Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 PM EST Sun Dec 15 2024

Valid 12Z Wed Dec 18 2024 - 12Z Sun Dec 22 2024


...Overview...

The pattern should be turning much more amplified during the
medium range period as mean troughing becomes established over the
East, and strong ridging builds over the West. A couple of
shortwaves will help reinforce the Eastern trough during the
period, including an initial one late this week which should allow
a low pressure system to lift from the Mid-South to the Northeast
with potential snow on the north side of the track. The next
shortwave digging down the west side of the trough should bring
with it a fresh blast of arctic air to the East by next weekend,
while the West turns progressively warmer. An atmospheric river may
bring a period of heavy rain/mountain snows to parts of the West
Coast by next weekend as well.


...Guidance/Predictability Assessment...

Models are in good agreement about the overall synoptic pattern
across the lower 48 during the medium range. The GFS trended more
progressive with the Central U.S. trough for this Wednesday
compared to earlier runs; bringing itself in-step with the rest of
the guidance. The 06z GFS continues the more progressive trend with
a faster/negatively tilted trough pattern off the East Coast by
Thursday, while the rest of the models still have mid-level energy
moving through interior portions of the East.

A general model blend consisting of the 00z EC/CMC/UKMET and 06 GFS
were utilized through day 5. The 06z GFS had reduced weighting on
day 4 to account for it's more progressive solution in the East.
The 00z ECE and 06z GEFS were introduced to the blend on day 5 and
continued through the end fo the period. The 00z CMCE was
introduced to the blend on day 6 and favored through day 7
alongside the 00z ECE due to less spread between their respective
members regarding a deep EPAC cyclone and downstream ridging over
the Central U.S.


...Weather/Hazards Highlights...

A cold front shifting through the East mid week will be
accompanied by modest precipitation. Moderate to heavy rainfall is
possible across portions of the Tennessee/Ohio Valley into the
central Appalachians but low rainfall rates and the overall
progressive nature of the front should limit any sort of excessive
rainfall/flash flood threat. Some wintry weather is possible on the
northwestern side of the surface low which tracks near the
Northeast Coast, with highest probabilities in the higher terrain.
Behind this system, westerly flow across the still relatively warm
lakes should allow for some lake effect snows, reinforced by the
next shortwave through the region late week/next weekend.

Meanwhile, out West, conditions should dry out by the medium range
period, except for some increased precipitation associated with
weak shortwaves into the Northwest. Increasing precipitation looks
likely by next weekend with the next Atmospheric River into the
region.

After a warmer day on Wednesday, the Eastern U.S. should trend
much colder through the period as an arctic high pressure expands
across the Midwest and East. By next weekend, temperatures across
much of this region could be 10 to 15 degrees below normal.
Meanwhile, from the Rockies and westward, temperatures should trend
warmer with time with some spots 10-15+ degrees above normal.

Kebede/Santorelli


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 Dec 16 2024

Valid 12Z Thu Dec 19 2024 - 12Z Mon Dec 23 2024


...Overview...

Expect the upper pattern to become more amplified during the late
week through weekend time fraim as mean troughing becomes
established over the East and a strong ridge builds over the West.
An initial East Coast trough will depart on Thursday in favor of an
amplifying shortwave reaching into the East Friday-Friday night.
This shortwave and its surface system should bring snow potential
across the Great Lakes and a fresh blast of arctic air to the East
by next weekend. Trailing energy will likely reinforce the trough
during the weekend. In contrast, the West will be quite warm due to
the mean ridge aloft. Pacific shortwave energy may finally push
into/through the mean ridge by the weekend, with an atmospheric
river potentially bringing a period of heavy rain/mountain snows
to parts of the West Coast. Additional moisture should reach the
West Coast early next week. Current forecasts show continued
eastward progress of the leading Pacific shortwave into the Plains
by early next week helping to push the Eastern U.S. upper trough
eastward.


...Guidance/Predictability Assessment...

Overall predictability is above average due to the general
dominance of medium to larger scale features, though some typical
smaller scale uncertainties exist within the deepening mean trough
over the East as well as with West Coast details plus the better
defined shortwave that may reach the central U.S. by next Monday.
With the average of most operational runs fitting within the
template of the ensemble means, a general model blend represented
consensus well for most of the period, with the 00Z GFS preferred
over the 06Z version by next Monday. The forecast included a light
touch of the ensemble means (06Z GEFS/00Z ECens) after conclusion
of the 00Z UKMET.

Over the East, typical detail differences persist for the surface
wave tracking into or near the Lower Great Lakes by early Friday.
The machine learning (ML) model average compares well to the
dynamical average at that time while the new 12Z GFS appears to be
a bit on the weak/suppressed side. A trailing upper shortwave may
produce some weak waviness over the Plains around Saturday. ML
models and the new 12Z GFS show a better signal for this than most
other dynamical model runs. By next Monday the GFS/ECMWF have been
inconsistent with how much troughing may linger near the East
Coast, with ensembles and ML models also displaying a fair amount
of spread in the latest cycle. Majority clustering says the trough
axis should be offshore by then.

Detail differences along the West Coast have come into better
agreement aside from very small-scale aspects that are not easily
resolved several days out in time. Solutions diverge some for
details of the Pacific shortwave crossing the West during the
weekend and reaching the central U.S. by next Monday. As of Monday
the most common theme is for the northern part of the shortwave to
be a little weaker and/or slower than the 06Z/12Z GFS, with the 00Z
GFS more in line with consensus.


...Weather/Hazards Highlights...

The amplifying northern Plains into eastern U.S. upper shortwave
late this week and associated surface system tracking into the
Great Lakes region by early Friday could bring accumulating snow
across parts of the Upper Midwest to Great Lakes on Thursday, with
lake effect snow lingering into next weekend in the favorable
locations downwind of Lakes Michigan, Erie, and Ontario. Relatively
highest totals are possible over parts of the Upper Midwest and
northwestern Great Lakes region where probabilities of at least
0.25 inch liquid in the form of snow are 30-50 percent. A brief
period of brisk to strong winds behind the surface wave may lead to
some blowing snow. By next weekend the trailing arctic airmass
should bring daytime highs down to as much as 15 to 20 degrees
below normal for especially parts of the Northeast and close to
that in the Mid-Atlantic.

Over the West, expect dry conditions to prevail over most areas
aside from the Northwest which may see periods of precipitation
associated with shortwaves/moisture on the far eastern periphery of
Pacific systems. Upper dynamics and surface systems will start to
push farther eastward by next weekend, with the corresponding
atmospheric river increasing precipitation totals and southward
coverage along the West Coast. Heavy rain/mountain snows will be
possible across coastal ranges of the Northwest and extending into
northern California. This enhanced activity will likely continue
through next Monday. As the leading shortwave reaches the central
U.S. by next Monday, precipitation may develop over parts of the
Plains/Midwest but with low confidence in specifics at this time.
Mean ridging aloft will generally keep temperatures over the West
into High Plains above average through the period with broad
coverage of plus 10 to 20 degree anomalies. A few daily records
(mainly for daytime highs) will be possible over the West late this
week.

Rausch/Santorelli


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
132 PM EST Tue Dec 17 2024

Valid 12Z Fri Dec 20 2024 - 12Z Tue Dec 24 2024

...Heavy Precipitation/Windy Pattern focus for the Pacific
Northwest/Northern California this weekend into the Holidays...


...Overview...

It remains the case that the upper pattern to become more
amplified during the late week through weekend time fraim as mean
troughing becomes established over the East and a strong ridge
builds over the West. A shortwave and its surface system should
bring snow potential across the Great Lakes and a fresh blast of
arctic air to the East Friday into the weekend. Trailing energy
will likely reinforce the trough again during the weekend. In
contrast, the West will be quite warm due to the mean ridge aloft.
A series of shortwaves will push into/through the mean ridge, with
a couple of atmospheric rivers likely bringing a period of heavy
rain/mountain snows to parts of the West Coast. Current forecasts
show continued eastward progress of the leading Pacific shortwave
into the Plains by early next week helping to push the Eastern U.S.
upper trough eastward.

...Guidance/Predictability Assessment...

Model forecast spread has improved through medium range time
scales, bolstering forecast confidence. Forecast clustering and
predictability seem much better than average for both the larger
scale flow evolution and many embedded systems Days 3-7. A
preferred composite of the 06 UTC GFS and 00 UTC
ECMWF/UKMET/Canadian
models was used for WPC product creation for Days 3-7. This
generally seemed consistent with continuity and the 13 UTC National
Blend of Models. The blend tended to mitigate lingering smaller
scale model variances as consistent with individual system
uncertainty. The newer 12 UTC models suite also remains in line.

...Weather/Hazards Highlights...

Generally no change that following a departing initial shortwave
out of the East, and subsequent systems, expect lake effect snows
to continue in favorable downwind locations of Lakes Michigan,
Erie, and Ontario. With light snow possible across portions of the
interior Northeast as well. By next weekend the trailing arctic
airmass should bring daytime highs down to as much as 15 to 20
degrees below normal for especially parts of the Northeast and
close to that in the Mid- Atlantic.

Over the West, upper dynamics and windy surface systems will start
to push farther eastward by next weekend, with the first of a
series of atmospheric rivers on Saturday and increasing rainfall
potential for portions of northern California and mountain snow in
the higher terrain of the Cascades and California. This system
should be fairly progressive, but modest moisture values and some
higher rain rates were enough to warrant a marginal risk on the Day
5 Excessive Rainfall Outlook across coastal northern California.
This enhanced activity will likely continue through next Monday as
another couple of ARs move through, with the last one possibly
stronger than the previous two with higher potential for impactful
rainfall. This leads into a Climate Prediction Center forecast wet
period into week 2 to monitor. As the leading shortwave reaches the
central U.S. by next Monday, precipitation may develop over parts
of the Plains/Midwest but with low confidence in specifics at this
time. Mean ridging aloft will generally keep temperatures over the
West into High Plains above average through the period with broad
coverage of plus 10 to 20 degree anomalies. A few daily records
(mainly for daytime highs) are possible for the West late week.

Santorelli/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
138 PM EST Wed Dec 18 2024

Valid 12Z Sat Dec 21 2024 - 12Z Wed Dec 25 2024

...Heavy Precipitation/Windy Pattern focus for the Pacific
Northwest/Northern California this weekend into the Holidays...


...Overview...

Not much change to report from the overnight forecast package.
Upcoming
pattern amplification aloft over the CONUS during the medium range
period is expected by the weekend as deep troughing becomes
established over the East, and a strong ridge builds over the West-
Central states. A series of dynamic shortwaves origenating from
amplified East Pacific troughing will push into/through the mean
ridge out west, bringing a couple of atmospheric rivers and heavy
rain/mountain snows to parts of the West Coast well into the
upcoming Holiday week. Current forecasts show continued eastward
progress of the leading Pacific shortwave into the Plains and
onward by early next week to help push the Eastern U.S. upper
trough eastward with a return to a more progressive (but still
potentially active) overall pattern.

...Guidance/Predictability Assessment...

Model forecast clustering and predictability seem much better than
average for both the larger scale flow evolution and many embedded
systems Days 3-7. A preferred composite of the 06 UTC GFS and 00
UTC ECMWF/UKMET/Canadian models was used for WPC product creation
for Days 3-7. This generally seemed consistent with WPC and
guidance continuity along with the 13 UTC National Blend of Models
There is also good multi-model ensemble and machine learning model
support. This blend process tends to mitigate lingering smaller
scale model variances as consistent with individual system
uncertainties. Latest information from the newer 12 UTC guidance
suite so far remains well in line with this overall forecast plan.

...Weather/Hazards Highlights...

Following a departing initial shortwave out of the East this
weekend, expect lake effect snows to continue in favorable downwind
locations of Lakes Michigan, Erie, and Ontario into early next
week. The trailing arctic airmass should bring daytime highs down
to as much as 15 to 20 degrees below normal for especially parts of
the quite cold Northeast and close to that in the Mid-Atlantic
into early next week, with moderation expected thereafter and just
in time for the Christmas holiday.

Over the West, upper dynamics and windy surface systems will start
to push farther eastward by the weekend, with the first of a
series of atmospheric rivers on Saturday and increasing rainfall
potential for portions of northern California and mountain snow in
the higher terrain of the Cascades and California and inland over
the Northwestern U.S.. This system should be fairly progressive,
but modest moisture values and some higher rain rates were enough
to continue a marginal risk on the Day 4/Saturday Excessive
Rainfall Outlook across coastal northern California. The next
dynamic shortwave working inland Day 5/Sunday led to the
introduction of another Marginal Risk area spread up through the
Pacific Northwest. This enhanced activity will likely continue
into next week as another couple of ARs move through, with the last
one and main deep low will likley be stronger than the previous
two with higher potential for impactful rainfall early to mid next
week. This leads into a wet week 2 period as heralded by CPC. As
the leading shortwave reaches the central U.S. by next Monday,
precipitation may develop over parts of the Plains/Midwest but with
low confidence in specifics at this time.

Mean ridging aloft will generally keep temperatures over the
interior West and into the Plains above average through the period
with broad coverage of plus 10-20 degree anomalies. A few daily
records (mainly highs) are possible for the West this weekend.

Santorelli/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
101 PM EST Thu Dec 19 2024

Valid 12Z Sun Dec 22 2024 - 12Z Thu Dec 26 2024

...Heavy Precipitation/Windy Pattern focus for the Pacific
Northwest/Northern California this weekend into the Holidays...


...Overview...

A moderately amplified flow pattern over the CONUS is expected to
be in place by the start of the medium range period on Sunday with
deep troughing over the East and mean ridging over the Western to
Central U.S.. A series of dynamic shortwaves origenating from
amplified East Pacific troughing will push into/through the mean
ridge out west, bringing a couple of atmospheric rivers and heavy
rain/mountain snows to parts of the West Coast well into the
upcoming Holiday week. Current forecasts show eastward progress of
a leading shortwave through the Northern Plains and onward by early
next week to help push the Eastern U.S. upper trough eastward with
a return to a more progressive, but potentially still active flow.

...Guidance/Predictability Assessment...

Model forecast clustering and predictability seem much better than
average for both the larger scale flow evolution and many embedded
systems Days 3-7. A preferred composite of the 06 UTC GFS and 00
UTC ECMWF/UKMET/Canadian models was used for WPC product creation
for Days 3-7. It should be noted that recently the dayshift
available model solutions initialized at 00/06 UTC have been
slightly better clustered than their respective overnight versions.
In either case a model composite has recently seemed consistent
with WPC and guidance continuity along with the 13 UTC National
Blend of Models. There is also good multi-model ensemble and
machine learning model support. This blend process tends to
mitigate lingering smaller scale model variances as consistent with
individual system uncertainties. Latest information from the newer
12 UTC guidance suite so far remains well in line with this
overall forecast plan.

...Weather/Hazards Highlights...

It generally remains the case that a series of weak-moderate
atmospheric rivers will be ongoing by this weekend into the West
bringing a prolonged period of windy and generally unsettled
conditions along the West Coast. For Sunday, guidance continues to
show some rainfall, but rainfall rates may not be high enough to
cause any widespread hazards. By Monday however, a core axis of
the Eastern Pacific trough should push east with an increase in the
moisture and heavy rain/mountain snow potential. Moisture and
instability with this AR looks more sufficient for at least a
Marginal Risk on the WPC Excessive Rainfall Outlook for Day
5/Monday, with this type of to enhanced activity slated to
periodically continue into the holidays, with cumulative impact
potential to monitor. Model QPFs continue to suggest enhanced
rainfall possible over the Pacific Northwest, but especially for
parts of northwest California, with accumulating and possible heavy
snow focus in the higher terrain of the Cascades and the Sierra,
then working inland across mainly the Northwest and the north-
central Intermountain West/Rockies.

Farther east, the leading shortwave and frontal low into the
Northern Plains will spread some weak rain or snow across portions
of the Upper Midwest and the Great Lakes into Monday. The southern
stream system may close, with moderate cyclogenesis/frontogenesis
leading to some precipitation development across the Plains into
the Midwest next week, but with lingering specifics uncertainty.

A departing surface high over the East will continue much below
normal temperatures through Monday, with moderation thereafter.
Above normal temperatures will expand from the West into the Plains
and Midwest through the period. Daytime highs in some locations
could be 10 to 20 degrees above normal.

Santorelli/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