Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Sat Mar 18 2023

Valid 00Z Sun Mar 19 2023 - 00Z Wed Mar 22 2023

...Great Lakes & Interior Northeast...
Day 1...

Lake-effect snow and the potential for squalls continues this
afternoon across the Great Lakes, extending south down the
Appalachians through WV. Any snow squall threat remains along and
ahead of the cold front, which at the time of this writing is most
of the way across Ohio, tracking east. The snow squall threat
should diminish after sunset. Any areas impacted by snow squalls
will see very rapid diminishing of visibility to near whiteout
conditions along with sudden onset of very gusty winds.
Fortunately, any squalls are not expected to last long, so
localized accumulations will be limited. The impact on travel
through any squalls will be greatest due to the rapidly
deteriorating conditions.

The lake-effect will continue well into Sunday, especially east of
Lake Ontario. A strong low drawing incredibly cold air for this
time of year (850 temperatures as low as -20C) is moving that cold
air across the wide open waters of the Great Lakes, which remain
largely ice-free. The 13 degree Celsius difference threshold
needed for lake-effect between the LST (lake surface temperature)
and 850 mb air temperatures is therefore well exceeded.  With such
extreme instability over the lakes, the predominant lake-effect
type has been cellular. Thus far the primary flow over the upper
lakes has been largely perpendicular to the long axis of those
lakes, which further supports the widespread cellular nature of
the lake-effect.

Overnight tonight into the early morning Sunday, the predominant
surface low will track northeastward, the loss of solar heating
will allow the lake-effect to better organize over the lakes,
though the westerly flow over Lake Ontario will be most favored
for the heaviest lake-effect snows east of the lake. Drier air
will begin shutting down the lake-effect over Lake Superior, and
sufficiently perpendicular northwesterly flow will keep Lakes
Michigan and Erie lake-effect rather disorganized. Lake
Huron/Georgian Bay's lake-effect band could very possibly help
intensify the Lake Ontario band in this flow pattern. Thus, WPC
probabilities for locally high snowfall are greatest into the Tug
Hill Plateau region of upstate New York east of Lake Ontario.

...The West...
Days 1-4...

A pair of lows are expected to impact the western US through this
time period. The first will be the weaker of the two, with the
trailing cold front causing the most impacts. In fact, the parent
low will turn north and never make landfall in the Pacific
Northwest. Nevertheless, the cold front certainly will. Expect
precipitation to break out in California late tonight, which will
track eastward, and with upper level support, will help a lee-side
low to develop in Colorado Monday night. Widespread largely light
snow is expected at the higher elevations, with locally stronger
upslope resulting in higher snow totals along the highest peaks.
Snow levels will remain relatively low in California as compared
with the previous atmospheric river events to impact the state, so
much of the Sierras and points inland will see mostly snow from
this first round. As usual, the Sierras will be the big winners as
far as snowfall totals are concerned, with amounts up to 3 feet
possible. Further inland, up to 18 inches of snow are possible
across the UT and southwestern CO mountains, with CO's getting
additional support from the developing lee-side low just to the
east, helping squeeze out additional snowfall. Snow is also
expected further north from WA through MT, though amounts will
generally diminish the further north you go, due to less moisture
availability and significantly weaker forcing. California through
Colorado will have the benefit of the left exit region of a 130 kt
jet helping support the snow.


There will be very little if any break in the snow in the
mountains of UT and especially CO on Tuesday between the two
systems, as the lee-side low takes its time moving away from the
Front Range. A strong digging longwave trough will force the
aforementioned 130 kt jet southward, giving it more of a U shape
with time. This will increase the divergence, which in addition to
an injection of plentiful Pacific moisture will make for more
widespread heavier snow from CA east across much of the mountains
of the Southwest and through the intermountain west. Much of OR,
northern ID and much of MT may not see much if any snow from this
storm at all. The parent surface low, unlike its predecessor, will
move ashore in CA and turn northward with the curving jet streak
and track north across NV, while rapidly weakening as it does so.
This again will allow much more Pacific moisture to penetrate much
further inland, allowing inland areas and higher elevations to get
much heavier snow with the second round Tuesday into Wednesday.
There remains significant uncertainty with the track and snowfall
amounts with this second storm, since as mentioned a lot of
ingredients are all coming into place, so expect winter headlines
with this storm to be hoisted in the next day or two. 

...Northern Plains through the Upper Great Lakes...
Days 3,4...

The same low described in the first paragraph of "The West"
section above will continue causing hazardous winter weather as it
tracks northeastward across the northern Plains through the upper
Great Lakes Tuesday and Wednesday. Fortunately, the trends in the
models have been decreasing with total snowfall from this low,
likely because it weakens considerably once it moves away from CO
due to the lack of forcing. Nevertheless it will still have
sufficient moisture to result in a widespread generally light
snowfall from the Dakotas through the U.P. of Michigan. Certainly
local bands may develop, along with ever-present lake influences
around Lake Superior, but for most areas, a general 1-4 inch
snowfall is expected along and northwest of the low center. Thus,
potential for warning criteria snow at this point remains low for
all of the above areas, but worth noting as even those smaller
snowfall amounts will have impacts, especially to travel with any
locally heavier snow bands.

Wegman