Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Sun Feb 21 2021

Valid 00Z Mon Feb 22 2021 - 00Z Thu Feb 25 2021

...Eastern Great Lakes and Northeast...
Days 1-2...

A shortwave moving through the Midwest and into the Great Lakes
will drape a vorticity lobe southward swinging across the
Mid-Atlantic Monday aftn. A wave of low pressure beneath this
feature will weaken and shift across the Great Lakes, while a warm
front draped to its east will lift northward. The primary surface
low will occlude to a triple point, and this feature, along with
the warm front, will move across the region Monday. Ahead of this
system, cold high pressure will be in place, but pronounced ascent
through strong WAA and mid-level divergence will weaken the high
and cause it to retreat quickly eastward.

Intense WAA noted by 60+ kts of wind at 850mb from the S/SW will
drive robust ascent forecast to exceed 20 micro-bars within the
DGZ Monday. While this warm advection will gradually erode the
cold air and cause a changeover from snow to rain as far north as
PA, initially the entire area should see snowfall. Some of this
could be heavy, with the HREF mean showing high probabilities for
1"/hr across PA/MD and into NY/NJ. The temporal duration of heavy
snow should be limited, especially where rain changeover occurs,
but some light accumulations of 1" or more are likely even in the
I-95 corridor Monday. Further north and into the terrain of PA,
NY, WV, and New England, WPC probabilities are moderate for 4".
This system pulls away early Tuesday, and cold advection in its
wake moving atop the Great Lakes will produce LES in the favored
W/NW snow belts. The heaviest snow D1-2 is likely SE of Lake
Ontario where a few inches of WAA synoptic snow Monday will be
followed by heavy LES Monday night and Tuesday. WPC probabilities
for 6" are low to moderate in the vicinity of the Tug Hill Plateau
both Monday and Tuesday.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

An atmospheric river (AR) ongoing this afternoon will persist
through Monday and into Tuesday across the Pacific Northwest, with
moisture spillover into the Northern Rockies likely as mid-level
confluence drives moisture robustly to the east. Embedded within
this flow, two distinct shortwaves will rotate southeast through a
broad trough to enhance ascent, and drive periods of WAA during
the greatest height falls. Snow levels will climb through the AR
as the moisture becomes more tropically sourced, and this will
limit significant accumulations to the higher terrain. However,
within the high terrain of the Olympics, Cascades, Northern
Rockies, and into the Tetons (D2), heavy snow is likely with WPC
probabilities for 8 inches high on both days. Two-day
accumulations could exceed 3 ft in the Cascades where the best
overlap of mid-level fgen and upslope flow combine with the deep
layer synoptic lift. By D3 /Wednesday/ the second shortwave will
dive into the Four Corners region leaving broad shortwave ridging
across the area. This should cut off the moisture and shunt the
ascent southeastward, bringing an end to the heavy snow, although
lingering moderate snow may persist across Idaho during Wednesday.


The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.


Weiss