Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Mar 28 2022

Valid 12Z Mon Mar 28 2022 - 12Z Thu Mar 31 2022

...California to the Rockies...
Days 1-2...

A deep closed low approaching CA this aftn will gradually weaken
as it lifts onshore and digs into the Great Basin by tonight. This
low is expected to fill into an open trough Tuesday aftn as it
moves into the southwest, but a persistent subtropical jet streak
arcing anticyclonically through the region will maintain upper
diffluence and aid in moist advection through mid-week. The
combination of the upper jet, and briefly enhanced warm/moist
advection will drive a modest AR onshore today, and PWs are
progged to reach near +2 standard deviations on both D1 and D2 for
CA and into the Four Corners region. The overlap of ascent and
moisture will lead to heavy snow, generally above 6000 ft or so in
the terrain across this area. On D1, this will manifest as heavy
snow in the Sierra and the San Bernadino/San Gabriel ranges of
southern CA where WPC probabilities for more than 6 inches are
moderate to high, and over 1 foot of snow is possible in the
higher terrain of the Sierra. This will also produce significant
accumulations at the important Sierra Passes, including Donner
Pass.

By D2 as the best forcing and moisture shift eastward, heavy snow
will overspread the terrain from the White Mountains of AZ east of
the Mogollon Rim, northward through the San Juans, CO Rockies
including the Front Range, Uintas, and Wind Rivers, where WPC
probabilities for more than 6 inches are as high as 70% in the San
Juans, and 30-50% elsewhere.


...Upper Midwest through the Mid-Atlantic...
Days 2-3...

Complex longwave trough development through the interaction of at
least 3 mid-level shortwaves will produce a long duration winter
weather event across portions of the Upper Midwest and Great
Lakes, with light wintry precipitation extending into the
Mid-Atlantic states. Guidance still features a wide variation in
possible outcomes for this event, but it is likely that two
surface lows will emerge out of the High Plains, with a leading
low moving from CO to MN on D2, and a secondary low emerging out
of OK and lifting towards MI on D3. The guidance has trended a bit
east once again tonight, indicating of a slower phasing of
mid-level energy, and is also suggesting the secondary low may be
the deeper system owing to a stronger shortwave and more
pronounced upper diffluence as a coupled jet structure develops.
Regardless of the exact evolution, moist advection out of the Gulf
of Mexico is progged to be intense on 850mb winds surging towards
50kts producing IVT exceeding +4 standard deviations according to
the NAEFS ensemble tables lifting into the Great Lakes. This deep
moisture combined with impressive deep layer ascent will spread
rounds of moderate to heavy precipitation northward, and a mix of
snow, sleet, and freezing rain is likely late Tuesday through
Thursday.

With the dual low structure producing significant uncertainty,
pinning down the locations of heaviest wintry precipitation is
challenging even for D2. However, utilizing the NBM and WSE
probabilities indicates for D2 the best chance for heavy snow will
be well north of the leading surface low, which is reflected by
WPC probabilities for 4+ inches as high as 30% along the MN/Canada
international border. Potentially more significant on D2 will be
the freezing rain threat as a broad Canadian high pressure
retreats in advance of the surface low. With the intense WAA
surging northward ahead of this low, the warm nose will likely
reach the Canadian border, but wet bulb temps of 0C or less will
allow for a period of freezing rain which may accrete to more than
0.1" from central WI through the L.P. of MI and as far east as the
Laurel Highlands of PA.

For D3, the more significant low pressure is likely to emerge out
of the Southern Plains and lift quickly northeast while deepening.
There exists still considerable uncertainty into how this
secondary low will evolve, but an impressive theta-e ridge may
blossom into a TROWAL, supplying enhanced moisture and ascent into
a strengthening deformation axis across the Upper Midwest. The
NAM/RGEM are more aggressive with this deformation and displaced
west of the global consensus, so not preferred at this time, but
with yet another shortwave digging into the large gyre, many
solutions are still possible. CAA on the back side of this low
will also be more impressive, so the column should be cold enough
to support snow from eastern MN through the western U.P. of MI,
and if the deformation axis pivots in this area, heavy snowfall is
possible. At this time, WPC probabilities indicate a moderate risk
for more than 6 inches on D3, with moderate icing of 0.1" or more
possible across the eastern U.P. and northern L.P. of MI within
the better WAA.


Weiss