Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Fri Dec 06 2024

Valid 12Z Fri Dec 06 2024 - 12Z Sun Dec 08 2024

...More lake-effect/lake-enhanced snow downwind from Lakes Erie and
Ontario...

...Arctic air currently engulfing much of the eastern U.S. will gradually
moderate over the next couple of days...

...Dry and milder than average temperatures in the western U.S. will
spread into the northern and central U.S. through the next couple of
days...

Under an intense surge of arctic air, Friday morning will begin with the
coldest temperatures so far this season across much of the central and
eastern U.S. with blustery conditions and a piercing wind chill. The
persistent flow of arctic over the relatively warm waters of the Great
Lakes has continued to bring lake-effect snows downwind into the Snow
Belt. By later today into tonight, another clipper currently forming
along the arctic front will spread more snow across the Great Lakes from
northwest to southeast. By Saturday, still another clipper will bring
more widespread snowfall across the upper Great Lakes, reaching into the
lower lakes Saturday night. Milder air could change some of the snow to
rain Saturday afternoon near the western fringe of these areas. As much
as two additional feet of new snow is possible near the eastern shore of
Lake Ontario through the next couple of days. Lighter snowfall amounts
can be expected elsewhere along the Snow Belt.

After a morning with wind chills possibly falling below zero across the
mountains of the Applachains, conditions are expected to improve through
the next couple of days as southwesterly winds begin to bring milder air
from the western U.S. into the northern and central Plains. The most
drastic recovery will be found over the northern High Plains where high
temperatures could top 60 degrees by Saturday afternoon.

The retreating arctic high pressure system that brings the milder air into
the northern U.S. will also bring increasing moisture into Texas. It
appears that rain will expand in coverage across southern to eastern Texas
through Saturday ahead of an upper trough. Additional influx of moisture
from the Gulf of Mexico could begin to raise the threat of heavy rain from
eastern Texas into Louisiana by early on Sunday.

After a tranquil Friday, increasingly unsettled weather is expected for
the Pacific Northwest by the weekend. Rain showers and some high elevation
snow will return to Washington State and Oregon by Saturaday as the next
front moves through. The mountain snow and low-elevation rain will
progress farther inland, reaching into the northern Rockies by early
Sunday as a low pressure system begins to develop across the northern High
Plains into Alberta Province of Canada. Much of the remainder of the
western U.S. will remain dry and milder than normal as high pressure
dominates the region. High temperatures will be in the 80s across the
Southwest while 70s and 60s will prevail from southern California to the
Pacific Northwest.

Kong


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Fri Dec 6 2024

Valid 12Z Mon Dec 09 2024 - 12Z Fri Dec 13 2024

***Heavy rain threat across the central Gulf Coast and Southeast
early next week***

19Z Update: The 12Z models are in excellent synoptic scale
agreement with the amplifying deep trough over the Central U.S.
through the middle of the week, and the evolving storm system and
cold front that will affect the East Coast. There are more
questions that remain regarding Pacific shortwave energy reaching
the West Coast and the eventual pattern evolution across the
Western U.S. for the end of next week, and how the ridge axis over
the West decays and becomes more zonal. The ECMWF and GFS are
seemingly at odds by Friday, with the GFS maintaining the ridge
axis longer and the ECMWF bringing in a well defined trough. An
examination of the ML guidance at the time of forecast preparation
did not strongly support the operational ECMWF with regards to
trough placement, and therefore a solution closer to the CMC/GFS
was preferred going into Thursday and Friday, with below average
forecast confidence by this time. Ensemble means were gradually
increased to about half by Friday amid growing uncertainty. The
previous forecast discussion is appended below. /Hamrick
-----------------------

...Overview...

The South and East will be the focus for potentially significant
rainfall during the early-middle part of next week as a northern
Plains into western U.S. upper trough as of early Monday heads into
the east-central U.S. The leading wavy cold front will likely reach
the East Coast around Wednesday with potential for stronger wave
development over/near the Northeast later Wednesday into early
Thursday. Upper troughing should linger over the East into next
Friday but with a weakening trend by then. While most of the East
dries out behind this system (aside from snow to the lee of the
Great Lakes), expect the Pacific Northwest to see increasing
precipitation after midweek as a mean trough aloft approaches.
System progression will lead to an area of well above normal
temperatures in the warm sector ahead of the central-eastern U.S.
cold front, followed by an area of below normal readings and then a
rebound to above normal over the Plains late in the week.

...Guidance/Predictability Assessment...

Very early in the forecast, 12Z/18Z guidance as well as the new 00Z
runs have trended stronger/faster with low pressure now forecast to
reach the Canadian Maritimes by early Monday. As a result the
trailing front pushes farther south along the Northeast coast with
more high pressure wedging into New England. Latest consensus
suggests the GFS could be a little overdone with its southward
extent of the front though.

For the upper trough progressing into the East and associated
surface front/wave development, guidance agrees well in terms of
the large scale evolution but shortwave differences lead to some
variability for wave details and frontal timing. Still, there is
decent model clustering toward expected Mid-Atlantic through New
England wave development by Wednesday into early Thursday, with the
majority of 12Z machine learning (ML) models also showing this low
reaching just north or northeast of Maine by early Thursday. Some
ensemble means are not as defined with this low yet due to spread
among individual members. Within the upper trough, one notable
questionable detail is the 12Z ECMWF's upper low that drops into
the northern Plains/Midwest on Wednesday, with most other guidance
and the new 00Z ECMWF run showing a more sheared depiction of
energy digging south from Canada.

Guidance spread increases noticeably over the eastern Pacific and
western U.S. during the latter half of the week. The most common
theme among dynamical and ML guidance at the moment is for a
possible weak system or two to arrive from the Pacific and then for
a better defined eastern Pacific upper trough to approach the West
Coast by next Friday, with a surface front and anchoring surface
low of very uncertain latitude. The ensemble means, 12Z and 00Z
GFS, and 00Z/05 ECMWF plus a relative majority of ML models fit
into this majority cluster. Other model runs and some ensemble
members show different ideas so confidence in specifics is below
average for now.

Guidance comparisons among the 12Z/18Z runs led to the updated
forecast starting out with an operational model blend followed by a
transition toward some ensemble mean input (18Z GEFS/12Z ECens) as
well as a late-period shift of the ECMWF component more toward the
00Z/05 run and GFS toward the 12Z run due to better comparisons to
the ensemble means/ML guidance along the West Coast.

...Weather/Hazards Highlights...

Early next week, upper trough dynamics reaching the West will
support a brief period of enhanced snow over the central Rockies,
primarily in Colorado. This energy will swing eastward within a
larger scale upper trough eventually reaching the East, supporting
a wavy cold front and eventual stronger Northeast surface
development by around late Wednesday into early Thursday. Leading
moist flow from the Gulf of Mexico will interact with the upper
trough/front to generate an area of potentially heavy rainfall from
the central Gulf Coast into Southeast/southern Appalachians
Monday-Tuesday and continuing into parts of the Mid-Atlantic and
Northeast on Wednesday. During the Monday-Tuesday night period
covered by the Days 4-5 Excessive Rainfall Outlooks, locations
along and a little inland from the central/east-central Gulf Coast
are within a Slight Risk area for both days, given a favorable
combination of anomalous moisture and some instability plus a
brief period of slower frontal progression as western U.S. dynamics
approach--thus leading to the possibility of some training/repeat
convection. The Day 5 Sight Risk area also extends a little farther
northeast into north-central Georgia. The updated Day 5 outlook has
the Slight Risk area adjusted about a row of counties eastward
compared to the initial overnight issuance to account for latest
trends in the 12z guidance. The Storm Prediction Center is
monitoring the potential for severe weather across this region.

As the midweek wave develops near the East Coast, the far
northwestern part of the moisture shield may contain some snow.
Then expect the cold air arriving behind this system to produce
another episode of lake effect snow during Wednesday-Thursday.
Otherwise expect mostly dry conditions over the East late in the
week.

Rain and higher elevation snow should return to the Pacific
Northwest and perhaps into the northern Rockies later in the week
as one or more system affect the region. There is a lot of guidance
spread for specifics at the moment, so it will take some time to
get better resolution of details in terms of precipitation coverage
and amounts.

An area of well above normal temperatures will spread across the
eastern half of the country early next week in the warm sector of
the wavy cold front likely to reach the East Coast around
Wednesday. Greatest anomalies should be for morning lows, reaching
plus 15-20F or more at some locations. Then an area of below normal
temperatures (generally negative 5-15F anomalies) will progress
from the central-southern Rockies into the East. Then the West and
Plains will rebound to above normal, with parts of the Plains
seeing plus 10-15F anomalies by late next week.

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

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Fri Dec 06 2024

Valid 00Z Sat Dec 07 2024 - 00Z Sat Dec 14 2024

In a broad sense the guidance agrees fairly well for the pattern
through the period, though some detail differences emerge next
week. A front with associated band of moisture should weaken
and/or retreat westward through the end of this week, followed by
a front approaching from the northwest Sunday into Monday but
likely not quite reaching the main islands. These features will
tend to keep winds on the lighter side and somewhat variable in
direction at times, allowing for varied shower focus. Then for
most of next week consensus shows high pressure becoming better
established north of the state, leading to a period of fairly
strong and gusty trades which may peak around Wednesday-Thursday.
This will support more windward/mountain focus for rainfall.
Meanwhile, guidance shows energy aloft pulling southward to the
east of the state early next week and ultimately closing off an
upper low that tracks westward along 10-15N latitude mid-late
week. There is good agreement that this feature should increase
moisture from the southeast after midweek, likely leading to
higher rainfall totals over the Big Island in particular.
However, the current majority cluster for the upper low track
(among both dynamical and machine learning guidance) would at
least keep the best defined band of heaviest rainfall just south
of the Big Island. As for westward progression of the upper low,
the 00Z machine learning models generally favor an intermediate
solution like the 00Z ECMWF/12Z ECens mean (and the UKMET through
the end of its run) versus the fast 00Z GFS. The 00Z CMC/CMCens
mean are a bit on the slower side.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1033 AM EST Fri Dec 6 2024
Day 1
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

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

...Far Deep South Texas...
The slow sagging frontal zone is nearly flat west to east across
the Northern Gulf of Mexico and a weak surface to boundary layer
cyclone has developed along the upstream edge near the southern tip
of Texas. Enhanced deep layer moisture resides along and north of
the boundary across this section of the front with total PWat
values ranging from 1.5-1.75". Warm sea surface and proximity to
the front allow for some weak instability in proximity to that low
with values of 750-1000 J/kg available along/offshore. So there
will remain some solid potential for thunderstorm activity across
far south Texas. However, the vast majority of guidance continues
to suggest best overall convergence and strongest updrafts will
remain offshore or at the near coast. It is possible some weakening
showers or rouge thunderstorm may make an appearance onshore in
proximity to the urban corridor of the lower Rio Grande Valley,
making the risk for flash flooding non-zero, but not rising to high
enough probability/coverage to reach 5% threshold for delineating
a Marginal Risk at this time.

Gallina


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1033 AM EST Fri Dec 6 2024
Day 2
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN TEXAS AND WESTERN LOUISIANA...

A low-level jet streaming Gulf moisture northward across eastern
Texas will interact with a strong shortwave trough which will move
northeastward out of New Mexico and into the central Plains. PWATs
along the Upper Texas coast are expected to increase to 1.5"+ and
instability should remain minimal (and mostly confined to the
coast). The heaviest rain is forecast to be on Saturday night due
to the typical nocturnal strengthening of the low-level jet. Soils
have become a bit more saturated in this region due to the recent
rainfall, and some rainfall is expected on Friday. Isolated flash
flooding is possible, especially in areas of more saturated soils,
urban areas, and flood prone/low lying areas.

Churchill/Roth


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1033 AM EST Fri Dec 6 2024
Day 3
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST...

Continued moisture flux transport via a moderately strong low-
level jet is expected to result in increasing coverage of flash
flooding on Sunday, as the bulk of the forcing and jet dynamics
via the aforementioned strong shortwave trough will not arrive
until midday (along with maximized instability). Localized training
is expected to result in 2-4" totals, suggesting a lower-end
Slight Risk based on the probability spectrum (which ranges from
15-40%). Given that most of the region is experiencing very dry
antecedent soil conditions (10th percentile or lower for much of
MS/AR/LA, per NASA SPoRT-LIS 0-100 cm soil moisture), any flooding
should be relegated to flood-prone areas (with the greatest risk
for any localized significant flash flooding over west-central LA,
where antecedent soil conditions and streamflows are locally higher).


Churchill

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 Fri Dec 6 2024


Early next week, upper trough dynamics reaching the West will
support a brief period of enhanced snow over the central Rockies,
primarily in Colorado. This energy will swing eastward within a
larger scale upper trough eventually reaching the East, supporting
a wavy cold front and eventual stronger Northeast surface
development by around late Wednesday into early Thursday. Leading
moist flow from the Gulf of Mexico will interact with the upper
trough/front to generate an area of potentially heavy rainfall from
the central Gulf Coast into Southeast/southern Appalachians
Monday-Tuesday and continuing into parts of the Mid-Atlantic and
Northeast on Wednesday. During the Monday-Tuesday night period
covered by the Days 4-5 Excessive Rainfall Outlooks, locations
along and a little inland from the central/east-central Gulf Coast
are within a Slight Risk area for both days, given a favorable
combination of anomalous moisture and some instability plus a
brief period of slower frontal progression as western U.S. dynamics
approach--thus leading to the possibility of some training/repeat
convection. The Day 5 Sight Risk area also extends a little farther
northeast into north-central Georgia. The Storm Prediction Center
is monitoring the potential for severe weather across this region.

As the midweek wave develops near the East Coast, the far
northwestern part of the moisture shield may contain some snow.
Then expect the cold air arriving behind this system to produce
another episode of lake effect snow during Wednesday-Thursday.
Otherwise expect mostly dry conditions over the East late in the
week.

Rain and higher elevation snow should return to the Pacific
Northwest and perhaps into the northern Rockies later in the week
as one or more system affect the region. There is a lot of guidance
spread for specifics at the moment, so it will take some time to
get better resolution of details in terms of precipitation coverage
and amounts.

An area of well above normal temperatures will spread across the
eastern half of the country early next week in the warm sector of
the wavy cold front likely to reach the East Coast around
Wednesday. Greatest anomalies should be for morning lows, reaching
plus 15-20F or more at some locations. Then an area of below normal
temperatures (generally negative 5-15F anomalies) will progress
from the central-southern Rockies into the East. Then the West and
Plains will rebound to above normal, with parts of the Plains
seeing plus 10-15F anomalies by late next week.

Rausch

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Fri Dec 6 2024


Early next week, upper trough dynamics reaching the West will
support a brief period of enhanced snow over the central Rockies,
primarily in Colorado. This energy will swing eastward within a
larger scale upper trough eventually reaching the East, supporting
a wavy cold front and eventual stronger Northeast surface
development by around late Wednesday into early Thursday. Leading
moist flow from the Gulf of Mexico will interact with the upper
trough/front to generate an area of potentially heavy rainfall from
the central Gulf Coast into Southeast/southern Appalachians
Monday-Tuesday and continuing into parts of the Mid-Atlantic and
Northeast on Wednesday. During the Monday-Tuesday night period
covered by the Days 4-5 Excessive Rainfall Outlooks, locations
along and a little inland from the central/east-central Gulf Coast
are within a Slight Risk area for both days, given a favorable
combination of anomalous moisture and some instability plus a
brief period of slower frontal progression as western U.S. dynamics
approach--thus leading to the possibility of some training/repeat
convection. The Day 5 Sight Risk area also extends a little farther
northeast into north-central Georgia. The Storm Prediction Center
is monitoring the potential for severe weather across this region.

As the midweek wave develops near the East Coast, the far
northwestern part of the moisture shield may contain some snow.
Then expect the cold air arriving behind this system to produce
another episode of lake effect snow during Wednesday-Thursday.
Otherwise expect mostly dry conditions over the East late in the
week.

Rain and higher elevation snow should return to the Pacific
Northwest and perhaps into the northern Rockies later in the week
as one or more system affect the region. There is a lot of guidance
spread for specifics at the moment, so it will take some time to
get better resolution of details in terms of precipitation coverage
and amounts.

An area of well above normal temperatures will spread across the
eastern half of the country early next week in the warm sector of
the wavy cold front likely to reach the East Coast around
Wednesday. Greatest anomalies should be for morning lows, reaching
plus 15-20F or more at some locations. Then an area of below normal
temperatures (generally negative 5-15F anomalies) will progress
from the central-southern Rockies into the East. Then the West and
Plains will rebound to above normal, with parts of the Plains
seeing plus 10-15F anomalies by late next week.

Rausch

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 AM EST Fri Dec 6 2024

Valid 12Z Fri Dec 06 2024 - 12Z Mon Dec 09 2024

...Great Lakes and Northeast...
Days 1-2...

Departing area of low pressure over Atlantic Canada and modest
high pressure over the Mid-MS Valley will maintain cyclonic flow
across the Great Lakes, especially off Lakes Erie/Ontario, for the
next day or two. A shortwave within the broad trough over the
region will help back the flow enough to instigate another strong
single band off Ontario into the Tug Hill between Watertown and
Syracuse this afternoon/evening and overnight. Robust snowfall
rates of 1-3"/hr (per the HREF) will help deposit another 12-18"+
of snow over favored areas before waning on Saturday. Downwind of
Lake Erie, multi-band streamers will result in a larger area of
light to moderate snow with WPC probabilities of at least another
6" of snow >50% from Erie, PA northeastward to the Chautauqua Ridge
in NY.

Another shortwave rounding the base of the longwave trough will
slip through the Great Lakes late Sat into early Sun with generally
light snow for the U.P., northern Lower MI, into NYS and northern
New England. WPC probabilities for at least 4 inches of snow are
generally low (10-40%) except for areas with lake enhancement and
parts of the Adirondacks, Green, and White Mountains where
probabilities are moderate (40-70%).


...Pacific Northwest/Northern Rockies...
Days 2-3...

An upper low currently south of the Gulf of Alaska will move
eastward over the next day or so, aimed toward British Columbia.
Ahead of it, a plume of moisture will be directed into the Pacific
Northwest starting early Saturday. Snow levels will start quite
high (around 8000-9000ft at precip onset very early Saturday) but
fall to around pass level (~4000ft) late Saturday into the
overnight hours. East of the crest, cold air in place at the
surface with overrunning WAA-driven precip supports a wintry mix of
snow/sleet/freezing rain for eastern WA. Some icing is possible,
from a few hundredths to around a tenth of an inch.

A second surge in moisture will occur on Sunday associated with
the parent upper low as it opens up across the region. With snow
levels already around pass level, snow should accumulate on Sunday
with WPC probabilities for at least 6 inches of snow >50% above
about 3500ft or so.

Farther east, moisture will spread across northeastern OR and into
the northern Rockies from late Saturday through the end of the
period early Monday. Snow levels will lower through time, and
nearly all areas except the lowest valleys will change from rain to
snow. WPC probabilities for at least 8 inches of snow days 2-3 are
>50% above about 5000ft or so, mostly in the Lewis Range and
across northern ID, the Blue Mountains, and into northwestern WY.


...Northern Minnesota and northeastern North Dakota...
Day 3...

Eastern extension of the northern Rockies system will manifest
itself as another upper low forming by Sunday over southern
Saskatchewan. That will move east-southeastward into early Monday
with WAA-driven precipitation to its east from northeastern ND
eastward across northern MN. Ptype will be an issue with milder air
overtopping the colder boundary layer, especially where precip
arrives near/after sunset Sunday. NBM ptype uncertainty maximizes
between all 4 types in a zone along the U.S.-Canadian border though
the majority of probabilities are either rain or snow. Still,
cannot rule out some light icing but will have time to narrow the
forecast with time. WPC probabilities for at least 4 inches of snow
are moderate (40-70%) over northern MN.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Fri Dec 6 2024

Valid 12Z Mon Dec 09 2024 - 12Z Fri Dec 13 2024

***Heavy rain threat across the central Gulf Coast and Southeast
early next week***

19Z Update: The 12Z models are in excellent synoptic scale
agreement with the amplifying deep trough over the Central U.S.
through the middle of the week, and the evolving storm system and
cold front that will affect the East Coast. There are more
questions that remain regarding Pacific shortwave energy reaching
the West Coast and the eventual pattern evolution across the
Western U.S. for the end of next week, and how the ridge axis over
the West decays and becomes more zonal. The ECMWF and GFS are
seemingly at odds by Friday, with the GFS maintaining the ridge
axis longer and the ECMWF bringing in a well defined trough. An
examination of the ML guidance at the time of forecast preparation
did not strongly support the operational ECMWF with regards to
trough placement, and therefore a solution closer to the CMC/GFS
was preferred going into Thursday and Friday, with below average
forecast confidence by this time. Ensemble means were gradually
increased to about half by Friday amid growing uncertainty. The
previous forecast discussion is appended below. /Hamrick
-----------------------

...Overview...

The South and East will be the focus for potentially significant
rainfall during the early-middle part of next week as a northern
Plains into western U.S. upper trough as of early Monday heads into
the east-central U.S. The leading wavy cold front will likely reach
the East Coast around Wednesday with potential for stronger wave
development over/near the Northeast later Wednesday into early
Thursday. Upper troughing should linger over the East into next
Friday but with a weakening trend by then. While most of the East
dries out behind this system (aside from snow to the lee of the
Great Lakes), expect the Pacific Northwest to see increasing
precipitation after midweek as a mean trough aloft approaches.
System progression will lead to an area of well above normal
temperatures in the warm sector ahead of the central-eastern U.S.
cold front, followed by an area of below normal readings and then a
rebound to above normal over the Plains late in the week.

...Guidance/Predictability Assessment...

Very early in the forecast, 12Z/18Z guidance as well as the new 00Z
runs have trended stronger/faster with low pressure now forecast to
reach the Canadian Maritimes by early Monday. As a result the
trailing front pushes farther south along the Northeast coast with
more high pressure wedging into New England. Latest consensus
suggests the GFS could be a little overdone with its southward
extent of the front though.

For the upper trough progressing into the East and associated
surface front/wave development, guidance agrees well in terms of
the large scale evolution but shortwave differences lead to some
variability for wave details and frontal timing. Still, there is
decent model clustering toward expected Mid-Atlantic through New
England wave development by Wednesday into early Thursday, with the
majority of 12Z machine learning (ML) models also showing this low
reaching just north or northeast of Maine by early Thursday. Some
ensemble means are not as defined with this low yet due to spread
among individual members. Within the upper trough, one notable
questionable detail is the 12Z ECMWF's upper low that drops into
the northern Plains/Midwest on Wednesday, with most other guidance
and the new 00Z ECMWF run showing a more sheared depiction of
energy digging south from Canada.

Guidance spread increases noticeably over the eastern Pacific and
western U.S. during the latter half of the week. The most common
theme among dynamical and ML guidance at the moment is for a
possible weak system or two to arrive from the Pacific and then for
a better defined eastern Pacific upper trough to approach the West
Coast by next Friday, with a surface front and anchoring surface
low of very uncertain latitude. The ensemble means, 12Z and 00Z
GFS, and 00Z/05 ECMWF plus a relative majority of ML models fit
into this majority cluster. Other model runs and some ensemble
members show different ideas so confidence in specifics is below
average for now.

Guidance comparisons among the 12Z/18Z runs led to the updated
forecast starting out with an operational model blend followed by a
transition toward some ensemble mean input (18Z GEFS/12Z ECens) as
well as a late-period shift of the ECMWF component more toward the
00Z/05 run and GFS toward the 12Z run due to better comparisons to
the ensemble means/ML guidance along the West Coast.

...Weather/Hazards Highlights...

Early next week, upper trough dynamics reaching the West will
support a brief period of enhanced snow over the central Rockies,
primarily in Colorado. This energy will swing eastward within a
larger scale upper trough eventually reaching the East, supporting
a wavy cold front and eventual stronger Northeast surface
development by around late Wednesday into early Thursday. Leading
moist flow from the Gulf of Mexico will interact with the upper
trough/front to generate an area of potentially heavy rainfall from
the central Gulf Coast into Southeast/southern Appalachians
Monday-Tuesday and continuing into parts of the Mid-Atlantic and
Northeast on Wednesday. During the Monday-Tuesday night period
covered by the Days 4-5 Excessive Rainfall Outlooks, locations
along and a little inland from the central/east-central Gulf Coast
are within a Slight Risk area for both days, given a favorable
combination of anomalous moisture and some instability plus a
brief period of slower frontal progression as western U.S. dynamics
approach--thus leading to the possibility of some training/repeat
convection. The Day 5 Sight Risk area also extends a little farther
northeast into north-central Georgia. The updated Day 5 outlook has
the Slight Risk area adjusted about a row of counties eastward
compared to the initial overnight issuance to account for latest
trends in the 12z guidance. The Storm Prediction Center is
monitoring the potential for severe weather across this region.

As the midweek wave develops near the East Coast, the far
northwestern part of the moisture shield may contain some snow.
Then expect the cold air arriving behind this system to produce
another episode of lake effect snow during Wednesday-Thursday.
Otherwise expect mostly dry conditions over the East late in the
week.

Rain and higher elevation snow should return to the Pacific
Northwest and perhaps into the northern Rockies later in the week
as one or more system affect the region. There is a lot of guidance
spread for specifics at the moment, so it will take some time to
get better resolution of details in terms of precipitation coverage
and amounts.

An area of well above normal temperatures will spread across the
eastern half of the country early next week in the warm sector of
the wavy cold front likely to reach the East Coast around
Wednesday. Greatest anomalies should be for morning lows, reaching
plus 15-20F or more at some locations. Then an area of below normal
temperatures (generally negative 5-15F anomalies) will progress
from the central-southern Rockies into the East. Then the West and
Plains will rebound to above normal, with parts of the Plains
seeing plus 10-15F anomalies by late next week.

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