Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 AM EST Mon Feb 22 2021

Valid 12Z Mon Feb 22 2021 - 12Z Thu Feb 25 2021

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

A 700 mb shortwave moving through the Great Lakes and Ohio Valley
this morning continues across New York and the Mid Atlantic today,
and then northern New England tonight.  Intense warm/moisture
advection results from a forecast 60+ kts of wind at 850mb from
the SW.  This will drive robust ascent forecast to exceed 20
micro-bars today.  While this warm advection will gradually erode
the cold air and cause a changeover from snow to rain in NJ/New
York City and parts of southern New England, snow could be briefly
heavy further inland in eastern PA/adjacent northern MD, with the
HREF mean showing high probabilities for 1"/hr. 

Because the wave moves steadily, the event should be relatively
short lived. The higher amounts are forecast in the preferred
upslope areas of the southern Adirondacks of NY, where
probabilities are moderate for 8 inches due to the strong jet
resulting in strong upslope ascent in windward locations.
Secondary maxima are expected int he mountains of southern VT, the
White Mountains of NH, and along the 700 mb frontogenesis maximum
in downeast Maine.  Areas along the immediate Maine coast appear
to be too warm for snow, so areas 25 to 50 miles inland have
better odds to see accumulating snow.

The 00z ECMWF shows a second 700 mb wave crossing the lower lakes
and Adirondacks/Green Mountains on Tue. Cold advection in the wake
moving across 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
lake effect snow Tuesday in southwest to northern NY. The wave
departing the Upper Great Lakes crosses northern NY and northern
New England on Wed., bringing additional snow showers to the
Adirondacks and northern Green Mountains of VT, followed by the
northern White Mountains and ranges of western Maine.


...Pacific Northwest through the Northern Rockies...
Days 1-2...

An atmospheric river (AR) will persist today 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. The ECMWF shows a 300 mb jet peaking near
150 kt in northern WA 00z Tue. Embedded within this fast flow, two
distinct shortwaves will enhance ascent, and drive periods of warm
air advection/moisture advection. Snow levels will climb through
the AR, and this will limit significant accumulations to the
higher terrain. However, within the high terrain of the Olympics,
Cascades, and Northern Rockies, heavy snow is likely with WPC
probabilities for 8 inches high on both days. Two-day
accumulations could exceed 3 ft with isolated peaks receiving 4 ft
in the WA Cascades where the best overlap of mid-level fgen and
upslope flow combine with the deep layer synoptic lift.
Accumulations of 2-3 feet are forecast across the Bitterroots of
ID and ranges of northwest MT, with 1-2 feet in the WY Tetons. A
700 mb ridge bridging across the northwest brings the event to a
close Wed.

CO Mountains and high Plains...
Day 3/Wednesday...

The models indicate a front passes south down the high Plains,
with ridging in Wyoming leading to a return flow that has an early
component that provide upslope flow from the CO high plains into
the foothills and front Range.
Low level frontogenesis and ascent from orographics should lead to
several inches of snow in the foothills and front range of CO.
The 00z Canadian, UKMET, ECMWF and 03z SREF Mean bring the highest
QPF and heaviest snow totals to the Sangre DeCristo mountains of
southeast CO.

The upper trough is further west towards southwest CO.  The 00z
Nam has more precip and thus more snow potential for southwest CO,
as the majority of models show the 700 mb front sliding south
across southwest CO, with an inverted sfc trough to focus
convergence and lift in the San Juan Mountains.

...Northern Plains to Upper MS Valley and Upper Great Lakes Day 2
to early Day 3...
The models have slightly amplified an 850-700 mb wave that moves
out of southwest Canada across the northern Plains and Upper MS
Valley on Day 2, departing after crossing the Upper Great Lakes. 
A stripe of about a quarter inch of liquid to a third of an inch
equivalent QPF is shown in the 00z GFS and ECMWF.  This should
lead to a likely band of 2-4 inches of snow along the wave's path
in the northern Plains to Upper MS Valley and then the Upper Great
Lakes. The slightly more amplified NAM and UKMET show potential
for 6 inches of snow in the Upper Great Lakes/northern WI, so a
low probability was shown there in case these solutions come to
fruition.

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

Petersen