050
FOUS11 KWBC 221908
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
208 PM EST Sat Feb 22 2025

Valid 00Z Sun Feb 23 2025 - 00Z Wed Feb 26 2025

...Pacific Northwest through the Northern Rockies...
Days 1-3...

The overall synoptic evolution forecast remains pretty consistent
among guidance as of the 12z NWP suite. A closed mid- level low
tracking slowly south of the Aleutians will continue shedding
lobes of vorticity eastward and onshore the Pacific Coast,
spreading precipitation from the Pacific Northwest through the
Northern Rockies into early next week.

Generally W to SW flow will persist across the Pacific during this
time, with enhanced confluence south of the Aleutian Low enhancing
moisture advection and IVT as an atmospheric river (AR). Both ECENS
and GEFS probabilities indicate that IVT will almost certainly
(>90%) exceed 500 kg/m/s, with 3 consecutive days of IVT exceeding
250 kg/m/s surging onshore beginning this evening. Within this
moisture plume, PWs are progged by NAEFS to exceed the 99.5
percentile of the CFSR database, especially late D1 through the end
of D2 from the WA/OR coasts to as far inland as the Central
Rockies. Shortwaves/vorticity spokes rotating within this flow will
cause periodic enhancement to ascent leading to heavier and more
widespread precipitation. Although the timing of these features is
not fully agreed upon by the guidance, consensus suggests bouts of
heavier precipitation are likely beginning late D1 and again late
D2. While this will result in heavy snow in higher terrain features
across the region through the period, snow levels will climb
steadily within the AR/accompanying WAA. Snow levels begin the
period around 4000-6000 ft, then climb to as high as 7000-9000 ft,
especially during D2. This will keep most significant snow
accumulations above area pass levels.

As me move into D3, a tertiary shortwave trough will pivot
underneath the prominent gyre to the north generating a more
significant height fall pattern as we step into Tuesday morning.
Snow levels that rose considerably in the periods prior will see
them decrease rapidly as the advancing shortwave trough allows for
colder temps to seep down into the previous 4000-6000 ft range
culminating in more widespread heavy snowfall across the Northern
Cascades and higher elevations of the Olympics in WA. Steady
progression of colder air mixing further into the column will
protrude areas east of the coastal ranges with areas inland
towards northern ID and northwestern MT seeing an uptick in
snowfall for the 12-00z time frame Tuesday into Wednesday. Heaviest
snow will still be confined to the Cascades, but some areas east
of the mountain chain will see >4" of snowfall, namely over the
Bitterroot Range extending northeast towards the Cabinet Mtns., and
southeast into the Absaroka Range.

The heavy snow footprint continues to be most prominent in that D2
to D3 time frame with the highest probs (>70%) for >4" located over
the ranges above. Additional high probs over 70% will exist in the
highest elevations of the Olympics by D3 due to the incoming
disturbance. Totals >8" are split between the remainder of D1 with
a break between D1 and part of D2, but will pick up significantly
in both magnitude (50-80%) and spatial coverage of the higher probs
across the PAC Northwest by the end of D2 into D3 within the
vicinity of the Cascades, Olympics, and Bitterroot Ranges. This
will culminate in the highest elevations encountering several (4+)
feet of snow over the next 72 hrs. with 1-3 feet forecast for areas
between 4000-7000 ft MSL. Additionally, some light icing is
expected in the Columbia Basin, as well as parts of eastern OR
where WPC probabilities D1 into early D2 reach 10-30% for up to
0.1" of ice.

Weiss/Kleebauer

...Tug Hill through Northern Green & White Mountains...
Day 1-2...

Shortwave trough analyzed over the northern Great Lakes will
continue pushing downstream into neighboring Quebec with the
southern fringes of the energy bisecting Upstate NY through
Northern New England by this evening. Sufficient low-level
buoyancy generated by modest delta-T`s over Lake Ontario will
create a moderately strong singular band that will align downwind
of the lake with the fetch aimed into the Central and Southern Tug
Hill Plateau. Combined moisture fetch and local upslope component
will allow for a targeted area of heavy snow to those areas within
the band placement. High probs of 70-90% for >4" exist within the
window of 00-18z Sunday with a very small area of 40-70% probs for
>6" intersecting the Tug Hill area between Watertown to Pulaski and
points inland. This is the primary zone for heavy snow within the
Northeastern U.S, but some upslope snows within the Northern Green
and White Mountains of VT/NH also point to some modest
probabilities of >4", especially as you enter the Greens from Mount
Mansfield and points north.

Kleebauer

$$