Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Thu Mar 23 2023

Valid 12Z Thu Mar 23 2023 - 12Z Sun Mar 26 2023

...The West...
Days 1-3...

Continued and reloading troughing out of the Gulf of Alaska will
maintain an active pattern for much of the West, especially from
the northern Sierra eastward to the central Rockies and northward
to the Canadian border. With the source region from the northwest
rather than southwest, the moisture levels will be near to below
average with the QPF driven by incoming height falls/PVA and
orographic enhancement/upslope. Incoming ~150kt jet west of 130W
this morning will move southeastward into CA and weaken as a
surface cold front pushes moves ashore D1 and then through the
Great Basin thereafter. Focus will be squarely over the WA/OR
Cascades and Coastal Range/Olympics as multiple vort maxes slide
through the area, especially this afternoon and again early
Friday. Snow levels around 2000-3000ft today will lower behind the
cold front to around 1000ft and below 500ft in some places,
bringing snow to some lowland areas. All mountains passes will
likely see appreciable or significant snow accumulations across
the Cascades with some snowfall rates of 1-2"/hr likely per the
WPC Snowband Tool. For the next three days, WPC probabilities of
at least 24 inches are highest (>50%) over the OR Cascades, and
are high (>70%) for the rest of the WA Cascades, Olympics, and
Coastal Ranges as well as into the Blue Mountains. WPC
probabilities for at least 4 inches of snow over the next 3 days
are >30% as low as 500-1000ft.

East of the Cascades, broad NW to SE flow aloft will carry the
vorticity train across the Great Basin into the central Rockies as
a closed mid-level low likely develops over southern ID early
Saturday and continues eastward into WY. With high pressure moving
through northeastern MT into the Dakotas, some easterly upslope
enhancement is forecast over southern MT into the WY ranges. WPC
probabilities of at least 8 inches of snow over the next 3 days
are highest (>60%) over these areas and into central ID. Lastly,
on the southern side of the height falls, the northern Sierra
eastward to the Wasatch into the CO Rockies will see several
rounds of snow, with the most on D1 (west) into early D2 (east) as
the front moves in. Snowfall amounts will be more modest here,
around 6-12" with higher totals in the mountain peaks.


...Northern Maine...
Day 1...

Low pressure moving through Quebec will drag its cold front
eastward across the Northeast D1, with a warm front across Maine,
bringing the rain/snow line quite far north. Northern Maine will
likely stay all snow, where WPC probabilities for at least 4
inches are highest (>50%) across far northern Aroostook County.


...Upper Midwest/Great Lakes into the Northeast...
Days 2-3...

Trough axis over CO/NM at the start of D2 (12Z Fri) will lift
northeastward and strengthen beneath an increasingly S-shaped
upper jet, favoring cyclogenesis out of the Ozarks/Mid-MS Valley
late Friday and continuing across the Midwest/eastern Lower MI
early Saturday. Thermal profile will be marginal across the Corn
Belt across WI, but with precipitation commencing after dark and
continued deepening of the surface low should allow for column
cooling amid a deformation axis over southern WI northeastward
across northern Lower MI (cooler overall but later arrival time
into Saturday daytime). Though there remains uncertainty in the
track of the low, amount of QPF, and temperature profile (not to
mention SLRs, ground temperatures, and snowfall rates),
ingredients support a stripe of heavier snowfall in this general
region and is supported by the higher NBM probabilities. CAM
guidance will fully get into the picture with the next iteration,
and may shed more light on specifics/trends/placement. For now,
WPC probabilities for at least 4 inches of snow D2-3 are >30% from
along the WI/IL border northeastward across northern Lower
Michigan where they increase to >70%.

Farther east, WAA-driven precip will move into the Northeast north
of a warm front that will likely linger across the Mid-Atlantic as
the triple point starts to develop near NJ Saturday night. Air
mass ahead of the precipitation will be near to sub-freezing and a
wintry mix is likely for some areas away from the coast to start.
But with the parent low track into southern Quebec, a changeover
from southwest to northeast will likely take place through much of
NY and into central New England as well as coastal Maine with
easterly flow. By the end of D3, triple point will be in the
process of taking over from the parent low near the Gulf of Maine,
which may help to keep in colder air for the Adirondacks and Green
and White Mountains into interior Maine as the closed low in the
mid-levels opens up. WPC probabilities for at least 4 inches of
snow are highest in these terrain locations and are highest (>50%)
over interior Maine.


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


Fracasso