Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Sat Mar 30 2019

Valid 12Z Sat Mar 30 2019 - 12Z Tue Apr 02 2019


...Great Lakes and Northeast...
Days 1-2...

Weakly coupled jets and mid-level PVA associated with a shortwave
lifting along the downwind side of a sharpening trough will drive
weak low pressure development along a stalled baroclinic boundary
today and Sunday. While initially precipitation will be rain as
the airmass is well too warm for frozen p-type, as the low pulls
away cold advection will develop behind the finally-eastward
progressing cold front. As this occurs, intense frontogenesis will
drive ascent to further cool the column, and precipitation is
likely to changeover to snow tonight into Sunday. The challenge is
determining when this will occur, how strong the ascent will be
during this transition, and how much moisture will linger once
snow begins. The best chance for heavy snowfall appears to be in a
narrow band along Lake Erie where robust fgen occurs to cool the
column before the mid-level trough axis shifts east. This will
enhance the potential for accumulating snow despite inhospitable
antecedent conditions due to warmth and rainfall. WPC
probabilities are high for 4 inches in the immediate vicinity of
the Lakeshore, with moderate chances for 4 inches into Sunday
across the high terrain of the Adirondacks and into Northern New
England.

Some lake effect snow downwind of Lake Ontario may enhance
snowfall accumulations into the Tug Hill Plateau post-FROPA as
well, but the temporal extent of robust forcing looks limited,
keeping accumulations generally less than 6 inches.


...Southern Rockies...
Days 1-2...

Sheared energy from the Pacific Northwest will dive southeast
across the Great Basin and close off briefly overhead the Four
Corners today into Sunday. Modest height falls and persistent LFQ
jet level diffluence will produce ascent in a moistening column on
low level return flow. Despite snow levels generally above 5000
ft, moderate to heavy snow is likely, especially on day 2 /Sunday/
where WPC probabilities feature a moderate risk for 8 inches in
the San Juans and Sangre De Cristos. Lighter accumulations are
likely elsewhere across the terrain of CO and NM, with WPC
probabilities indicating a 10-20% chance for 4 inches as far south
as the Sacramento Mountains.


...Northern Rockies...
Day 3...

An upper level jet stream diving from Western Canada into the
Northern Rockies will induce lift, coincident with moisture ahead
of a system approaching the West Coast will spilling into the
region. Forcing is generally transient, and snow levels are quite
high, 5000-7000 ft south of Montana, but moderate to heavy snow is
possible Monday. The heaviest amounts are forecast in the ranges
of NW WY, where WPC probabilities are high for 4 inches across the
Big Horn, Wind River, and Absaroka Mountains.


The probability for significant freezing rain (0.25 inches) is
less than 10 percent all 3 days.


Weiss