Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Sun Apr 30 2023

Valid 12Z Sun Apr 30 2023 - 12Z Wed May 03 2023

...Western Great Lakes...
Days 1-2...

Anomalously intense upper level low with 700-500mb heights falling
to -4 standard deviations according to the NAEFS ensemble tables
will expand a longwave trough across the entire eastern third of
the CONUS this weekend through early next week. This strong upper
low will move very slowly eastward with time in response to the
exceptionally amplified pattern across the area, 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 across MI, loop to the east, and then may retrograde a
second time and loop over Ontario next week, leaving a cold
airmass across Lake Superior and the surrounding land. 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, precluding heavy lake effect snow, 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 snow with rates greater than
1"/hr (as reflected by the WPC prototype snowband tool) will
occur.

Although the overall column appears marginal for cold enough air
for all snow, in higher elevations or during periods of more
intense ascent to produce dynamic cooling, heavy snow will be the
primary p-type. However, in lower terrain, especially during
lighter precipitation rates, a mix of rain and snow may occur.
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 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, and the guidance
has continued to increase its snowfall, especially in the U.P.
During Sunday and Monday. It is likely that many elevated areas
will receive 1-2 feet of snow, and despite the low SLR, the WSE
and NBM 90th-percentile suggests isolated amounts to 3 feet. With
this being a heavy and wet snow, this will likely result in
extreme impacts as noted by the WSSI/pWSSI, with tree damage and
power outages possible.


...Central Appalachians...
Days 2-3...

An extremely anomalous closed mid-level low characterized by
700-500mb heights of more than 4 standard deviations below the
climo mean will expand across the eastern CONUS through early next
week. The core of this trough will be a closed low which will
drift slowly from Michigan today into New England on Tuesday,
resulting in sharp cyclonic flow across the Central Appalachians.
Lobes of vorticity swinging through the flow will drive periodic
enhanced ascent, aided by steep lapse rates and subtle LFQ
diffluence as a jet streak develops across the Southeast. The
column will be generally quite cold for early May, with 850mb
temps falling to 0C or below, but still generally marginal for
wintry precipitation. However, snow levels beneath the core of the
trough will be 2500-3500 ft, which when combined with upslope flow
into the terrain should produce periods of moderate to heavy
snowfall, especially in the higher terrain from the MD Panhandle
through WV Monday and Tuesday, and this is where WPC probabilities
for 4 inches or more of snow are greatest, reaching 20-40% D2 and
D3. Locally, this could result in around 12 inches of snow in the
highest terrain during the 2-day event. Depending on how intense
the ascent can be, some higher accumulations are possible, as are
some light snow accumulations even in the lower elevations. At
this time range it is too early to determine exactly how this will
evolve, but an impactful snow event is possible in some areas
early next week, with impacts worsened by low SLR on trees that
have already leafed-out.


...California...
Days 2-3...

Amplified closed mid-level low just off the Oregon coast Monday
morning will dig southward, blocked from making any eastward
progression by an impressive omega block ridge centered over the
Intermountain West. This slow moving low will continue to amplify
as it digs southward, directing increasing moist/onshore flow
through downstream confluent flow beneath a modest jet streak
rotating around the base of the trough, especially after 12Z
Tuesday. A surface low tracking southward in tandem will help
enhance ascent, resulting in an expanding shield of precipitation
spreading across CA, which will be snow in the higher elevations.
Snow levels Monday night will be generally 5000-7000 ft, but will
fall beneath the upper low to 4000-5000 ft, with steep lapse rates
beneath the upper low potentially allowing precipitation to drag
down snow levels even towards 3000 ft on D3. This will yield heavy
snow above these elevations, primarily in the Sierra where upslope
flow will additionally enhance ascent. WPC probabilities for more
than 4 inches of snow on D2 are minimal, but expand and increase
considerably during D3, reaching above 80% in the highest terrain,
and extending along much of the Crest. Local snowfall exceeding 12
inches is possible, and impactful snow is likely at the Sierra
Passes.


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

Weiss