Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
247 AM EST Sat Dec 04 2021

Valid 12Z Sat Dec 04 2021 - 12Z Mon Dec 06 2021

...Winter storm to produce heavy snow and gusty winds from the Northern
Plains to the Upper Great Lakes this weekend...

...A strong cold front entering the Midwest and Southern Plains by Sunday
evening could spark isolated severe thunderstorms and instances of flash
flooding from the lower Ohio Valley to eastern Texas...

Much of the active weather located across CONUS through the beginning of
next week will be associated with a strong area of low pressure forecast
to traverse the northern tier. Upper level energy combined with Pacific
moisture are currently entering the Northwest and will spread into the
northern High Plains this morning. As it does so, the Washington Cascades
and far northern Rockies are expected to pick up around 6 to 12 inches of
snow today, with locally higher amounts. By this evening and into early
Sunday, a developing area of low pressure over central Montana will move
eastward and spread moderate-to-heavy snowfall just to the south of the
U.S.-Canada border. This storm system is then forecast to continue
strengthening on Sunday as it enters the Upper Midwest and Great Lakes,
with gusty winds and bitterly cold air trailing closely behind. By Monday
morning, storm total snowfall amounts over 6 inches are expected to
stretch through northern North Dakota/Minnesota and into portions of far
northern Wisconsin and the U.P. of Michigan. The combination of heavy snow
and gusty winds will likely make travel treacherous at times for this
region. Winter Storm Watches and Warnings, as well as Winter Weather
Advisories have been issued.

The other facet of this early-December winter storm will be found along
and ahead of an attached cold front that is forecast to reach the Midwest
and Southern Plains by Sunday evening. Warm and well above average
temperatures out ahead of this cold front will clash with the approaching
arctic airmass and likely spark a line of showers and thunderstorms
extending from the Ohio Valley to eastern Texas. Isolated instances of
severe weather and flash flooding are possible into Monday morning for
this section of the country. Correspondingly, the Storm Prediction Center
has issued a Marginal Risk (level 1/5) of severe weather from far southern
Indiana to East Texas. A Marginal Risk (level 1/4) of excessive rainfall
has been issued between far southwest Ohio and central Arkansas.

Elsewhere, light snow showers and lake effect snow is likely throughout
the Interior Northeast and northern New England later today as a weak area
of low pressure swings into the region from Canada. For the Northwest,
another round of wintry weather is forecast to enter by Monday morning.
Air temperatures associated with this system could be cold enough to
support light snow in the lower elevations of Washington.

Snell

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Sat Dec 04 2021

Valid 12Z Tue Dec 07 2021 - 12Z Sat Dec 11 2021

...Notable snow in the interior Northeast is currently expected
Wednesday, with wintry impacts potentially reaching southward into
the I-95 corridor and possibly the Mid-Atlantic states, while
locally heavy rain may occur over the Southeast...

...Considerable mountain snows are likely across the Cascades,
Sierra Nevada, Intermountain West, and Rockies during the latter
half of next week...

...Overview...

Expect a broad mean trough aloft to prevail across much of the
lower 48 next week, with an axis generally extending from
central/east-central Canada into the western U.S. Embedded
shortwaves will push low pressure/frontal systems across the
country, leading to a somewhat more active weather pattern than
has been the case lately. In particular, parts of the West should
see meaningful precipitation while the intensity/coverage of
rainfall should increase over the southeastern quadrant of the
country and some impactful snow may fall over northern parts of
the East.

...Guidance/Predictability Assessment...

After multiple days of GFS runs had been significantly stronger
with shortwave energy coming into the West Day 3/Tuesday, the 18Z
and 00Z GFS have come in weaker with the feature, more in line
with other guidance. So there seems to be an increasing consensus
with this shortwave, but uncertainty is still somewhat high given
the energy stems from the less predictable high latitudes. The
uncertainty in the details of the forecast continues Wednesday
onward despite reasonable consensus for the overall pattern.
Differences in the handling of this shortwave energy as well as
northern stream/Great Lakes energy lead to evolution and
track/timing uncertainty for an area of low pressure from the
eastern U.S. into the western Atlantic Wednesday-Thursday. Overall
the low pressure system continued its overall trend more
suppressed, but with considerable variability in its track and
timing with the 12/18Z model cycle and the incoming 00Z cycle,
which leads to uncertainty with precipitation amounts across the
Mid-Atlantic and Northeast.

Overall there is fairly good agreement on another bundle of
Pacific energy coming into North America Wednesday, with less
amplification than its predecessor, producing southern Canada low
pressure that anchors a front reaching into the West/Plains/Great
Lakes. The 12Z ECMWF appears to be the outlier with this surface
low, faster/farther east compared to consensus and the previous
model cycles. Then, energy is yet again expected to dig southward
and amplify troughing in the West by Friday, though the degree of
amplification differs among guidance, with the 12Z CMC depth a bit
on the more extreme side.

Model preferences led to starting with a composite of 12Z/18Z
operational models for about the first half of the period. Then
the forecast blend transitioned to a blend of the operational
12/18Z GFS and 12Z ECMWF and the 18Z GEFS/12Z ECMWF mean.

...Weather/Hazards Highlights...

Gulf moisture should increase by Tuesday into Wednesday ahead of
frontal systems and cause moderate to possibly locally heavy
rainfall from the Lower Mississippi Valley into the Tennessee
Valley and Southeast. Moisture spreading farther north into cooler
air should lead to snow across the northern latitudes, with the
highest probabilities of notable snow across the Northeast. There
is still considerable uncertainty over how low pressure from the
eastern U.S. into the western Atlantic will evolve/track, so
confidence is fairly low for exactly how much precipitation may
fall over the Northeast, and how far south significant snow
reaches. Some rain could continue late next week near a wavy front
lingering over the extreme Southeast.

Fairly light precipitation is forecast for the Pacific Northwest
and the Four Corners states on Tuesday, before precipitation
chances increase from midweek onward. As shortwave energy and
troughing drop southward through the West, precipitation will
spread from the Pacific Northwest and Northern Rockies Wednesday
southward to California into the Great Basin to Central Rockies
Thursday and Friday. Generally expect higher elevation snow and
lower elevation rain, with snow levels likely to decline later in
the week as upper troughing amplifies over the West. Parts of
Southern California may see its first rainfall in over a month.

Chilly temperatures are likely across the far north-central U.S.
on Tuesday given snow cover from Sunday-Monday's system, with
temperatures 10-20F below normal. Other than that, temperatures
should be within about 10 degrees of normal in most places across
the lower 48 Tuesday and Wednesday before a warmup by Thursday
through Saturday from the Plains eastward, with 10-20F above
average temperatures. Cooler temperatures may spread into the West
late in the week (highs 5-15F or so below normal by Friday and
Saturday) with the potential for upper troughing to amplify over
the region.

Tate/Rausch


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Sat Dec 04 2021

Valid 00Z Sun Dec 05 2021 - 00Z Sun Dec 12 2021

Active weather will continue over Hawaii through this weekend and
well into next week. First, a cold front currently moving through
the state will continue to track southward Saturday before
stalling, which will help to focus moisture especially over the
Big Island and Maui. The enhanced moisture should lead to
increased coverage of showers for those areas. Meanwhile, drier
dew points and gusty northwesterly winds are occurring and
forecast behind the cold front. The highest elevations of the Big
Island have Blizzard Warnings issued by NWS Honolulu given the
moisture in place with the stalled front and windy conditions.
Meanwhile, there is a Flood Watch in place for most of the rest of
the state lasting through early next week.

By Sunday into Monday, an upper low is expected to reform around
25N/165W, while an upper high resides well east of the state, and
a surface low and high generally reflect the positions of the
upper-level features. This pattern will lead to mean southerly
flow across Hawaii through much of the period, directing deep
tropical moisture northward from the ITCZ. Model guidance agrees
that precipitable water values will be over the 95th percentile
(with the ECMWF showing periods above the 99th percentile) around
Monday-Wednesday, with the axis of highest PWs drifting slowly
east to west across the state as a couple of surface troughs could
focus the moisture. Heavy rainfall should follow this pattern and
several inches of rain can be expected, along with flooding/flash
flooding potential. By the latter part of next week, the upper low
is forecast to drift westward while an upper high encroaches
toward Hawaii, eventually lessening heavy rain chances.

Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Sat Dec 04 2021

Day 1
Valid 12Z Sat Dec 04 2021 - 12Z Sun Dec 05 2021

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
306 AM EST Sat Dec 04 2021

Day 2
Valid 12Z Sun Dec 05 2021 - 12Z Mon Dec 06 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID-SOUTH AND OHIO VALLEY...

Largely zonal flow aloft across North America will become more
amplified by Day 2 as a potent shortwave trough (originating near
the Gulf of Alaska) digs southward into the Northern Plains by
Sunday morning. This upper-level feature is progged to lead to
cyclogenesis near the Great Lakes with a relatively brief period
of strong Gulf moisture return expected for much of the central US
within the warm sector of the developing cyclone. The best, most
sustained low-level moisture return looks to occur across portions
of the Mid-South and Ohio Valley into Sunday night, coinciding
with the strongest dynamical forcing (i.e. best DPVA and
divergence aloft via the right-entrance region of a jet streak).
Precipitable water values are expected to surge to ~1.25 inches
per the GEFS, about a +1.5-2.0 anomaly (or in excess of the 75th
percentile climatologically). Guidance remains in fairly good
agreement with regard to QPF with an areal average of 1-2 inches
depicted from near Little Rock northeastward towards Louisville.
Some higher resolution guidance (e.g. the GEM regional, GFS, and
ECMWF) suggest the potential for localized totals of 2-3+ inches
in a relatively short period (which makes sense given the
expectation for warm sector convection). A couple of factors that
will work against excessive rainfall potential include the limited
residence time of high rainfall rates (due to the progressive
nature of this system) and the overall dry antecedent conditions
with the majority of the area currently experiencing soil moisture
anomalies in the bottom 10-20th percentile climatologically (per
NASA SPoRT-LIS 0-40 cm soil moisture data). While these factors
should act to significantly limit the spatial extent of any flash
flooding, a Marginal Risk was retained and expanded slightly given
continued model agreement and increasing confidence.

Churchill

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Sat Dec 04 2021

Day 3
Valid 12Z Mon Dec 06 2021 - 12Z Tue Dec 07 2021

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

Churchill

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

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Sat Dec 04 2021

Valid 12Z Sat Dec 04 2021 - 12Z Tue Dec 07 2021


...Pacific Northwest to the Northern Rockies...
Day 1...
A potent, quick-hitting shortwave trough crosses northern WA and
the northern Rockies today on a 130kt jet streak. Snow levels will
be around 2500ft in western WA and about 3500ft in the northern
Rockies. Day 1 WPC probabilities remain above 50 percent for
accumulations of 8 inches or more across the northern WA Cascades
including Snoqualmie and Stevens passes. Heavy accumulations also
remain likely for portions of the northern Rockies, with WPC
probabilities remaining high for accumulations of 8 inches or more
across the Lewis Range and Glacier National Park, including Marias
Pass.

Tonight into Sunday, the upper trough will amplify as it spills
onto the High Plains, pushing a strong cold front south through
the northern Rockies and High Plains. This will shift the focus
for better organized snows farther south along the northern
Rockies. Areas impacted include the Mission Ridge, and the Swan
and southern Lewis Ranges in northwestern Montana; the Little Belt
Mountains in central Montana; and the Bitterroot and Clearwater
mountains in northern Idaho. WPC probabilities are greater than
40 percent for snow accumulations of 4 inches or more across those
areas. However, widespread heavier amounts are not expected, with
only low probabilities indicated for snow accumulations of 8
inches or more.

Dry/ridging spreads east across the area Sunday. However, the next
shortwave trough plunges into the region late Sunday night.
Originating over the Gulf of Alaska, this system is expected to
draw colder air into the region, supporting low elevation snow
across western Washington on Sunday. The trough is forecast to
continue southeast, bringing snows and the potential for locally
heavy amounts into the Blue Mountains and the northern to central
Idaho ranges with Day 3 moderate to high probabilities for 6 or
more inches for higher terrain WA to western WY.

...Northern Plains to the Great Lakes...
Day 1...
Snows are developing and spreading east across MT this morning,
organizing along a northwest-southeast oriented baroclinic zone
extending across northeastern Montana that will soon reach into
the Dakotas. While widespread heavy accumulations are not
expected, locally heavy amounts are expected over far northeastern
Montana, near the U.S.-Canada border this afternoon before the
activity further develops as it shifts east tonight.
In fact, the threat for heavy snow is expected to expand markedly
across ND and northern MN tonight as the previously noted
shortwave trough moves east of the Rockies and amplifies over the
northern Plains tonight through Sunday. Models continue to show
strong low-to-mid level frontogenesis and favorable upper forcing
supporting a swath of moderate to locally heavy snow shifting east
across North Dakota and northern Minnesota overnight, before
reaching into northern Wisconsin and Michigan during the day. Day
1 WPC probabilities are moderate for 6 or more inches across
northern ND, though the true mesoscale banding potential is not
realized in the input guidance, so areas of 8 inches or more are
likely in at least a narrow strip. The highest probabilities on
Day 2 remain over the Minnesota Arrowhead, where heavy snow Sunday
is enhanced by Lake Superior and then the system pivots with wrap
around continuing through Sunday night There are moderate
probabilities for a foot or more over the northern North Shore.

The strong surface cyclone is expected to track across the U.P. of
Michigan and Lake Superior Sunday night. Additional synoptically
driven snows Sunday night will be followed by lake enhanced snow
showers of Superior and Michigan in the comma head through Monday
before slowly tapering off Monday night. Day 2 probabilities for 8
or more inches are moderate over the typical snow belts of the
Porcupines, Keweenaw, Huron Mtns, and southeast of Whitefish
Bay/SSM. Day 3 probabilities are moderate for 4 or more
additional inches off Superior and Michigan, but strong winds and
decent cold air should allow locally much more. A fairly brief
bout of LES off the eastern Great Lakes brings some low 4 inch
probabilities to the east sides of Lakes Erie and Ontario.

For Days 1-3, the probability of significant icing is less than 10
percent.

Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Sat Dec 04 2021

Valid 12Z Tue Dec 07 2021 - 12Z Sat Dec 11 2021

...Notable snow in the interior Northeast is currently expected
Wednesday, with wintry impacts potentially reaching southward into
the I-95 corridor and possibly the Mid-Atlantic states, while
locally heavy rain may occur over the Southeast...

...Considerable mountain snows are likely across the Cascades,
Sierra Nevada, Intermountain West, and Rockies during the latter
half of next week...

...Overview...

Expect a broad mean trough aloft to prevail across much of the
lower 48 next week, with an axis generally extending from
central/east-central Canada into the western U.S. Embedded
shortwaves will push low pressure/frontal systems across the
country, leading to a somewhat more active weather pattern than
has been the case lately. In particular, parts of the West should
see meaningful precipitation while the intensity/coverage of
rainfall should increase over the southeastern quadrant of the
country and some impactful snow may fall over northern parts of
the East.

...Guidance/Predictability Assessment...

After multiple days of GFS runs had been significantly stronger
with shortwave energy coming into the West Day 3/Tuesday, the 18Z
and 00Z GFS have come in weaker with the feature, more in line
with other guidance. So there seems to be an increasing consensus
with this shortwave, but uncertainty is still somewhat high given
the energy stems from the less predictable high latitudes. The
uncertainty in the details of the forecast continues Wednesday
onward despite reasonable consensus for the overall pattern.
Differences in the handling of this shortwave energy as well as
northern stream/Great Lakes energy lead to evolution and
track/timing uncertainty for an area of low pressure from the
eastern U.S. into the western Atlantic Wednesday-Thursday. Overall
the low pressure system continued its overall trend more
suppressed, but with considerable variability in its track and
timing with the 12/18Z model cycle and the incoming 00Z cycle,
which leads to uncertainty with precipitation amounts across the
Mid-Atlantic and Northeast.

Overall there is fairly good agreement on another bundle of
Pacific energy coming into North America Wednesday, with less
amplification than its predecessor, producing southern Canada low
pressure that anchors a front reaching into the West/Plains/Great
Lakes. The 12Z ECMWF appears to be the outlier with this surface
low, faster/farther east compared to consensus and the previous
model cycles. Then, energy is yet again expected to dig southward
and amplify troughing in the West by Friday, though the degree of
amplification differs among guidance, with the 12Z CMC depth a bit
on the more extreme side.

Model preferences led to starting with a composite of 12Z/18Z
operational models for about the first half of the period. Then
the forecast blend transitioned to a blend of the operational
12/18Z GFS and 12Z ECMWF and the 18Z GEFS/12Z ECMWF mean.

...Weather/Hazards Highlights...

Gulf moisture should increase by Tuesday into Wednesday ahead of
frontal systems and cause moderate to possibly locally heavy
rainfall from the Lower Mississippi Valley into the Tennessee
Valley and Southeast. Moisture spreading farther north into cooler
air should lead to snow across the northern latitudes, with the
highest probabilities of notable snow across the Northeast. There
is still considerable uncertainty over how low pressure from the
eastern U.S. into the western Atlantic will evolve/track, so
confidence is fairly low for exactly how much precipitation may
fall over the Northeast, and how far south significant snow
reaches. Some rain could continue late next week near a wavy front
lingering over the extreme Southeast.

Fairly light precipitation is forecast for the Pacific Northwest
and the Four Corners states on Tuesday, before precipitation
chances increase from midweek onward. As shortwave energy and
troughing drop southward through the West, precipitation will
spread from the Pacific Northwest and Northern Rockies Wednesday
southward to California into the Great Basin to Central Rockies
Thursday and Friday. Generally expect higher elevation snow and
lower elevation rain, with snow levels likely to decline later in
the week as upper troughing amplifies over the West. Parts of
Southern California may see its first rainfall in over a month.

Chilly temperatures are likely across the far north-central U.S.
on Tuesday given snow cover from Sunday-Monday's system, with
temperatures 10-20F below normal. Other than that, temperatures
should be within about 10 degrees of normal in most places across
the lower 48 Tuesday and Wednesday before a warmup by Thursday
through Saturday from the Plains eastward, with 10-20F above
average temperatures. Cooler temperatures may spread into the West
late in the week (highs 5-15F or so below normal by Friday and
Saturday) with the potential for upper troughing to amplify over
the region.

Tate/Rausch


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml