Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
145 PM EST Thu Feb 27 2025

Valid 00Z Fri Feb 28 2025 - 00Z Mon Mar 03 2025


...Upper Great Lakes through the Northeast...
Days 1-2...

An unusually strong clipper system (MSLP below the first
climatological percentile per the ECMWF) diving southeast from
southern Canada will head for the Upper Great Lakes on Friday. The
storm will reside beneath the diffluent left- exit region of a
90-100 knot 500mb jet streak, while also sporting an unusually
strong NWrly IVT that surpasses 300 kg/m/s on the western flank of
the surface low. The low- level circulation at 850mb is also rather
potent with heights that are below the 0.5 climatological
percentile by 18Z Friday according to NAEFS. This equates to what
would be a "front- end thump" of heavy snow late Thursday night
across the Minnesota Arrowhead, then across the Michigan U.P. and
far northern Michigan Friday morning where the best 850mb FGEN
ensues. Just north of the 850mb FGEN is where snowfall rates of
1-2"/hr are possible for several hours and supported by the latest
12z HREF, which has trended slightly north over the last few
forecast cycles. A dry slot and warming boundary layer should
limit snowfall over the Michigan U.P. and northern Michigan Friday
afternoon, leading to a relatively narrow swath of heavy snow. As
the storm heads east though, additional snow is likely on the back
side of the storm as low- level CAA over Lake Superior increases
Friday evening. Snow should taper off by Saturday morning in the
Upper Great Lakes. WPC probabilities show moderate-to-high chances
(50-70%) for snowfall totals >6" in the MN Arrowhead with similar
chances for >8" in the Keweenaw Peninsula, Huron Mountains, and
eastern Michigan U.P.. Localized amounts approaching a foot are
possible closer to Sault Ste. Marie, MI and locations just south
and east.

The storm continues its march east into southern Ontario Friday
evening with the same method of producing periods of heavy snow as
far east as the Tug Hill and Adirondacks of northern New York.
Unlike the Upper Great Lakes, however, the storm track is farther
north through the St. Lawrence Valley. WPC's Snowband Probability
Tracker, utilizing the 12Z HREF guidance, shows the potential for
1"/hr rates from the Tug Hill to as far east as the Green and White
Mountains Friday night. The dryslot and warming low level-
temperatures arrive Saturday morning with only northern Maine
likely to still see periods of moderate-to-heavy snow through rest
of the day Saturday. Snow should taper off throughout the Interior
Northeast by Saturday night, with some lake-effect showers
lingering into Sunday. WPC probabilities show moderate- to- high
chances (50-70%) for snowfall >6" in the Tug Hill and Adirondacks
neighboring the Tug Hill through Saturday morning. These chances
drop across the remainder of New England, with 30-50% probabilities
of 6+ inches found only across northern ME.

Additionally, some lingering upslope/lake-effect snow is likely
tonight across the Interior Northeast following the passage of a
strong cold front, extending from as far south as the central
Appalachians. WPC probabilities for at least 4 inches of snow are
low (10-30%) across the central Apps, western Upstate New York, the
Adirondacks, Green Mts and White Mts of New England.


...California...
Day 3...

An upper trough traversing the eastern Pacific Saturday is forecast
to close-off by Sunday and follow a weakness in a western U.S.
ridge behind a weaker upper low swinging across the Four Corners.
This will allow for a period of light to moderate precipitation to
spread onshore the West Coast on Sunday. IVT values are expected to
remain weak (below 250 kg/m/s), so overall amounts shouldn't add
up to much. Nonetheless, with snow levels around 4000-5000 feet,
WPC probabilities for at least 6 inches of snow through Sunday
night are medium (30-50%) across the Sierra Nevada and Shasta
Siskiyous of northern California, highest probs mainly above 8000
feet.


Snell/Mullinax