Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Fri Mar 29 2019

Valid 12Z Fri Mar 29 2019 - 12Z Mon Apr 01 2019


...Central Rockies and High Plains...
Days 1-3...

Energy ejecting from a filling low off the Pacific Northwest coast
will drop southeast through the Four Corners and into the Central
Plains as an elongated upper ridge dives southward from Western
Canada. This energy will be riding along a 100+kt Pacific Jet max
such that height falls and divergence aloft will drive synoptic
ascent to produce heavy snowfall across WY/CO and into SD/NE.
Coincident with this large scale forcing, a stripe of intensifying
850-700mb frontogenesis will dig southward to enhance lift while
increasing low-level northerly flow will produce some orographic
ascent in the favored upslope regions as well. This all suggests
the potential for significant snowfall from the Wind River range
of WY eastward into the Laramies and southward to the CO Rockies
where WPC probabilities are moderate to high for 8 inches of
accumulations.

Further east into NE, the guidance has trended upward with its
snowfall where the best combination of synoptic and mesoscale
ascent overlaps. The WPC super-ensemble mean continues to increase
across this area, and nearly all ensemble members have 4 inches or
more, with higher totals likely near the Pine Ridge of NE where
WPC probabilities feature a slight risk for 8 inches. Even further
south and east, jet level diffluence and the sinking frontogenesis
may allow for some lighter accumulations all the way into northern
KS, but this will be highly dependent on the snowfall rates as
surface temperatures are marginal for snowfall.

By day 2, the ridge pushing southward drives the best jet
diffluence and baroclinic gradient south of the area while drying
occurs from the north. This will continue snow, but of a much
lighter intensity, across the mountains from WY southward into NM.
A secondary wave of low pressure developing south of the ridge on
Sunday may once again enhance snowfall locally across the San
Juans and Sangre De Cristos, and WPC probabilities increase to
high for 4 inches.


...Great Lakes and Northeast...
Days 2-3...

Slow moving cold front and waves of low pressure moving along it
produce a challenging snow forecast from Eastern MI through PA and
into Northern New England. A wave of low pressure is expected to
develop along this boundary Saturday in response to height falls
and a weakly coupled jet structure, and as this low moves
northeast it will drag the front to the east as a cold front
Saturday night. As this occurs, the mid-level trough axis lags
such that moist advection and isentropic lift persist to produce
precipitation behind the front, and as the column cools due to CAA
a changeover from rain to snow is likely. However, the rate at
which this changeover occurs, and the intensity of the
precipitation after the column cools enough for snow, remains
uncertain and model spread is significant. The NAM continues to be
the strong outlier, likely due to a slower frontal movement
leaving more precipitation in the cooling air mass, and is not
preferred. This keeps WPC probabilities at 20-40% for 4 inches
from extreme eastern MI, along the lake shores of Erie and
Ontario, and into the terrain of Northern New England late
Saturday and Sunday. With the potential for strong frontogenesis,
some higher amounts of snow are possible where any banding sets
up, but with a warm and wet antecedent airmass, accumulations more
than 4 inches should be the exception rather than the rule.


The probability for significant freezing rain is less than 10
percent all 3 days.

Weiss