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

Valid 12Z Mon May 01 2023 - 12Z Thu May 04 2023

...Upper Great Lakes...
Days 1-2...

A cold core and expansive upper low will move very slowly eastward
from MI to Upstate NY through midweek in response to the
exceptionally amplified pattern across North America, and work in
tandem with coupled jet streaks to drive pronounced synoptic
ascent across the Great Lakes. The result of this synoptic lift
will be a deepening area of surface low pressure which will likely
retrograde north of Lake Huron today before tracking over southern
Ontario Tuesday, leaving a cold airmass across Lake Superior and
areas south. As this low deepens, moisture advection will increase
as 285-290K isentropic ascent surges a theta-e ridge cyclonically
into the Great Lakes, and where this overlaps with a cooling
column on intensifying northerly flow /CAA/, heavy snow will be
the result. Although 850mb - lake sfc temperature differences are
expected to be generally less than 10C, the persistent fetch
across Lake Superior will drive lake enhanced precipitation. Where
this flow then intersects some of the higher terrain of the U.P.,
including the Gogebic, Porcupine, and Huron ranges, a long
duration of heavy and dense snow with rates greater than 1"/hr in
spite of SLRs generally 6-10:1 will occur.

The overall column remains marginally cold for all snow, but in
higher elevations and/or during periods of more intense ascent
sufficient dynamic cooling is expected to produce heavy snow. So
near lake level, expect a mix of rain and snow at times. This
leads to a challenging accumulation forecast, and a situation
where "effective"SLR due to melting/daytime sun angle could be
much lower than the "true" SLR otherwise supported by the
atmosphere. Expect then that this snow will be very heavy and wet,
but still be able to accumulate rapidly and efficiently at times.
Although guidance continues to be exceptional with its snowfall
output today and into Tuesday, have lowered amounts a bit through
lower SLR and it is likely that snow depth will be much less than
measured snowfall for this event, which should be 1-2 feet across
much of the central U.P., with local amounts potentially up to 3
feet across the Huron Mountains. With this being a heavy and wet
snow, expect extreme impacts as noted by the WSSI/PWSSI, with tree
damage and power outages expected.


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

Anomalously cold airmass will settle across the eastern CONUS as a
closed mid-level low with height anomalies exceeding -4 standard
deviations with respect to 700-500mb heights on the NAEFS ensemble
tables drifts across the region. Impressive cyclonic flow around
this feature will drive spokes of vorticity from the Great Lakes
through the Mid-Atlantic, and as 850mb temps crash below 0C, this
will support waves of snowfall, especially in the higher terrain
above 2500 ft. Despite the airmass being quite cold, it is still
marginal for snowfall across much of the east (it is May), but
periods of more intense ascent, especially where mid-level lapse
rates are the steepest, could result in snow accumulating even
below 2500 ft. However, the heaviest snow is likely in the higher
elevations of WV and the MD Panhandle above 3500 ft where upslope
flow will help drive more intense ascent and more rapid
accumulations despite SLRs that should remain quite low. With leaf
out already occurring, heavy snow could result in significant
impacts as noted by WSSI/PWSSI suggesting major impacts in the
highest terrain. Expect this event to materialize as generally
longer periods of moderate snow instead, which will also inhibit
more significant accumulations due to compaction and melting,
lowering the measured SLR, but in a few areas warning level
snowfall is still possible. WPC probabilities reflect this, with
small areas of 40-60% for more than 4 inches of snowfall D1 and D2
in the highest terrain of WV.


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

Extremely amplified mid-level pattern across the CONUS will
manifest as an anomalous closed low with 700-500mb heights falling
to -3 standard deviations below the climo mean according to the
NAEFS ensemble tables. As this closed low sinks slowly southward
along the CA coast through mid-week, downstream confluence
overlapped with a modest subtropical jet streak will spread
moisture onshore, but with near normal PW anomalies. However,
ascent across CA will increase through height falls and modest
upper diffluence, helping to deepen a surface low which will track
southward beneath the mid-level feature. Onshore S/SE flow
downstream of this wave will help surge moisture onshore, with
flow favoring upslope ascent into the Sierra through the period.
With snow levels falling beneath the cold core low, from around
6000 ft early to as low as 4000 at the coldest, this will support
waves of heavy snow in the Sierra, with snowfall rates at times
likely reaching 1"/hr. Although snow will likely not be continuous
during the three days, periods of heavy snow will result in
significant accumulations in the Sierra, including at pass level.
The greatest WPC probabilities for more than 6 inches occur on D2,
centered in the higher terrain of the Sierra when they reach
50-60%, but are otherwise generally 10-30% both D1 and D3. This
could result in significant 3-day total snowfall accumulations of
more than 12 inches in some areas.


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

Weiss