Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Sat Dec 21 2024

Valid 00Z Sun Dec 22 2024 - 00Z Wed Dec 25 2024


...California, Pacific Northwest & Northern Rockies...
Days 1-3...

A particularly active pattern over the north Pacific continues
through the week ahead with a weakening wave over the interior
Northwest this evening, the next wave reaching the PacNW coast late
Sunday, followed by a more potent trough that arrives Monday night
and digs down the CA coast Tuesday.

Snow levels will be around 4000ft tonight as the ongoing wave
lifts across the northern Rockies. Day 1 PWPF are 30-60% for an
additional 6" for the ridges in and around Glacier NP.

A lull under brief ridging can be expected late tonight into Sunday
before a moisture plume ahead of the next wave reaches the PacNW
coast around midday. Snow levels surge above 6000ft with this plume
and generally drop to around 5000ft under the trough axis late
Sunday night when precip begins to taper off as flow backs
southerly. Day 2 PWPF for >6" are generally limited to the
northern WA Cascades, though they are 50-80% for the Tetons and
immediate ranges to the south as sufficient moisture streams inland
with the wave.

A potent atmospheric river arrives Monday early evening with a
focus on far northern CA. Snow Levels understandably surge up over
8000ft over the CA and southern OR Cascades Monday evening, but
crash below 5000ft Tuesday morning under the trough axis that digs
down to the central CA coast before rushing onshore Tuesday
afternoon. A wave of heavy precip shifts south down the Sierra
Nevada on Tuesday with snow levels generally dropping to 7000ft
during times of heaviest rates. Day 3 PWPF for >8" are 30-70% in
the higher WA Cascades and the High Sierra.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

First, ongoing LES in NNWly flow off the eastern Great Lakes will
slowly diminish overnight as flow gradually weakens as high
pressure shifts over central Ontario. Day 1 PWPF for an additional
4" after 00Z are 20-50% mainly off central and eastern Lake
Ontario.

A wave over the interior Northwest this afternoon develops into a
mid-level low over north-central MT tonight in the left- exit
region of a 110kt jet streak. Southerly low level ahead of the
wave
will intersect a frontal boundary and spread a wintry mix from
northeast MT across much of ND through Sunday and then central
MN/northern WI Sunday night. The moisture source and DGZ aloft are
marginal, while 850mb temps rise >0C despite surface temps
remaining below freezing. Expect a swath of light icing
from northeast Montana and northern North Dakota to central
Minnesota with PWPF for >0.1" around 10% in northwest ND. Narrow
bands of locally moderate snow can be expected north of the
surface wave and the mix and may produce an inch or two of
accumulation in spots through Sunday night.

By Monday morning, a more consolidated surface low will form in
the Upper Mississippi Valley at the same time 925-850mb moisture
(with origins out of the Gulf of Mexico) streams into the Great
Lakes. 850mb WAA over a boundary layer air-mass that is cold and
quite dry will support wet-bulb temperatures that support mostly
snow from northern Wisconsin/the Michigan U.P. to the northern
half of Michigan's Mitten. Broad 250-500mb troughing will also
provide adequate lift, giving rise to a more solid shield of
snowfall over the region. Day 2 WPC probabilities for >4" remain 
moderate (40-60%) in the northern portions of Michigan's Mitten
and in the eastern third of the U.P.

Monday night, this wave shifts east over the Northeast with 1-2"
of synoptically driven (not LES) likely for areas east of Lake
Erie over the eastern Great Lakes Monday night with potentially
1-3". Snow spreads Monday evening over much of the rest of NY and
central/northern New England. The progressive nature keeps max
totals limited with Day 3 PWPF for >4" generally 20-40% over the
Adirondacks, Greens and Whites as well as Down East Maine. The
highest probabilities are around 50% in the Tug Hill region where
upslope effects and some lingering LES maximize snow there. The
surface low develops a bit over the Gulf of Maine, so interests in
coastal Maine should watch for banding snow (like last night) as
the system pulls away Tuesday morning.


Jackson