Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Fri Mar 24 2023

Valid 00Z Sat Mar 25 2023 - 00Z Tue Mar 28 2023

...Pacific Northwest, Northern & Central Rockies/Plains...
Days 1-3...

A longwave trough will setup shop over the western U.S. through
the weekend and into the start of next week with a series of upper
level disturbances revolving beneath the base of the trough. These
disturbances will be responsible for periods of mountain snow from
the Cascade Range to the northern Rockies and even the Colorado
High Plains. The latter is likely to see snow as a result of a
vigorous and negatively tilted 500mb trough axis pivoting through
Colorado early Saturday morning. The GFS depicted strong mean
layer 700-300mb Q-vector convergence originating over the high
plains of eastern CO and WY early Saturday morning, then moving
east into western NE and western KS. Then between 12-18Z, the axis
of strongest Q-vector convergence stalls and pivots over
east-central WY and into western NE. This pivoting axis of
strongest low-mid level convergence still varies by location
across global and CAM guidance. Where the deformation zone sets up
will determine which areas see some of the heavier snowfall rates,
which could approach 1"/hr at times. Areas in CO and WY early on
have better odds at accumulating snowfall due to snow occurring at
night and into the early morning hours. By the daytime hours, snow
will be harder to accumulate on roadways given the added solar
radiation, limiting any accumulation to be more confined to where
the heavier snowfall rates setup. The experimental PWSSI does show
50-60% probabilities for Minor impacts in east-central CO and
along the I-70 corridor south and east of the Denver metro area.
There are also 40-50% probabilities as far north as the WY/NE
border and the Black Hills. This is a setup highly dependent upon
banding and its location/timing, so while most totals will likely
range between 1-3", there are higher end amounts that could
approach 5", as evident by WPC's PWPF showing 10% probabilities
for >6" of snowfall in east-central CO.

Meanwhile, the Absaroka, Tetons, and Wind River Ranges are better
bets for much heavier amounts given their locations north of this
anomalous 500mb low. WPC PWPF between 00Z Sat - 00Z Sun shows
probabilities for >12" of snowfall ranging between 50-70%. By
Saturday, a trio of 500mb lows look to take shape: one in southern
ID, one along the WA coast, and another diving south off the coast
of British Columbia. The moisture source is not particularly
impressive, but temperatures remain colder than normal and the
presence of these disturbances will support some modest synoptic
scale lift to go along with any minor upslope enhancement. Between
00Z Sun - 00Z Mon, WPC PWPF shows 40-60% odds for >6" of snowfall
in the higher terrain of southern MT and northern WY, as well as
the Wasatch just east of Great Salt Lake. From 00Z Mon - 00Z Tues,
the areal coverage for probabilities for >6" decreases, but are
still in the 10-30% range over the Trinity/Shasta of northern CA
and in the Absaroka and Wind River Ranges in MT and WY.

...Upper Midwest & Great Lakes...
Days 1-2...

An intensifying wave of low pressure is forecast to track into the
Great Lakes on Saturday and produce a swath of heavy, impactful
snowfall from eastern IA, northern IL, southeast WI, and into
northern MI. The setup synoptically is ripe for a late season
winter storm, highlighted by jet coupling from diffluent quadrants
of two jet streaks (one over the south central U.S., the other
more intense jet streak over southern Canada), exceptional PVA at
500mb out ahead of the upper trough, and a robust 60 knot low
level jet over the Mid-South creating a warm conveyor belt of
moisture on the north side of the 850mb low track tonight and into
Saturday morning. High pressure to the north is anchoring just
enough cold/dry air in place over the region to support snow,
particularly once intense vertical velocities aloft cause
subsequent dynamic cooling in the atmospheric column. As the storm
system deepens into the early morning hours, the deformation axis
which consists of a ribbon of 850-700mb frontogenesis from eastern
IA to the western shores of Lake Michigan will be the catalyst for
1-2"/hr snowfall rates through Saturday morning. Latest WPC
Snowband Probability Tracker shows HREF members not only producing
1-2"/hr rates, but could even approach 3"/hr at its peak. With
snowfall starting off during the overnight hours, snowfall will
have the opportunity to accumulate on all surfaces with rates that
intense. Even as the sun rises, so long as >1"/hr rates continue,
snowfall will be able to accumulate on roadways with slush having
already accumulated prior to sunrise. Not to mention, these
snowfall rates combined with wind gusts >30 mph can cause near
zero visibility and whiteout conditions.

Latest 12Z HREF shows its 24-hr mean snowfall ranging between
6-10" from northern IL to southeast WI and northern MI between 00Z
Sat - 00Z Sun. The 12Z HREF also depicted 24-hr snowfall
probabilities for the same time frame listed of 60-70% for >8" of
snowfall in these areas. It was also worth noting the latest WPC
PWPF even shows a 10-30% chance for >12" of snow in southeast WI,
where additional lake enhancement of Lake Michigan could aid in
locally heavier amounts. This time of year, snowfall is generally
a heavy/wet type, which combined with wind gusts of 30-40 mph will
likely result in power outages and tree damage to impacted areas.
These three regions that have been most routinely highlighted
(eastern IA, northern IL, southeast WI, and northern MI) features
at least 50% chances for Moderate impacts between 06Z Sat - 06Z
Sun. There are also swaths of 20% probabilities for Major impacts
near the Madison, WI metro and in eastern WI where the combination
of heavy snow and heavy snowfall rates are the primary factors
driving these impacts. It is worht noting there will be a sharp
northern and western edge in the snowfall footprint, so subtle
25-50 mile shifts in storm track can make a big difference in
forecast versus observed totals. Snow should conclude from IA to
the western shores of Lake Michigan by midday Saturday, then
concluding over northern MI Saturday early evening.

...Northeast...
Days 1-2...

The warm front associated with the storm system tracking through
the Great Lakes will advance north through the Mid-Atlantic on
Saturday. Strong 290k isentropic ascent will be catalyst for
precipitation to develop across the northern Mid-Atlantic tonight
and push into the Northeast on Saturday. The strong 50-60 knot
southerly flow at 850mb will result in a burgeoning warm nose at
low levels. However, temperatures in the surface-900mb layer are
likely to remain below freezing for a few hours before ultimately
giving in to the strong southerly flow aloft. This will support a
wintry mix of precipitation across northern PA, much of interior
NY, and into central New England. Some areas could pick up over a
0.1" of ice accumulation through Saturday evening. WPC PWPF does
show 20-40% probabilities for >0.1" of ice east of Buffalo, as
well as the Catskills, Adirondacks, Berkshires, and Green
mountains. Much farther north, northern ME has better odds of
staying all snow as a coastal low forms in the Gulf of Maine. WPC
PWPF between 00Z Sun - 00Z Mon suggest there is a 40-60% chance
for >8" of snowfall in far northern ME to the west of Caribou.
Some localized areas could even approach a foot of snow by the
time the storm exits Sunday evening. There is a large portion of
central ME and into the White mountains of NH that features 50-70%
probabilities for >4" of snowfall between 12Z Sat - 12Z Sun. The
experimental PWSSI shows 40-50% probabilities for Moderate impacts
over northern ME where there is the best opportunity for winter
weather to cause disruptions to daily life late Saturday into
Sunday.

Mullinax