Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 PM EDT Tue Mar 19 2024

Valid 00Z Wed Mar 20 2024 - 00Z Sat Mar 23 2024

...Eastern Great Lakes into New England...
Days 1-2...

Broad cyclonic flow across the Northeast will maintain westerly
flow and CAA across the Great Lakes tonight into Wednesday. 850mb
temps falling to around -10C will support steepened lapse rates and
a deep DGZ across primarily Lakes Erie and Ontario, which will
produce periods of lake effect snow (LES) through D1. Low-level
thermals are barely cold enough for snow, but regional hodographs
indicate nearly unidirectional westerly flow across the lakes which
will support long residence times across Lake Ontario, Lake
Superior, and favorable upslope into the Chautauqua Ridge east of
Lake Erie. Heavy snowfall rates at times are likely, and WPC
probabilities for more than 6 inches are above 70% in the eastern
U.P. and the Tug Hill Plateau, with several inches also possible
downstream of Lake Erie.

Within this cyclonic flow, a more intense shortwave and associated
vorticity maxima will rotate out of Ontario and amplify into a
closed low over Upstate NY and into New England by Thursday
morning. This will drive strengthening height falls and downstream
PVA, collocated with the modest LFQ of a pivoting jet streak
within the pinched flow southeast of the amplifying trough. This
will result in an occluding surface wave moving across Canada,
leading to a secondary intensifying surface low over eastern New
England late Wednesday. This low will cause a surge in moisture on
increasing warm/moist isentropic ascent ahead of the accompanying
cold front, with additional wrap-around moisture persisting in its
wake as the low pulls away late Thursday. The atmospheric column is
marginally cold for snow, but in the higher terrain of northern
New England, and especially northern ME, heavy snow should be the
result, and this is reflected by WPC probabilities reaching as high
as 50-70% for more than 6 inches in northern and central ME, with
slightly lower probabilities extending into the White Mountains of
NH and the Northeast Kingdom of VT.

Additionally, a strong cold front moving out of the Great Lakes
will race across the Mid-Atlantic and into New England on
Wednesday behind the low pressure. Along this front, the high-res
guidance is in good agreement that waves of snow showers will
occur, especially from northern PA/central NY into New England
Wednesday afternoon and Wednesday evening. The snow squall
parameter lights up across this area, driven by 200-400 SBCAPE
overlapped with 0-2km theta-e lapse rates falling well below 0C/km
and high low-level RH. This suggests that at least scattered snow
squalls may occur D1, with briefly heavy snow rates and gusty winds
resulting in hazardous travel.


...Northern Rockies through the Upper Midwest...
Days 2-3...

A southward advancing cold front will stall and waver as a
stationary front from the Northern Rockies into the Central High
Plains Wednesday and Thursday, while intensifying cyclonic flow
begins to envelop the region south of an amplifying closed low over
Manitoba. Spokes of vorticity shedding around this gyre will spread
southeast, with one of these dropping southeast and overlapping
with the RRQ of a modest jet streak pivoting eastward to produce
impressive synoptic ascent atop the baroclinic gradient to drive
surface low development. The combination of the jet streak and the
strengthening surface low will drive a west to east oriented band
of precipitation beneath the jet streak, followed by more expansive
precipitation northeast of the developing low. This more widespread
precip shield will be aided by intensifying moist isentropic
upglide on the 285-290K surfaces Thursday into Friday, driving PW
anomalies to around +1 sigma to support heavier precipitation.

The guidance has trended a bit southward today, but the ensemble
clusters are generally well aligned overall with the low track.
This low will initially deepen as it drops into the Central Plains
by Friday morning before shearing and weakening D3. Despite this,
an axis of heavy precipitation, likely falling as snow in many
areas, will develop in response to increasing fgen aided by the
elevated warm front and the response to the jet streak position
aloft. The DGZ appears subjectively elevated, but the WAA beneath
it results in a near isothermal layer which could maintain
aggregates, and lead to a fast moving axis of heavy snowfall,
especially D3 from ND through MI. Farther west, the snowfall
intensity may be somewhat less, but of longer duration due to
prolonged forcing beneath the upper jet streak.

WPC probabilities for 6+ inches D2 are above 70% in a small
corridor near Glacier N.P., and also in a narrow strip beneath the
best jet-level ascent, reaching 10-20% across central and southern
ND. The fast movement of this system pushes the highest
probabilities for 6+ inches into WI/MI D3 where they reach above
60% due to the expansion of precipitation and intensification of
forcing, although it remains overall modest, while high
probabilities continue in the Northern Rockies.


Weiss