Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 PM EDT Fri Mar 17 2023

Valid 00Z Sat Mar 18 2023 - 00Z Tue Mar 21 2023

...Great Lakes & Interior Northeast...
Days 1-2...

As a deep cyclone tracks into southeast Canada this evening, brisk
cyclonic flow revolving around the western flank of the storm and
associated anomalous low-level cold temperatures aloft will
continue to kick up lake effect snow bands and snow showers
through tonight and into Saturday. 850mb winds of 25-40 knot
running over Lake Superior and Lake Michigan will keep the lake
effect snow machine cranking tonight and into Saturday, only to
pick up more during the daytime hours Saturday when surface based
heating leads to greater instability levels over both land and the
Great Lakes. Delta-Ts in the surface-850mb layer will be as high
as 25-30F in some cases with MUCAPE of 50-100 J/kg available over
Lakes Erie and Ontario. While there will be single band lake
effect streamers off the two lakes, the strong lapse rates and
sufficient moisture through much of the low-mid levels of the
atmosphere will support numerous snow showers that could contain
heavy bursts of snow. It is in the usual suspect areas (Michigan
U.P., northern MI, downwind of Lakes Erie and Ontario) that will
have the best odds for >6" of snowfall. WPC PWPF shows up to 70%
probabilities for >6" of snow along the northern shore of the
Michigan U.P. and in NY's Tug Hill Plateau. Locally, some areas
could approach another foot or so in accumulation in the Michigan
U.P. or in Tug Hill as well. Far western NY and northwest PA will
have lesser chances for the higher amounts, but still contain
40-60% probabilities for >4". Snowfall rates within these bands
are likely to contain 1-2"/hr snowfall rates with perhaps even
higher rates possible given the available instability. The winds
associated with these bands and the intense snowfall rates are
likely to cause whiteout conditions within them, as well as
drifting and rapid accumulation on roadways.

In addition to the lake effect snow, a secondary cold front
tracking through the Great Lakes on south into the Ohio Valley
will become the focus for the formation of snow squalls as far
west as central OH by Saturday morning. Squalls are likely to form
in western NY and both western and central PA, then race north and
east through the interior Northeast Saturday afternoon. These
squalls will have the benefit of having available low level
moisture in place, as well as quality surface based heating ahead
of the approaching cold front. Steepening surface-3km lapse rates
will lead to some meager MUCAPE to develop and allow for these
convective snow squalls to pack some punch. These squalls are
forecast to race east over the interior Northeast into early
Saturday evening, reaching as far east as VT/NH. Snow squalls are
notorious for causing dramatic changes in weather conditions, from
sunny/pleasant conditions to whiteout conditions and rapid
accumulations/drifting on roads. Travelers from the Lower Great
Lakes to the Northeast should be aware these squalls can lead to
rapid reductions in visibility. WPC is continuing to produce Key
Messages for the ongoing heavy snowfall in the northern Great
Lakes and the potential for snow squalls tonight into Saturday.
They can be found at the bottom of this discussion.

...Southern Rockies & West Texas...
Days 1-2...

An upper level disturbance tracking across the lower Four Corners
region coinciding with a upsloping NE low level flow into the
Sangre De Cristo mountains will lead to periods of snow in the
Southern Rockies this evening. Snow will be winding down from the
daytime hours Friday, but WPC PWPF still suggests there are 30-50%
odds for >4" of snowfall along the Sangre De Cristo, as well as
the San Juan of southwest CO. Farther south in Texas, temperatures
have cooled off considerably in wake of a cold frontal passage. To
the west, a broad, weak, and slow moving 500mb disturbance through
northern Mexico is set to induce diffluent flow over TX tonight
and into Saturday. A strengthening jet streak Friday night will
also place its diffluent right-entrance region over West TX and as
far east as central TX. At 850mb, a ribbon of moisture flux
advected via ESE flow into the Davis and Glass mountains will
coincide around the same time that 700mb moisture is directed into
south-central TX. The easterly 850mb flow will cause upslope
enhancement for precipitation along the eastern slopes of the
Davis and Glass mountains, while westerly 700mb flow also leads to
some upslope flow on the western slopes. This will continue into
Saturday night and early Sunday as well, making for a multi-day
event for periods of snow. This is a recipe for periods of heavy
snow in the Davis and Glass mountains, as well as potentially
minor accumulations in the foothills to the south and east of
there.

Elevation and timing of snowfall (night vs daytime) will be
critical in accumulations. Given the prolonged stretch of snowfall
in West TX, it will be late tonight and Saturday night that have
the best odds for accumulation. Rates will likely be light enough
that accumulations will be limited to primarily grassy surfaces
and trees. The Davis and Glass mountains are the areas that have
the elevation to support heavier totals. WPC PWPF shows 10-30%
probabilities for >4" in these mountain ranges while there are
similar probabilities at lower elevations for >2" near the Big
Bend. The WSSI does feature some Major impacts in the Davis and
Glass mountains, suggesting considerable disruptions to daily life
are likely in some of these regions through Saturday night.

...The West...
Days 2-3...

The western U.S. will see a more active pattern return starting
Saturday night when the first in a series of Pacific storm systems
bring more rounds or mountain snow to the region. The first storm
system is not overly impressive, but it will deliver a slug of
850-700mb moisture into the Sierra Nevada and the Trinity
mountains. Snowfall amounts between 00Z Sun - 00Z Mon are forecast
to range between 6-12" above 6,000 feet, with the higher totals
occurring in the tallest peaks of the Trinity and Sierra Nevada.
The next storm system will be quick to track into the West Coast
on Monday thanks to the strong 150 knot jet streak in the
northeast Pacific. California will reside beneath this jet
streak's left-exit region, maximizing synoptic scale lift aloft
while a steady diet of Pacific moisture is fed into the West
Coast. These two storm systems and the jet streak will also send
Pacific moisture north and east with measurable snowfall expected
in ranges such as the Cascade, the Wasatch, and in mountains such
as the Blue of northeast OR and the San Juan in southwest CO. The
heaviest totals will reside in the Sierra Nevada where over a foot
of snow is possible, but snowfall accumulations of up to a foot
are possible in the other mountain ranges previously mentioned.
The northern Great Basin can also expect locally heavy snowfall
>6". In terms of impacts, the experimental PWSSI shows a swath of
50-60% probabilities for Moderate impacts along the northern and
central Sierra Nevada on Monday, meaning additional impacts to
travel and infrastructure are possible.


...Key Messages for March 16-18 Winter Storm...

--Lake effect snow will expand downwind from Lakes Superior and
Michigan overnight and into Saturday. East of Lakes Erie and
Ontario, lake effect snow will develop and become heavier by
tomorrow night.

--Snow may be heavy at times with rates in excess of 1"/hr. Total
snowfall may exceed eight inches in favored locations across the
U.P. of Michigan, northwestern Lower Michigan, and into the NY Tug
Hill Plateau.

--More vigorous snow bands may produce low visibility, blowing
snow, and rapidly changing road conditions leading to hazardous
travel.

--A reinforcing cold front may cause snow squalls to develop from
the eastern Great Lakes through the interior Northeast Saturday
and Sunday. Briefly heavy snow rates and gusty winds within any
snow squalls will produce hazardous travel.


Mullinax