Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
511 PM EDT Wed Apr 15 2020

Valid 00Z Thu Apr 16 2020 - 00Z Sun Apr 19 2020

...Central Rockies through the Southern Great Lakes...
Days 1-3...

A shortwave dropping through the Northern Rockies will amplify
briefly as it enters the northern Great Basin Thursday night
before opening and shearing into the westerlies along the
periphery of the broad eastern CONUS trough. The combination of
PVA/height falls and jet level diffluence within the entrance
region to a jet streak will produce ascent in an increasingly
moist column. Upslope flow is also likely into the eastern slopes
of the Rockies and into the Front Range in response to increasing
easterly flow behind a cold front and around the developing low.
On day 1 /through Thursday evening/ WPC probabilities are high for
12 inches in the Front Range and other portions of the Rockies
along the CO/WY border, with high probabilities for 6 inches in
the other terrain as far west as the Grand Tetons.

Late D1 and through D2, forcing becomes strong across the Central
Plains. A warm front will lift across KS/MO/IL and likely stall as
the surface low develops and tracks along this baroclinic
gradient. At the same time, a strengthening LLJ from the S/SW will
transport anomalous moisture into the region, with ascent being
provided through isentropic lift atop this boundary. The
combination of isentropic ascent and intensifying frontogenesis
along this temperature gradient will drive heavy snow in a narrow
band from central NE, along the IA/MO border, and into northern
IL. There remains considerable spread in the latitudinal placement
of this heavy band, but it appears the heaviest snow will fall
invof the IA/MO border where the boundary may stall to prolong
intense ascent, with snowfall rates rising above 1"/hr. WPC
probabilities on D2 are high for 6 inches along this state border,
but there exists less than usual confidence at this time range and
the heaviest snow axis may shift north or south in the next day.
WPC probabilities for 4 inches are moderate as far east as NW Ohio
on D2, passing just south of Chicago.

By day 3, /Friday night into Saturday/, the low moves quickly
along the baroclinic gradient across the Southern Great Lakes and
into New England. Moisture should surge northward on robust WAA,
and where the column is cold enough for snow either through
advection or dynamic cooling, a swath of heavy snow is likely from
northern Ohio through the PA/NY border, and into Southern New
England. There remains considerable uncertainty into the exact
placement of the heaviest now, but WPC probabilities currently
indicate a moderate chance for 4 inches in the higher terrain of
PA and NY, and into the Berkshires of MA.


...New York/Pennsylvania...
Day 1...

A fast moving shortwave and associated weak surface low will move
in the vicinity of the NY/PA border tonight and then off the New
England coast Thursday morning. As this feature races east, it
will be accompanied by a brief burst of heavy snow. Intense
frontogenesis with overlapped -EPV combined with strong forcing
into the DGZ and an exceptionally deep isothermal theta-e layer
suggests convective snow rates are likely, possibly reaching
towards 2"/hr. The forcing will move quickly, and temps initially
are marginal, but dynamic cooling will allow rapid accumulation
across a narrow corridor. The highest accumulations may reach 4",
but in general should be more 2-3 as shown by WPC probabilities
for 2" of 70-90%. While this amount of snow is not generally
impactful, the rates and likely heavy-wet consistency of the snow
could be problematic for a few areas, primarily west of Albany.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss