Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Tue May 02 2023

Valid 12Z Tue May 02 2023 - 12Z Fri May 05 2023

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

Persistent closed low plaguing the Great Lakes the past few days
will gradually pivot east-southeast through mid-week, with the
associated surface low also drifting eastward with time. Continued
northerly flow behind this low as it ejects eastward will produce
persistent CAA across Lake Superior. This northerly flow will have
a two-pronged effect on snowfall during D1, as it maintains modest
isentropic lift and advection of the theta-e ridge cyclonically
around the low, while also providing at least modest moisture
enhancement from the lake. 850mb to lake sfc temperature
differences are modest, only around 7C, so true lake effect snow
is not expected. However, ascent beneath steep lapse rates within
th core of the upper low, the aforementioned isentropic upglide,
and some upslope component, especially in the Huron Mountains,
will produce periods of heavy snow the first half of D1 before
waning as the column dries. SLRs will likely remain quite low,
especially after sunrise in the May sun angle, but significant
accumulations are still likely before precip winds down tonight.
Additional snowfall of more than 4 inches is possible today,
primarily in the Huron Mountains as reflected by WPC probabilities
exceeding 50% in that range, which could bring event total
snowfall to 2-3 feet in some areas.


...Central Appalachians...
Day 1...

Anomalous mid-level low will continue to spin slowly across the
Great Lakes before finally diving southeast to the Mid-Atlantic
and beginning to shear out Wednesday night. Impressive cyclonic
flow around this feature will maintain westerly winds into the
Appalachians today, with spokes of vorticity rotating around the
gyre helping to drive ascent. The atmospheric column is quite cold
for early May, with 850mb temps falling below 0C all the way into
Virginia through Wednesday. The lobes of vorticity will cause
periodic fluctuations in the 850mb temperatures, and despite
persistent W/NW flow, this may create weak WAA at times into the
region which will not only provide additional support for lift,
but also some enhanced moisture which will be wrung out during
periods of upslope flow. Snow levels through Tuesday should be
around 3000 ft, which will result in moderate to heavy snow in the
higher terrain of WV/MD, with some lighter accumulations likely as
low as 2000 ft into western PA as dynamic cooling and
precipitation loading help drag down some colder air at times. The
intensity of snow will likely be quite variable at times, causing
a lower than typical confidence in forecast amounts, especially
during the day as May sun angle causes some melting and compaction
resulting in low effective SLR. Still, significant accumulations
are likely in the higher terrain as reflected by WPC probabilities
exceeding 80% above generally 4000 ft in the WV/Central
Appalachians, with locally more than 8 inches possible.


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

Omega block centered across the High Plains will force an
amplified closed low to drop slowly southward along the coast of
CA today and Wednesday. As the omega block finally breaks down
into Thursday, this low will begin to eject into more pronounced
westerlies and shift onshore and into the Great Basin on Thursday.
Ascent surrounding this feature will be sufficient to produce an
expanding area of precipitation today, which will likely fall as
snow above modest freezing levels of 4000-5000 ft. While PW is
progged to be near to even slightly below normal, the mid-level
height falls, PVA, and periodic upper diffluence will combine with
upslope flow into the Sierra to wring out this PW as snow, and
rates may reach 1"/hr at times where upslope is most pronounced.
The most significant precipitation is expected D1 before some
drying occurs D2 leaving just lingering light snow in the terrain.
However, by D3 as the entire system shifts eastward, including the
weakening surface low beneath the closed low aloft, a renewed
surge of moisture and precipitation is expected with additional
rounds of heavy snow. WPC probabilities D1 and D2 for more than 4
inches are confined to the Sierra and generally lower and sink
southward during the period, but locally more than 8 inches of
snowfall is likely in the higher terrain these first 2 days. By
D3, as the surface low ejects onshore bringing renewed moisture
and ascent, snowfall will increase once again across the Sierra
but also expand eastward as spillover becomes more impressive. WPC
probabilities for more than 4 inches on D3 reach 30-50% in the
Sierra, with some low probabilities also extending into the Great
Basin ranges in NV.


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


Weiss