Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 PM EST Fri Jan 31 2025

Valid 00Z Sat Feb 01 2025 - 00Z Tue Feb 04 2025

...Northeast...
Days 1 & 3...

Mid-level low pressure over the Mid-Mississippi Valley early this
afternoon will shift ENE over the Midwest today and continue to
direct broad and strong moisture advection to the east. This
feature will then become strung-out as an open, but sharp wave as
it exits the East Coast early Saturday. A few pockets of freezing
rain may linger over northern PA and Upstate NY through this
evening with a few hundredths of an inch possible. Meanwhile, the
northern edge of the precip shield will feature snow bands east
from Buffalo across the Adirondacks, Green, and White Mountains,
along with southern NH and far southern Maine. This expanding
northern snow shield is being driven by a ribbon of confluent flow
between the approaching shortwave and a deeper trough to the north
over southeast Canada, which promotes extended mid-level fgen and
dynamic cooling throughout the column at least initially tonight.
Cold air advection will shortly trail the system late tonight as a
surface low pressure system transfers to the coast. This may lead
to locally heavy east-west oriented bands of snow from Upstate NY
through southern VT/NH and northern MA, which is highlighted by 12z
HREF 1-hr snow rates up to 1". Day 1 snow probabilities (beginning
00z tonight) for >4" are around 10-30%, perhaps 40% for the higher
terrain. Snow tapers off by Saturday morning.

The next shortwave trough to impact the Northeast is currently
entering the PacNW, but rides a strong jet over the Great Lakes on
Sunday and reaches the Northeast Sunday evening. This allows a warm
front to lift over the Northeast with a fairly quick hitting round
of snow. Day 3 snow probabilities for >4" are currently 20 to 40%
for only the Tug Hill, Presidential Range of the Whites in NH and
southern Maine.


...Upper Great Lakes...
Day 2...

A fast moving upper level shortwave trough tracking across the
Northern Plains Saturday night will coincide with the diffluent
left- exit region of a 250mb jet streak to provide modest ascent
over the Upper Midwest with an eastward surface low track over
northern MN Saturday night. The warm front ahead of the low lifts
north Saturday evening, increasing 850-700mb WAA to provide
additional low- level ascent and periods of snow from northern MN
on east across the Upper Midwest Saturday afternoon/evening.
Northern WI and the southern MI Upper Peninsula see moderate snow
with locally enhanced lift on the North Shore of Lake Superior on
southeast-to- easterly flow. Light snow spreads over much of MI
early Sunday while the heavier snowfall over northern MN/northern
WI tapers off by Sunday afternoon. WPC snow probabilities for >4"
are 20-50% over northern WI/western U.P. and 40-60% in the MN
Arrowhead with 70% probabilities for >6" along the North Shore
escarpment.

Snell/Jackson

...Western U.S. into the Northern Plains...
Days 1-3...

An unsettled and wintry pattern is underway for the Pacific
Northwest and will be quickly spreading into the interior/northern
Rockies this evening. Mild temperatures coincident with the
atmospheric river (AR) moisture plume axis will support snow levels
4000-5000ft across the Cascades this evening before dropping after
midnight as the cold air mass pushes in from the west. 160-170kt
jet will support broad lift on its poleward exit region across the
northern Great Basin into the Rockies tomorrow as QPF generally
decreases and focuses farther south into NorCal. There, snow levels
will be quite high (5000-8000ft from north to south) with plenty
of rain initially even up toward the Sierra passes before changing
over to snow. With a strong baroclinic zone settling into
NorCal/southeastern OR later this weekend, snow levels will remain
high over the Sierra.

For D1, the heaviest snow will fall over the Cascades while
increasing into lower elevations, eventually reaching below 500ft
Saturday. Ratios will be on the lower side given the strong Pacific
influence esp along/west of the Cascades. WPC probabilities for at
least 8 inches of snow are >50% above 2000-3000ft or so over the
WA-OR Cascades. Eastward, jet-enhanced snowfall will focus over the
Bitterroots and western MT into the Absarokas/Tetons/Wasatch with
WPC probabilities of at least 12 inches >50% above 6000-7000ft or
so from north to south.

For D2-3 on the colder side of the AR, QPF will decrease as the
jet extends into the western High Plains and the upper low over
coastal BC slowly migrates to near Vancouver Island by the end of
this forecast period. The moisture plume is forecast to dip
southward, stall, then lift back north a bit over the Sun-Mon
period, focusing the heaviest snowfall over the CA/OR border
northeastward across central ID and into western WY (lifting back
into southwestern MT Monday). To the north, snow levels will remain
low (near sea-level or valley floors for the interior) over WA/OR
though 2m temperatures may limit snowfall accumulation along the
I-5 corridor especially during the day. WPC probabilities for at
least 8 inches of snow D2-3 are >50% above 1500ft or so across
western WA/OR but above 5000ft or so into the northern Great Basin
and northern Rockies. Areas of the Tetons and Wind River Range may
see several feet of snow over the next few days. Farther east over
the western High Plains into the northern Plains, combination of
the jet extension and increasing N-S baroclinicity will support
frontogenetically-forced snowfall in a west-to-east band from near
the Black Hills across SD late Sun into early Mon (north of a
forming area of low pressure). WPC probabilities for at least 4
inches of snow there D3 are 25-55% with potential for some higher
accumulations in any bands that may set up parallel to the frontal
zone.

To the south, higher snow levels will keep the highest snow totals
above 6000-8000ft from north to south across NorCal into the
Sierra as the warm side of the system remains more or less in place
there. Across northern NV and into UT, snow levels will remain
high as well with significant snow above 8000-9000ft.

For lowland areas in WA/OR, lighter QPF and marginal daytime
temperatures may preclude appreciable snow, but WPC probabilities
for at least two inches of snow are at least 20-30% over western WA
into northwestern OR.


Fracasso