Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Sat Apr 29 2023

Valid 00Z Sun Apr 30 2023 - 00Z Wed May 03 2023

...Heavy and wet snow expected south of Lake Superior through
Monday night...

...Western Great Lakes...
Days 1-3...

An upper level low over western Wisconsin will continue to deepen
as it drifts east, crossing MI Sunday night. By 12Z Monday,
700-500mb height anomalies reach 4 standard deviations below
normal according to the 00Z NAEFS ensemble tables. This strong
upper low will then stall or continue a slow drift east into/over
Ontario. Both upstream and downstream jet streaks pivot through
the trough with time, leading to periods of enhanced ascent
through local diffluence where the favorable right entrance or
left exit regions of the jet streaks overlap with PVA and height
falls. This synoptic ascent will lead to further surface
cyclogenesis over/near northern Lake Michigan tonight then
stalling and retrograding around northern MI as the system
occludes and then reinvigorates. The slow movement of this feature
results in a long duration of snow bands over the west sections of
the system through Monday night.

Expect dynamic cooling to result in a changeover to snow through
this evening in banded precip currently developing over northern
WI that expand east into the U.P. of MI per 12Z CAMs. Snowfall
rates reach 0.5-1"/hr per the 12Z HREF. This band is likely to
pivot in place over the western U.P. of MI through Sunday, which
should result in several inches of snowfall with Day 1 WPC snow
probabilities moderately high for >6" in the Gogebic Range along
the western WI/MI border into the Porkies in the U.P. of MI.

The surface low continues to drift eastward as further cool air
funnels southeastward out of Canada. This will allow the overall
environment to become increasingly supportive to periods of
moderate to heavy snow Sunday night through Monday night as
moisture increases through theta-e advection wrapping cyclonically
around the system. Isentropic ascent will begin to maximize during
this time as well, noted by an increasing pressure gradient along
the 285-290K isentropic surfaces, which will further aid to wrap
moisture and ascent around the west side of the low into the U.P.,
WI, and portions of the L.P. as well. Additionally, the increasing
northerly flow which could reach 30-40 kt at 850mb will help drive
modest CAA across Lake Superior. Water surface temperatures are
only around +2C according to GLERL, and the 850mb temps are only
progged to fall to around -5 to -7C, so the delta-T values are
expected to remain below 10C, resulting in some lake enhanced
precipitation, falling as snow across the U.P. into or through
Tuesday. Some uncertainty persists due to the likelihood that
precip will fluctuate at times between rain and snow, but where it
is all snow, especially in higher terrain, significant to locally
extreme accumulations are expected, especially Sunday night into
Monday. Day 2 WPC probabilities for >6" inches are high over the
western half of the U.P. into northern WI and moderate over the
Huron range for Day 3. There may be a relative minimum along the
immediate shoreline due to warmer temperatures, but where lake
enhanced moisture and some upslope flow can combine, especially
D3, more than a foot of snow is possible, and 3-day total snow may
exceed 2 feet in areas under the best banding and higher
elevations.


...Central Appalachians...
Day 3...

The occluding low over the northern Great Lakes will expand Sunday
through Monday with the combination of lowering heights and
reinforcing impulses bringing 850mb temps below 0C in westerly
flow that brings upslope snow to the Allegheny and Potomac
Highlands of east-central WV Monday night into Tuesday. There are
low to moderate Day 3 probabilities for >4" snow above about
3000ft there.


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

Low pressure that moves down the Pacific Northwest to the central
California coast Monday/Monday night will direct Pacific moisture
into the Sierra Nevada Monday night/Tuesday. Height falls will
allow snow levels to drop below 6000ft Monday night and Day 3 snow
probabilities for >4" are moderate over the northern half of the
Sierra Nevada.


The probability of significant icing greater than 0.25" is less
than 10 percent.

Jackson