Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
215 AM EST Thu Jan 16 2025

Valid 12Z Thu Jan 16 2025 - 12Z Sat Jan 18 2025

...Improvement in fire weather conditions across southern California...

...A brief moderation of temperatures across the central U.S. before an
arctic front plunges into the northern U.S. on Friday...

...Lake effect snow expected on Thursday and snow also expected for the
central Appalachians...

There will be a few things making weather headlines through Friday night
across the country. Lake effect snow is expected in the wake of a
shortwave passage, and the heaviest snowfall will be downwind of Lake Erie
across western New York on Thursday, where lake effect snow warnings are
now in effect. Moderate to heavy snow is also on the way for the central
Appalachians with moist upslope flow leading to 5-10 inches of
accumulation across the higher terrain of West Virginia into south-central
Pennsylvania, where winter storm warnings are in effect.

A pattern change will evolve across the Central U.S. and the Rockies going
into Friday and especially into early Saturday. After a brief moderation
in temperatures across the north-central U.S. on Thursday, an abrupt
change to reality is coming by Friday as a pair of strong cold fronts
heralds the arrival of frigid temperatures and brutally cold wind chills.
This will continue well beyond the short range forecast period, and the
Weather Prediction Center has Key Messages regarding this arctic blast.
This could also be accompanied by some snow showers and a few snow squalls
across the northern Rockies and western High Plains going into Friday and
Friday night, resulting in mainly light accumulations but accompanied with
poor visibilities and gusty winds.

For the Eastern U.S., a gradual moderation trend in the recent very cold
conditions is on the way to conclude the work week with readings returning
to near seasonal averages for the East Coast, and above average for much
of the Southeast states. A developing low pressure system over the Deep
South Friday night will likely lead to increasing showers and perhaps a
few thunderstorms near the central Gulf Coast region by Saturday morning,
but this region should remain dry until then.

Things are starting to look better in terms of the forecast across
California and the areas that have been devastated by the ongoing
wildfires. Even though no rain is in the forecast, the winds are expected
to switch to a more onshore flow late Thursday and into Friday, bringing
higher relative humidities and less chaotic wind flow, thus helping to
mitigate the wildfire threat compared to recent days.

Hamrick


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM 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
South next week, with some lingering issues also continuing for a
progressive wave tracking off the East Coast on Sunday.


...Guidance/Predictability Assessment...

Very early in the forecast, spread persists for the details of a
shortwave reaching the eastern half of the country by Sunday along
with its accompanying wave that should track off the East Coast and
most likely reach a position just south or east of Nova Scotia by
early Monday. The relative majority of guidance (also accounting
for the 12Z machine learning models) clusters within the realm of
the ECMWF/UKMET/ICON for track and timing. In contrast to recent
GFS tendencies to be on the faster side for the shortwave and
surface feature, latest runs have adjusted slower than other
guidance. Meanwhile the latest CMC runs are a deep and western
extreme with the surface low (leading to heavier Northeast
precipitation than other guidance), though their upper dynamics
finally look a lot better versus consensus than in recent days when
they were excessively slow/amplified with the originating energy
over the West.

Detail confidence drops precipitously behind this feature, as
models show wide variations among each other and from run to run
for important shortwaves. The first one should dig into the West by
next Monday, with its eventual evolution affecting how much wintry
precipitation may spread across the southern tier early-mid week.
Through the 12Z/18Z model runs, there was a prominent clustering
toward greater southwestward elongation of this shortwave versus
the GFS. This was due to the majority not dampening the northern
side of the upstream ridge as much as GFS runs. GEFS means have a
GFS flavor but show some nudge toward a little more trough
amplitude. Meanwhile the 12Z ML models also show more elongation
than the GFS but with faster timing than most of the dynamical
models. The past couple UKMET runs appeared closest to the ML model
ideas. The new 00Z ECMWF has introduced a complication in that it
now looks more like a slightly slower version of the GFS. As for
eventual effects across the South, the overall array of guidance
has supported a moisture shield somewhere in the realm of the prior
two ECMWF runs (12Z and 00Z/15) and the GEFS/ECens means--between
the extremes of the relatively dry GFS and heavy/northward CMC.
While some aspects of guidance actually trends a little closer
together as the next bundle of shortwave energy drops into the mean
trough by Wednesday-Thursday, significant differences develop
regarding whether/how much flow splits over the east-central
Pacific. This does not bode well for the forecast immediately
after early Thursday.

Based on the above considerations, the early part of the updated
forecast started with the 12Z ECMWF/12Z UKMET/12Z GFS in order from
more to less weight. The 12Z GFS was little more palatable than the
18Z run where differences existed. The blend started to introduce
the 18Z GEFS/12Z ECens means by Day 5 Tuesday and reached 60
percent total mean weight by the end of the period, with 30/10
lingering percentages of the 12Z ECMWF/GFS respectively. This
approach helped to provide the best continuity possible under the
circumstances while waiting for better eventual agreement for
details.


...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 may
produce a band of mostly light precipitation before its departure.
Some wintry weather is possible over parts of the Mid-Atlantic and
southern New England. Although there is an intermediate majority
cluster for this system in the guidance, there are still a couple
solutions on the extreme ends of the envelope so continue to
monitor forecasts.

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, now 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.


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








/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

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
208 AM EST Thu Jan 16 2025

Valid 00Z Fri Jan 17 2025 - 00Z Fri Jan 24 2025

Today's guidance continues to show good agreement and continuity
into early next week, followed by increasing divergence from
Tuesday onward. A front passing through the main islands
Thursday-Friday will disrupt the trades and possibly bring some
pockets of heavier rainfall, though higher totals should remain
well to the north. High pressure behind this front and a weaker
trailing front to the northwest will likely bring moderate to
brisk northeasterly winds for the weekend into early next week.
Guidance is consistent in showing below-climatology precipitable
water values behind the late-week front, keeping most showers
fairly light during the weekend and first part of next week.

Today's guidance shows a similar divergence from 24 hours ago
Tuesday onward with an amplifying upper trough and leading cold
front. Among the standard dynamical guidance, the ECMWF/CMC and
their ensemble means are still slower and more amplified with the
trough versus the latest GFS runs. The 00Z GEFS mean is somewhat
of a compromise. At the surface this makes the difference between
a slower/wetter frontal passage on Wednesday in the ECMWF cluster
versus a faster/drier one in the GFS, along with trailing high
pressure restoring trade flow more quickly. Recent machine
learning models have displayed typical spread, but on average
continue to recommend a compromise solution (which is still
preferred) for the upper trough and frontal progression. The 00Z
ECMWF run has trended lighter with its QPF along the front versus
previous runs, which is one step toward a compromise even if its
timing is still relatively slow.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1104 AM EST Thu Jan 16 2025
Day 1
Valid 16Z Thu Jan 16 2025 - 12Z Fri Jan 17 2025


The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1104 AM EST Thu Jan 16 2025
Day 2
Valid 12Z Fri Jan 17 2025 - 12Z Sat Jan 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1104 AM EST Thu Jan 16 2025
Day 3
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

PW values expected to rise to 2 to 2.5 standard deviations above
the mean Saturday into early Sunday along and ahead of the arctic
frontal boundary pushing across the northeast Gulf into the
Southeast and North Florida. The axis of strong low level west
south westerly flow along and ahead of the arctic front will slow
for a period late Saturday into early Sunday as mid level height
falls dive into the base of the broad vortex across eastern North
America. This will support potential for training of precip areas
along and just to the south of the arctic front across far southern
Georgia into North Florida. The non NCEP models...EC..CMC...CMC
GEM and UKMET...are showing a more organized axis of heavy rainfall
potential than the GFS and NAM. WPC qpf is leaning more toward the
non-NCEP guidance given the anomalous PW values and potential for
a period of training. The expected axis of heavy rains is forecast
across regions that have seen increases in relative soil moisture
over the past week from recent heavy rains that have stretched from
the central Gulf Coast into the FL Panhandle and North Florida.
Still, FFG values remain high, with the forecast additional
rainfall amounts of 1-2" likely only resulting in isolated runoff
issues.


Oravec

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Jan 16 2025


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 may
produce a band of mostly light precipitation before its departure.
Some wintry weather is possible over parts of the Mid-Atlantic and
southern New England. Although there is an intermediate majority
cluster for this system in the guidance, there are still a couple
solutions on the extreme ends of the envelope so continue to
monitor forecasts.

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, now 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.


Rausch

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Jan 16 2025


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 may
produce a band of mostly light precipitation before its departure.
Some wintry weather is possible over parts of the Mid-Atlantic and
southern New England. Although there is an intermediate majority
cluster for this system in the guidance, there are still a couple
solutions on the extreme ends of the envelope so continue to
monitor forecasts.

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, now 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.


Rausch

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Thu Jan 16 2025

Valid 12Z Thu Jan 16 2025 - 12Z Sun Jan 19 2025

...Great Lakes & Central Appalachians...
Days 1...

A potent 500mb trough diving south through the Great Lakes this
morning will foster healthy upper level divergence over the
Northeast and Central Appalachians today while also directing
plume of 700-300mb layer-averaged moisture through these regions.
In addition, the brief shot of low-level CAA will trigger some
lake-effect snow showers today with the Chautauqua Ridge sporting
moderate-to-high chances (50-70%) for snowfall totals >4" through
Thursday evening. The Central Appalachians are most favored for
heavy snowfall totals given their favorable position beneath the
diffluent left-exit region of a 100kt 500mb jet streak and upwards
of 40kt westerly winds at 850mb aiding in healthy upslope snowfall.
Snow looks to fall heaviest beginning late morning and lasting
through the afternoon with rates topping 0.5"/hr in some cases. In
fact, given the steepening lapse rates Thursday afternoon over the
Mid-Atlantic, residents in the region should be on the lookout for
possible snow squalls given the favorable time of day and more than
adequate upper-level divergence aloft. Snow in the Central
Appalachians looks to continue Thursday night and finally tapers
off by Friday morning. WPC probabilities depict high chances
(>70%) for snowfall totals >6" from the Laurel Highlands of
southern Pennsylvania and near the MD/WV border to the Allegheny
Highlands in West Virginia. Some of the taller peaks of eastern
West Virginia could see totals around a foot of snow by Friday
morning.

Day 3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that will deliver a frigid air-mass that
is the coldest and most dangerous of the season this weekend and
into next week (see Key Messages linked below). The Arctic front
will turn on the LES machine over Lake Superior by early Friday
morning while periods of snow develop along and in wake of the
Arctic frontal passage over the Great Lakes Saturday afternoon,
then over the eastern Great Lakes and central Appalachians Saturday
evening. LES bands look to form over parts of central and western
New York Saturday night that likely persist through the remainder
of the weekend. Through 12Z Sunday, WPC probabilities show
moderate-to-high chances (50-70%) for snowfall >4" and low chances
(10-30%) for snowfall >6" in the Tug Hill Plateau, but expect
these probabilities and snowfall totals to rise as the LES bands
that linger in the medium range enter the short range over the next
24-48 hours.

...Rockies and High Plains...
Days 2-3...

The impending snowfall in these region, starting late Thursday
night in the northern Rockies/High Plains, is driven by a
combination of both upper level disturbances and the arrival of a
bitterly cold Arctic air-mass that is destined to be the coldest
air-mass of the season to infiltrate the Lower 48. This
exceptional cold front (a "blue norther") out ahead of will push
south Thursday night at the same time as 500mb PVA ahead of an
approaching upper trough occurs over the northern Rockies/High
Plains. Snow showers will breakout across Montana early Friday
morning and make its way south throughout the day into Wyoming,
then into Colorado by Friday night. Southern Montana and into
Wyoming, in particular, sport notably higher snow squall parameters
given the stronger surface- based heating that will cause steeper
low-level lapse rates. Snow squalls would cause rapid reductions in
visibility due to a combination of heavy snow rates and whipping
wind gusts, as well as rapid accumulations on roads as temperatures
plummet well below freezing. Motorists in these areas should keep
a close eye on potential snow squalls as it can cause dangerous
travel conditions in a matter of seconds.

As the front plunges south Friday evening, the snow potential
shifts south into Colorado with periods of snow getting started
near the start of the evening rush hour in the Denver/Boulder metro
area. While the snow squall potential may not be as high, the
easterly upslope-enhancement of snowfall rates will make for
occasional periods of heavy snow Friday evening. Snowfall rates
will weaken as the front works its way south of the Palmer Divide
Friday night and down the spine of the Sangre De Cristo early
Saturday morning with some snow along the Front Range lingering
into the day on Saturday as a dome of ~1050mb high pressure builds
in from the Canadian Prairies. WPC probabilities are keying in on
the Front Range as the focus for the heaviest snowfall with
moderate-to-high chances (50-70%) for snowfall >4" from the Laramie
Range on south to parts of the Sangre De Cristo. The I-25 corridor
from Fort Collins on south to the Colorado Springs area Friday
have similar probabilities for >4" of snowfall with even low
chances (10-30%) for greater than 6" between Friday evening and
Saturday morning in spots. Some of the higher peaks (>10,000ft)
have moderate chances (40-60%) for snowfall >8" through Saturday
morning. The WSSI is currently depicting Minor Impacts for just
about all of these referenced areas, suggesting the potential for
winter weather conditions that would require enhanced caution while
driving in these affected areas.


Mullinax


...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM 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
South next week, with some lingering issues also continuing for a
progressive wave tracking off the East Coast on Sunday.


...Guidance/Predictability Assessment...

Very early in the forecast, spread persists for the details of a
shortwave reaching the eastern half of the country by Sunday along
with its accompanying wave that should track off the East Coast and
most likely reach a position just south or east of Nova Scotia by
early Monday. The relative majority of guidance (also accounting
for the 12Z machine learning models) clusters within the realm of
the ECMWF/UKMET/ICON for track and timing. In contrast to recent
GFS tendencies to be on the faster side for the shortwave and
surface feature, latest runs have adjusted slower than other
guidance. Meanwhile the latest CMC runs are a deep and western
extreme with the surface low (leading to heavier Northeast
precipitation than other guidance), though their upper dynamics
finally look a lot better versus consensus than in recent days when
they were excessively slow/amplified with the originating energy
over the West.

Detail confidence drops precipitously behind this feature, as
models show wide variations among each other and from run to run
for important shortwaves. The first one should dig into the West by
next Monday, with its eventual evolution affecting how much wintry
precipitation may spread across the southern tier early-mid week.
Through the 12Z/18Z model runs, there was a prominent clustering
toward greater southwestward elongation of this shortwave versus
the GFS. This was due to the majority not dampening the northern
side of the upstream ridge as much as GFS runs. GEFS means have a
GFS flavor but show some nudge toward a little more trough
amplitude. Meanwhile the 12Z ML models also show more elongation
than the GFS but with faster timing than most of the dynamical
models. The past couple UKMET runs appeared closest to the ML model
ideas. The new 00Z ECMWF has introduced a complication in that it
now looks more like a slightly slower version of the GFS. As for
eventual effects across the South, the overall array of guidance
has supported a moisture shield somewhere in the realm of the prior
two ECMWF runs (12Z and 00Z/15) and the GEFS/ECens means--between
the extremes of the relatively dry GFS and heavy/northward CMC.
While some aspects of guidance actually trends a little closer
together as the next bundle of shortwave energy drops into the mean
trough by Wednesday-Thursday, significant differences develop
regarding whether/how much flow splits over the east-central
Pacific. This does not bode well for the forecast immediately
after early Thursday.

Based on the above considerations, the early part of the updated
forecast started with the 12Z ECMWF/12Z UKMET/12Z GFS in order from
more to less weight. The 12Z GFS was little more palatable than the
18Z run where differences existed. The blend started to introduce
the 18Z GEFS/12Z ECens means by Day 5 Tuesday and reached 60
percent total mean weight by the end of the period, with 30/10
lingering percentages of the 12Z ECMWF/GFS respectively. This
approach helped to provide the best continuity possible under the
circumstances while waiting for better eventual agreement for
details.


...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 may
produce a band of mostly light precipitation before its departure.
Some wintry weather is possible over parts of the Mid-Atlantic and
southern New England. Although there is an intermediate majority
cluster for this system in the guidance, there are still a couple
solutions on the extreme ends of the envelope so continue to
monitor forecasts.

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, now 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.


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








/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