Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 AM EDT Sun Apr 28 2024

Valid 12Z Sun Apr 28 2024 - 12Z Wed May 01 2024

...Pacific Northwest and Northern Rockies...
Days 1-3...

An active period of late-season winter weather is likely for mid
and high terrain as the calendar approaches May, with heavy snow
likely each day through early next week.

The primary driver of this active weather is confluent westerly
flow across the Pacific and into the Pacific Northwest, which will
transport moisture onshore. The mid-level flow will generally be
W/NW through the period, transporting a weak a AR eastward as
reflected by just modest GEFS probabilities for IVT exceeding 150
kg/ms, and this direction has been shown to be less favorable for
significant moisture and associated QPF than those with a S/SW
direction. However, the persistence of this flow, lasting all 3
days of the forecast period, combined with subtle mid-level backing
at times in response to multiple potent shortwaves advecting
through the flow, will result in widespread precipitation from the
Olympics and Cascades into the Northern Rockies by Tuesday.

Forcing into the moisture will be driven by periodic divergence and
height falls ahead of each shortwave, aided by waves of LFQ
diffluence as a zonal jet streak pivots to the east. The generally
westerly low-to-mid level flow will additionally upslope into N-S
terrain features, producing enhanced upslope flow, and where this
interacts with the greater synoptic ascent, heavier precipitation
is likely. Snow levels will begin around 3500-4500 ft, but will
drop steadily behind a cold front, reaching as low as 2000 ft by
Monday evening, and then hovering around 2500-3000 ft through
Tuesday, although the heaviest snow accumulations should remain
above 3500 ft (near the NBM 75th percentile).

Most of the snow will be produced via the aforementioned overlap of
upslope flow and synoptic lift, but an interesting development on
D3 may enhance snowfall across the Northern Rockies, especially
near Glacier NP. During this time, an inverted trough extending
from a wave of low pressure moving across Saskatchewan will rotate
southward from Canada, producing enhanced lift through weak but
overlapped fgen/deformation, as well as causing a wind shift to the
E/NE to upslope some higher moisture content air. This could result
in heavier snow rates and hence accumulations D3, but spread
remains considerable in the model output. This will need to be
monitored for any hazards in the next few days as WPC probabilities
for 6+ inches of snow are already 70-80%, and locally more than 12
inches is becoming likely.

Otherwise, WPC probabilities for more than 6 inches of snow on D1
are 50-90% across the WA Cascades and in portions of the Olympics
and OR Cascades. By D2 the heavy snow spreads more extensively to
the east, reaching 30-50% in the Salmon River and Bitterroot
Ranges, while continuing another day across the Cascades. With
snow levels falling below pass levels, significant snowfall
exceeding 6 inches is becoming more likely at many of the Cascades
Passes as well, including Santiam, White, Snoqualmie, and Stevens
Passes.


...Minnesota...
Day 2...

A closed mid-level low and associated occluded surface low will
lift out of the Central Plains Sunday night and weaken while
pivoting into the Great Lakes Monday. Downstream of this feature,
impressive synoptic ascent through jet-level diffluence, mid-level
divergence, and WAA along the elevated front will spread
precipitation into the Upper Midwest and Great Lakes. This WAA is
likely to be intense, and accompanied by some weak deformation to
enhance omega into the moistening column. This will result in an
axis of heavy precipitation, which will initially fall as freezing
rain as surface wet-bulb temperatures remain just below 0C from
near Duluth, MN northward along the Arrowhead. The guidance has
become a bit more aggressive with icing accretion tonight, but
robust lift and wet-bulbs just near freezing without dry-advection
to offset warming due to the latent heat of freezing should limit
accretion below what the models are producing. There may be some
enhance icing in the higher terrain of the iron ranges, but after
coordination with WFO DLH, the preferred solutions are near the WSE
mean and NBM 75th percentile, which is reflected by WPC
probabilities for 0.1" of freezing rain that reach 10-20% in the
northern part of the MN Arrowhead.

Weiss