temperature anomalies are forecast to reach 15-35 degrees below
normal, with the most extreme anomalies in the Plains. Coldest
temperatures on an absolute basis should be over northern Plains
and Upper Midwest where subzero highs and lows in the -20s are
expected. Wind chills should be even colder and more dangerous. WPC
has begun posting Key Messages regarding this cold outbreak. The
northern-central Rockies/Plains should moderate considerably by
Tuesday-Wednesday while cold weather will persist into Wednesday
across the South and East. 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 slowly beginning to push eastward
from the West Coast.

Shortwave energy moving over the southern U.S. will push a low
pressure system across the Southeast this weekend. The leading
front may become east-west oriented, which would support training
showers and thunderstorms that could produce locally heavy
rainfall. Thus, there is a Marginal Risk of heavy rainfall for
portions of the Southeast on Saturday (Day 4 ERO). This system
should quickly move northeast ahead of an approaching cold front
later this weekend, which will lead to a decreased risk of heavy
rainfall in the Southeast for Sunday (Day 5 ERO). Precipitation
will also extend well to the north ahead of the approaching cold
front this weekend, with precipitation chances for most areas east
of the Mississippi River and chances for wintry precipitation for
much of the Northeast and central Appalachians.

Farther west, expect snow to move southward across the
Rockies/High Plains into Saturday in association with the Arctic
surge and digging shortwave energy. Upslope flow should help to
enhance accumulations over some areas. Another shortwave may
produce some snow over parts of the Rockies early next week. What
this shortwave does will determine how much precipitation emerges
over the southern tier from Texas eastward on the southern
periphery of the overall upper trough and surface high pressure.
However, the southern extent of the cold air at the surface means
that if there is precipitation it would mainly be in the form of
wintry precip, perhaps all the way to the Gulf Coast, and freezing
rain/ice is an increasing concern for parts of the South given the
possibility of a warm nose atop Arctic cold air. Southern tier snow
potential shows up in the winter weather probabilities for 0.25
inch liquid in the form of snow, now 10-15 percent over an area
just north of the western half of the Gulf Coast for the early
Monday-early Tuesday period and about 10-35 percent across a much
broader swath of the South into southern Mid-Atlantic early Tuesday
into early Wednesday.

Uncertainties in the setup aloft over the Southwest into next week
make the pattern at the surface somewhat uncertain as well.
However, there is a general trend for weaker winds over southern
California into the weekend, but offshore flow may strengthen again
by Monday as 850mb winds increase (dramatically in some models).
Keep an eye on SPC fire weather forecasts for further information.


Dolan/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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
209 PM EST Thu Jan 16 2025

Valid 12Z Sun Jan 19 2025 - 12Z Thu Jan 23 2025

...Strong Arctic surge will push into the central U.S. this
weekend and reach the East by early next week...

...Overview...

Models and ensembles continue to show a very amplified mean pattern
consisting of a deep trough from northeastern Canada through the
lower 48 and an eastern Pacific ridge extending well northward.
Thus confidence remains high in the forecast of Arctic air dropping
southward over the central U.S. this weekend and expanding to
include much of the East by early next week. By midweek most of the
Plains should see moderating temperatures while the cold will
likely linger over the South and East. In contrast, guidance is
having a lot of difficulty in resolving important embedded
shortwaves that will play an important role in determining the
coverage and amounts of potential wintry precipitation across the
East on Sunday and then the South into next week.


...Guidance/Predictability Assessment...

There is still a frustrating amount of model spread with a
shortwave and surface low/frontal wave impacting the East on Sunday
that affects snow chances and amounts. CMC runs have been farther
inland with the low track which in turn tracks QPF (snow) farther
west into the Mid-Atlantic and Northeast and even keeps the coastal
Mid-Atlantic with rain as the precipitation type. ECMWF runs have
generally been more suppressed with the low track to keep snow in
the southern Mid-Atlantic and offshore of New England farther
north, with the exception of the 06Z run, and the 12Z run trended a
little favorably. The EC-based AIFS on the other hand has been
fairly persistent in its precipitation axis across the Mid-Atlantic
and coastal Northeast, and the UKMET is similar. GFS runs had been
suppressed but have gradually been trending northward. Thus the
general trend is for increasing precipitation and snow
probabilities across the urban corridor from DC to NYC to Boston
with a low track that is in the middle of the model spread. But
small shifts in the low track will cause dramatic differences in
impactful snow, so continue to monitor forecasts as this system
moves into the short range.

Energy digging south through the western side of the broad trough
early next week will also be a factor in model uncertainty on the
periphery of the broad trough. Models as of about a day ago tended
to hold more energy back in the Southwest to produce positively
tilted troughing around Tuesday. Today's model guidance (with the
exception of the CMC) is trending away from that pattern in favor
of a small closed upper low to the west of the main trough in the
eastern Pacific to Southwest or Great Basin. There are, of course,
some model differences in when/how that feature gets absorbed back
into the mean flow. These model differences have ramifications for
how much precipitation (wintry in most areas) affects the south-
central U.S. to Southeast. In a general sense, CMC ensemble members
have more wintry precipitation across the South compared to the
more suppressed GEFS and ECMWF members. Then by Wednesday-
Thursday, the broad trough axis should finally lift and push
eastward, while there is general model agreement for more troughing
to drop into the northern Rockies or so.

The WPC forecast was based on a multi-model and ensemble mean
blend early in the forecast period given the model variations even
at the beginning. Increased the proportion of the ensemble means to
well over half by Days 6-7 due to the detail differences among the
relatively predictable large scale pattern.


...Weather/Hazards Highlights...

By Sunday a broad area of hazardously cold Arctic air will already
cover the central U.S. and should expand to cover much of the east
by Monday. Expect many areas to see highs at least 20F below
normal, with the coldest anomalies likely to be over the High
Plains and vicinity where highs could be 30-40F below normal.
Coldest temperatures on an absolute basis should be over northern
Plains and Upper Midwest where subzero highs and lows in the -20s
are likely. Wind chills should be even colder and more dangerous.
WPC is posting Key Messages regarding this cold outbreak. The
northern-central Rockies/Plains should moderate considerably by
Tuesday-Wednesday (though with a potential for high winds) while
cold weather may persist into Wednesday and possibly to some extent
Thursday across the South and East. The West will see a period of
cool but not extreme temperatures from the weekend into the start
of next week, with near normal and then slightly above normal
readings slowly beginning to push eastward from the West Coast.

The wave forecast to track quickly off the East Coast on Sunday is
likely to produce some possibly impactful precipitation before its
departure. As discussed in the above section, chances for plowable
snow are trending up in the urban corridor with a
compromise/middle ground forecast track for the low, but
uncertainty remains. After this system exits, the pattern will be
favorable for lake effect snow through midweek. Farther south,
fronts pushing through Florida could provide some focus for rain.

Farther west, a shortwave digging into the West early next week
may produce some scattered snow depending on its evolution. What
this shortwave does after Monday will determine how much
precipitation emerges over the southern tier from Texas eastward on
the southern periphery of the overall upper trough and surface
high pressure. The southern extent of the cold air at the surface
means that a majority of precipitation could be of a wintry
variety, perhaps all the way to the Gulf Coast. Freezing rain/ice
is also an increasing concern for parts of the South given the
possibility of a warm nose atop Arctic cold air. Southern tier snow
potential continues to show up in the winter weather probabilities
for 0.25 inch liquid in the form of snow, 10-20 percent over an
area just north of the western Gulf Coast for the early Monday-
early Tuesday period and 10-40 percent across a much broader swath
of the South into southern Mid-Atlantic early Tuesday into early
Wednesday. Some precipitation potential lingers thereafter, but
through this whole period confidence in details is below average
due to considerable model differences and variability.

Uncertainties in details aloft over the Southwest into next week
make the surface pattern uncertain as well. However, there is some
potential for a period of relatively stronger winds over parts of
California and vicinity around Monday-Tuesday. Continue to monitor
Storm Prediction Center fire weather forecasts for further
information.


Tate/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