Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Mon May 01 2023

Valid 00Z Tue May 02 2023 - 00Z Fri May 05 2023

...Michigan Upper Peninsula...
Day 1...

Atmospheric spindown from winter to summer typically results in
blocky flow, and the first few days of May are no exception. The
peak in closed low frequency over the CONUS is right on time as
the strong/deep/anomalous upper low (500mb heights near -4 sigma;
many 12Z sounding locations across MI to the Mid-Atlantic setting
monthly low 500mb heights going back 70 years per the SPC Sounding
Climo page) only slowly wobbles eastward over the next 36 hours.
The moisture source from the Atlantic will continue to wane as the
occlusion stretches out and dissolves, leaving a broad cyclonic
flow around the Great Lakes. This, however, will be enough to
sustain lake effect and enhanced precipitation, with snow
continuing over the western U.P. of Michigan through Tuesday.
Accumulations will be tied to existing snow cover, elevation, time
of day, and proximity to Lake Superior (water temperatures still
in the mid 30s) with a general heavy/wet snow on NNW flow. Heavier
bands of snow will result in higher chances of accumulation though
compaction will occur at times and especially during the day.
Nevertheless, snow rates >1"/hr will result in 6-12 inches for
many locations on top of what has already fallen since yesterday,
adding to tree stress and increasing the chance of power outages
with SLRs generally 6-10:1. Final vort max will spin around the
center of the upper low early Tuesday with a final push of
snowfall before all precipitation ends during the afternoon.
Through 36 hrs, WPC probabilities of at least 6 inches are highest
over the higher terrain of the U.P., including the Gogebic,
Porcupine, and Huron ranges.


...Central Appalachians...
Days 1-2...

Southern extent of the large upper low has brought in cold sub-0C
temperatures at 850mb across the central Appalachians and the
passage of vort maxes around the center of the upper low on 
persistent moist westerly will result in higher elevation snowfall
over the next two days. Snow levels will generally be around
2500ft +/- 500ft depending on time of day and rate of snowfall,
with higher accumulations above 3500ft. The upper low is forecast
to slowly move southeastward from southern Ontario this evening to
central PA by Wednesday morning as a final vort max swings around
the circulation. SW flow this evening will veer to westerly later
tonight through Tuesday, maximizing upslope into the mountains of
WV, western MD, and into south central PA/Laurel Highlands.
Marginal thermal profiles should limit SLRs to near and below
10:1, especially during the day, though may be quite variable
depending on snow rates that may be >1"/hr per the HREF. WPC
probabilities for at least 4 inches are highest (>50%) above
3000ft.


...California...
Days 1-3...

The West Coast will have its own closed low on the west side of
the omega block pattern, slipping down the CA coast through
Wednesday before turning east on Thursday into the southern
Sierra. 700-500mb heights are forecast to lower to -3 sigma
according to the NAEFS ensemble tables with continued
southwesterly flow aloft across much of the Sierra. Though
precipitable water values will likely be near normal (upstream
connection is from the mid-latitudes and not the subtropics), the
combination of broad upper diffluence around the nearly circular
jet, slowly approaching height falls, and upslope flow will
promote waves of snowfall as a lead vort max swings around the
main circulation followed by the upper low itself. Still,
sufficient moisture transport and flux into the terrain will
squeeze out modest amounts of snow, with snow levels wavering from
above 5000ft early to near 4000ft at times as the cold upper low
moves closer. Snow rates will likely exceed 1"/hr per the 12Z CAM
guidance with the heaviest amounts in the northern Sierra D1 and
shifting southward through D3 into the southern Sierra as the
closed low approaches. WPC probabilities of at least 6 inches are
generally above 50% above 7000ft. Three-day totals may exceed
12-18 inches at the highest peaks.


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


Fracasso