Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Fri Jan 29 2021

Valid 12Z Fri Jan 29 2021 - 12Z Mon Feb 01 2021

...Western U.S....
Days 1-3...

Longwave trough moving onshore CA this morning will quickly begin
to fill and weaken into split flow, with the southern stream
shortwave diving into the Southern Plains by Saturday. The waning
amplitude of this feature will lead to a weakening downstream
Pacific jet streak, and the combination of this less intense moist
advection and weaker ascent through height falls will finally
bring an end to the tremendous snowfall across CA the last few
days. While the total forcing and moisture weakens, there is
likely to still be sufficient overlap for heavy snowfall on D1
from the San Bernadinos and Sierra, eastward through portions of
the Wasatch, Mogollon Rim, White Mountains, and extending into the
San Juans and even the Tetons of WY. WPC probabilities are
moderate for 8 inches in these ranges, generally above 4000-5000
ft as warm advection ahead of the trough drives most of the
precipitation.

This first system exits to the east on Saturday bringing an end to
heavy snow across the Southwest and Four Corners. However, a
second impulse follows quickly behind it, approaching the Pacific
Northwest Sunday and Monday. This next impulse will likely remain
offshore through the end of the forecast period, but increasingly
confluent and moist mid-level flow beneath an intensifying Pacific
jet streak will spread moisture across the Northwest and as far
south as northern CA. Warm advection, modest height falls, and
upslope flow will lead to renewed heavy snow developing across the
Olympics, Cascades, and down into the Shasta/Trinity/Siskiyou
ranges. WPC probabilities on D2 are modest for 8 inches, but
increase by D3 to show high chances, especially in northern CA and
the Olympics.


...Midwest through the Mid-Atlantic and Central Appalachians...
Days 2/3...

A large and impactful storm system is likely to develop across the
Southern Plains early on Saturday, and then expand into the
eastern US through early next week. The primary low pressure
responsible for this winter storm will be driven by a residual but
strong shortwave ejecting from California, and becoming a closed
low near Missouri on Saturday. This low will then once again open
into a wave Sunday over the Great Lakes, while secondary vorticity
lobes rotate into the longwave trough and off the East Coast. This
implies that the primary low will begin to weaken while secondary
low development begins off the VA coast D3. The timing and
intensity with which this second low develops has significant
implications on the wintry precipitation for the east.

Across the Midwest, precipitation will begin to expand and lift
northward as WAA sourced from the Gulf of Mexico rises
isentropically to the north. There is likely to be plenty of
moisture, and an expansive precip shield is likely. As this lifts
northward ahead of the surface low and parent mid-level wave, it
will spread rain and snow northward, with heavy snow developing
where the column is cold enough due to 290K isentropic lift
intersecting the saturating DGZ. There remains some uncertainty
into how far north the warm nose will lift and mixed precip will
occur, but soundings indicate that some dynamical cooling will
offset the warm nose, and the unfolding event looks to be more
rain and snow, with just a narrow corridor of freezing rain
accumulating to 0.1" or less. The highest probabilities for this
freezing rain are across Iowa, and are less than 20%. North of
this region, WPC probabilities indicate a high chance for 6" of
snow D2 into D3 from far NW IL southeast into western OH,
including The Chicago and Milwaukee metro areas where some lake
enhancement is likely. Locally higher amounts are possible should
any banding develop NW of the 700mb low, which is likely to pass
just south of Chicago.

Further east into the Appalachians and Mid-Atlantic, precipitation
will also expand as WAA/isentropic ascent intensifies atop a cold
dry wedge of high pressure. The forcing for ascent late Saturday
into Sunday is quite strong, and the guidance has trended a bit
cooler and stronger overnight. This suggests that the initial
precipitation will be a burst of snowfall as far south as the
NC/VA border, expanding northeast into PA and NY by the end of the
period. In the Appalachians, a period of heavy snow is likely to
accumulate to more than 6" before a changeover occurs to freezing
rain as the warm nose blossoms northward. WPC probabilities for
0.1" of accretion are as high as 40% in SW VA and eastern WV.
Further north, the forecast becomes much less certain as a dry
slot is likely to lift northeast across VA and into the I-95
corridor from DC to Philadelphia, creating a loss of cloud ice and
turning snow to freezing drizzle. Much uncertainty still exists in
how this will evolve, and it is significantly dependent on how the
aforementioned secondary low develops offshore. At this time the
guidance indicates that a burst of snow is likely Sunday before
the dry slot occurs, and WPC probabilities for 4 inches of snow
are moderate from central VA into central PA, and eastward
including the I-95 corridor, before transition to a period of
light freezing rain.


...Eastern New England...
Day 1...

An inverted trough extending for a low pressure well east of the
Gulf of Maine will pivot W/SW this morning and then drop southward
near Cape Cod this evening. As this pivots across the waters, a
potent mid-level closed low will drop across New England, with the
combination of low-level convergence and mid-level height falls
driving intense ascent and increased instability. Along this
trough, a band of intense snow showers or snow squalls is likely,
and while the most intense snowfall is likely to remain just east
of the shore, a period of heavy snow is likely, especially along
the Outer Cape. Impressive low-level fgen overlapped with theta-e
lapse rates of less than 0 suggest convective snow is possible,
and HREF probabilities indicate a chance for 1"+/hr snowfall
rates. WPC probabilities for 4 inches are as high as 50%, confined
to the immediate Cape Cod National Seashore from Truro to Chatham.


Weiss