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

Valid 00Z Mon May 01 2023 - 00Z 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 lakes MI/Huron Monday 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 rather wey/very dense snow
with rates greater than 1"/hr in spite of SLRs generally 7:1 to
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 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, as already seen
last night with 6-8" reported over much of the Interior western
U.P. into northern WI. 12Z guidance continues the trend of
increased snowfall, especially in the western and increasingly
central U.P. through Monday night. It is likely that many areas a
few hundred feet above lake level will receive 1-2 feet of snow,
and despite the low SLR and high sun angle, see continued increase
in snow depth through the event. There is an isolated risk for 4
feet of snow, particularly in the highest portions of the Huron
Mtns in central U.P. where 12Z guidance focuses the prolific
precipitation with this event. 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 2-3...

A cold core low will drift east from Michigan to Upstate New York
through 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 left exit diffluence as a jet streak develops across the
Southeast. The column will be generally quite cold for early May,
with 850mb temps falling below 0C, but still generally marginal
for wintry precipitation below about 2500ft. The westerly upslope
flow into the terrain of east-central West Virginia should produce
periods of moderate to heavy snowfall Monday into Wednesday. WPC
probabilities for >6" are 30 to 40 percent for areas above about
3000ft on Day 2 with probabilities 10 to 30 percent for an
additional >6" for Day 3. There is a risk for 24" storm total snow
above 4000ft which occupies little territory in WV. Some light
snow accumulations are likely above areas over 2500ft with
moderate probabilities for >2" extending up Garrett Co MD and into
the Laurels in SW PA. For higher elevations, impacts will worsened
by low SLR on trees that have already leafed-out.


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

Strong low off the WA coast this afternoon will eject south as a
reinforcing shortwave trough plunges from the Gulf of Alaska on
account of the omega blocking ridge centered over the
Intermountain West. This low will continue to deepen as it digs
southward to the north-central CA coast, directing increasingly
moist onshore flow through downstream confluent flow beneath a
modest jet streak rotating around the base of the trough into CA,
especially after 00Z 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 will be generally 5000-6000 ft over
the Sierra Nevada Monday night into Wednesday, with steep lapse
rates beneath the upper low potentially allowing precipitation to
drag down snow levels toward 4500ft Tuesday. Locally heavy snow
can be expected above these elevations, primarily in the Sierra
Nevada where upslope flow will additionally enhance ascent. WPC
probabilities for >6" are moderately for the central and northern
Sierra Nevada for both Days 2 and 3. Storm total snowfall in the
central High Sierra exceeding 12 inches is likely, and impactful
snow is likely at the Sierra Passes, including I-80 between Blue
Canyon and Truckee.


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

Jackson