Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Mon Mar 27 2023

Valid 00Z Tue Mar 28 2023 - 00Z Fri Mar 31 2023

...Northeast...
Day 1 & Day 3...

A progressive but potent mid-level vort max will foster a wave of
low pressure over the Mid-Atlantic this afternoon that will track
off the Northeast coast later this evening. High pressure over
James Bay is helping to anchor just enough sub-freezing air to
support periods of snow over the tallest mountain ranges tonight.
Latest WPC PWPF shows 10-30% odds for >4" of snowfall in the Green
and White Mountains with 5-10% probabilities in the Catskills. The
tallest peaks could approach 6" in some spots in central New
England. Totals will be minor to even little accumulations at
lower levels. Periods of snow will dissipate by early Tuesday
morning as a weak bubble of high pressure build in from the Great
Lakes.

By Wednesday, a strong cold front will swing through the Great
Lakes and head for the Northeast Wednesday evening and into the
overnight hours. There are some hints on some guidance that this
cold front could cause a rapid drops in temperatures that may
result in a quick changeover from heavy rain to a heavy burst of
snow. In fact, guidance such as the hi-res NAM and FV3 are cold
enough to where the line looks more like an organized line of snow
squalls. It is still early so there is time to further monitor
trends in guidance, but there is the potential for a burst of snow
to push through causing reduced visibility from heavy snow rates
and wind gusts. Cannot rule out the possibility for quick snow
accumulations in affected areas as well.

...Great Lakes...
Days 2-3...

A fast moving cold front taking shape over the Midwest Tuesday
night will race across the Great Lakes Wednesday morning. Lapse
rates in wake of the cold frontal passage are steep enough, along
with  to where some brief lake effect streamers are possible
Wednesday morning and through the midday hours. Lake effect bands
will linger longest over the U.P. of Michigan and northern
mainland Michigan. WPC PWPF show 20-40% probabilities for >4" of
snowfall in these areas with snow concluding Wednesday afternoon
as high pressure quickly builds in overhead by Wednesday evening.
Look for some slick spots on roads and visibilities could be
greatly reduced in the heaviest bands of snow.

...The West...
Days 1-3...

The western third of the Lower 48 is set to see the most active
winter weather pattern in the short term. A barreling upper level
low off the West Coast (forecast to be at or below the 1st
climatological percentile at just about every mandatory height
level according to NAEFS between 12Z Tuesday - 12Z Wednesday) will
direct a conveyor belt of moisture at California and Oregon on
Tuesday. The IVT according to NAEFS is shown to be up to the 99th
climatological percentile up and down the coast of California
starting tonight and lingering through Wednesday morning. These
anomalous IVT values (still projected to be above the 90th
climatological percentile) will work their way into the Southwest
and the central Rockies Wednesday and into Thursday. PWs up to
0.75" will also be above the 90th climatological percentile along
the California coastal range north of San Francisco and work their
way south through central and southern California through
Wednesday.

With such anomalous moisture and strong onshore flow, heavy
mountain snow is anticipated from the Sierra Nevada and
Trinity/Salmon Mountains of California on north along the Cascade
Range into southern Washington. The former ranges mentioned in
California will see the heaviest snowfall through mid-week.
Snowfall will be measured in feet there, ranging between 2-4 feet
in these ranges. Farther north, the Cascade Range can expect 1-2
feet of snow through mid-week. Even as far south as southern
California, the higher terrain of the San Gabriels and San
Bernadinos could see up to a foot of snow. The Days 1-3 WSSI show
an extensive area of Major to Extreme impacts in the Sierra Nevada
and northern California mountains, primarily driven by the
combination of snow amount and snow load. Strong winds will also
contribute to whiteout conditions with impossible travel for
roadways at and above 4,000 feet in elevation.

The moisture from this upper low, including the upper low itself,
will spill over into the Great Basin and eventually the
Intermountain West Tuesday and into both Wednesday and Thursday.
Snowfall between 1-2 feet is anticipated in the higher terrain of
eastern OR, the Sawtooth, central Nevada, the Wasatch, and into
both the Tetons and Wind River Ranges. The Days 1-3 WSSI does show
some Minor to Moderate impacts in some of these aforementioned
areas, suggesting some impacts to daily life and travel are
expected through mid-week.


For Days 1-3, the probability of significant icing greater than
0.25" is less than 10%.


Mullinax