Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
230 PM EDT Mon Apr 14 2025

Valid 00Z Tue Apr 15 2025 - 00Z Fri Apr 18 2025


...Great Lakes & Interior Northeast...
Days 1-3...

The combination of a deep upper trough moving across the Great
Lakes and Northeast as well as an associated surface low sliding
from southeast Canada on Tuesday towards the Gulf of St. Lawrence
will usher in strong CAA and lake-effect snow. An upper shortwave
diving over the Midwest this evening will kick start the
changeover from rain to snow across the MI U.P. and eventually the
northern MI L.P. by early Tuesday. This snowfall may be boosted by
some lake- induced steeper lapse rates (lake temps 4-5C with 850mb
temps below -6C). Some heavier rates of >1"/hr are possible
overnight tonight in the western U.P. (30-60% probs from the HREF),
which could lead to modest snow totals there along with gusty
winds. On Tuesday, snow will end from west to east during the
afternoon into the evening as heights rise from the west. WPC
probabilities for at least 4 inches of snow are highest (>70%) over
the U.P. and low (10-30%) over the northern L.P. of MI. In
addition, probabilities for at least 8 inches of snow are 40-50%%
over some of the favored lake areas around the Porcupine and Huron
Mountains.

As the Canadian low pressure continues through Quebec its strong
cold front will sweep colder air throughout the eastern Great Lakes
and Northeast on Tuesday. Once the front clears the coast, another
area of low pressure may develop along the occlusion near northern
Maine late Tue/early Wed before finally moving away on Wednesday
mid- day, bringing the potential for accumulating snowfall for far
northern Maine. Rain will change to snow from west to east as the
column cools post- FROPA, setting up some lake- enhanced/effect
snow for areas east of Lakes Erie/Ontario and upslope snow for the
central Appalachians and into the Adirondacks and Green/White
Mountains. Lapse rates will be steep and around 7-9C/km across
much of the Northeast, but warm boundary layer temperatures should
keep snowfall confined to higher elevations and areas downwind of
the Great Lakes. WPC probabilities for at least 4 inches of snow in
Days 2-3 are 20-50% in far western WY south of BUF, 50-90% in the
Adirondacks and Tug Hill Plateau, and 40-80% in the northern Green
and White Mountains into far northwestern Maine. Low (10-20%)
probabilities for at least 4 inches of snow are present over the
central Appalachians and far northern Maine.


...Northern Rockies...
Days 2-3...

Strong height falls plunging southward out of southwestern Canada
late Wed will drive a cold front through Montana and Wyoming,
setting up a period of wintry weather through Thursday.
Northeasterly flow post- FROPA will provide plenty of upslope
enhancement into northwestern MT on Day 2 amid ample lift in the
column and lowering snow levels. Moderate to perhaps heavy snow is
likely along the Lewis and Flathead Ranges while also extending
southward and southeastward into WY on Day 3. Snow levels are
expected to drop to below 3,000 ft by Thursday across MT, but
remain slightly higher across WY. There is the potential for a
corridor of snowfall to reach into portions of the MT High Plains
on Weds night associated with modest mid-level fgen, but
uncertainty remains high with these details. WPC probabilities for
at least 6 inches of snow are high (>70%) over much of the MT and
northern WY mountains, including areas extending from the Lewis
Range to the Absarokas and Big Horns (mostly above 6,000 ft).


...Central Sierra...
Day 3...

Meandering upper low west of California will finally move into
SoCal late Wednesday night/early Thursday morning, with some
downstream generally light precipitation into the Sierra. Snow
levels will be quite high (>8000ft) and there are low probabilities
(30-60%) of at least 4 inches of snow through 00Z Friday in the
high central Sierra above 10,000ft.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Snell/Fracasso