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

Valid 12Z Sat Apr 29 2023 - 12Z Tue May 02 2023

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

A complex, evolving, mid-level pattern will result in a
challenging forecast for parts of the Great Lakes, but it is
becoming increasingly likely that a significant storm will bring
heavy late season snow to areas around Lake Superior through the
weekend.

A shortwave with an intense, strung out, vorticity lobe will dig
rapidly out of Saskatchewan and then intensify into a potent
closed low over WI/MI on Sunday. This low will become anomalously
deep as reflected by 700-500mb height anomalies dropping to -4
standard deviations according to the NAEFS ensemble tables. This
strong upper low will then pivot only slowly northeast through
Monday as the resultant longwave pattern becomes increasingly
amplified and stagnant. Aloft, the amplifying low will result in
both upstream and downstream jet streaks pivoting through the
trough with time, leading to periods of enhanced ascent through
local diffluence where the favorable RRQ or LFQ of the jet streaks
overlap with the best PVA and height falls. This synoptic ascent
will yield surface cyclogenesis across the Great Lakes Saturday,
and this low will deepen slowly with time while stalling and
retrograding in response to the increasingly vertically stacked
pattern. The slow movement of this feature suggests a long
duration of precipitation, including snow, is likely beginning
early Saturday, and persisting through early next week.

The challenge for this event will revolve around the thermal
structure of the column and timing/placement of more intense omega
to dynamically cool the column. On Saturday, a band of moderate to
heavy precipitation is likely to form in response to an overlap of
deformation and increasingly strong fgen driven by the low-level
baroclinic gradient and subtle ageostrophic response to the upper
level jet position across Lake Superior. Regional soundings in the
vicinity of this band are marginally supportive for snow, but with
the best fgen driving pronounced UVVs directly into the saturated
DGZ, expect dynamic cooling to result in a changeover to heavy
snow during the periods of most intense ascent, with snowfall
rates likely reaching 0.5-1"/hr as reflected by the WPC prototype
snow band tool. This band is likely to pivot in place near the
western U.P. of MI, which could result in significant snowfall
accumulations, and WPC probabilities for more than 4 inches are
above 50% surrounding the Gogebic range of WI/MI, with locally 6-8
inches possible.

As the event unfolds into Sunday and Monday, the surface low
begins to drift eastward as cooler air funnels southeastward out
of Canada. This will allow the overall environment to become
increasingly supportive to periods of moderate to heavy snow 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 kts 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, less than ideal for lake effect
snow. However, flow across the lakes should increase available
moisture despite limited residence times on the strengthening
winds, and it is likely there will be some lake enhanced
precipitation, falling as snow across the U.P. 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 accumulations are
likely, especially on Tuesday. WPC probabilities for more than 4
inches remain quite high, above 70% both D2 and D3, spreading from
the Gogebic/Porcupine ranges on D2 through much of the central
U.P. and into the Huron range on D3. 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 approach 2 feet in some areas.

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

Weiss