Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Fri Apr 16 2021

Valid 00Z Sat Apr 17 2021 - 00Z Tue Apr 20 2021

...Northeast...
Day 1...

A potent late season nor'easter will pivot slowly away from the
New England coast tonight. Banded snowfall NW of the low which has
been prevalent this aftn will begin to wane as the most robust
ascent weakens and the low pulls away. The intense closed low
aloft will continue to work in tandem with upper diffluence on the
LFQ of a slowly departing jet streak to drive omega across the
region, with a slow exit from west to east through Saturday
morning. Additional accumulations should be light outside of the
terrain, but could reach all the way to the coast of Maine and New
Hampshire overnight. WPC probabilities for significant snow (>4")
are confined to the highest peaks of the White Mountains and into
northern Maine where the chance is as a high as 40%.


...CO/NM Rockies...
Days 1-2...

A sharp upper trough digging over CO today will shift east, but
persistent longwave troughing will be reinforced by a secondary
shortwave shifting across the Great Basin and into the Four
Corners on Saturday, while persistent but weakening LFQ diffluence
from a subtropical jet streak aids in deep layer ascent. In the
low levels, a cold front will be well south of the region into
Texas, but continued cold low-level flow from the N/NE will drive
upslope precipitation into the Rockies and Sangre De Cristos, with
additional snowfall spreading into the San Juans as overrunning
and moist mid-level flow persists. WPC probabilities on D1 are
high for 6 inches in the San Juans and Sangre De Cristos, with
locally 12 inches likely in the best upslope regions. By D2, the
best forcing shunts southward, but moderate probabilities for 6
inches continue in the San Juans and New Mexico portion of the
Sangre De Cristos.


...Northern/Central Rockies...
Day 3...

An amplified mid-level flow pattern across the CONUS will be
characterized by a short-wavelength but impressive ridge along the
west coast, with large cyclonic troughing across the eastern 2/3
of the CONUS. Embedded within this trough, a shortwave is progged
to dig out of the Alberta/Saskatchewan Sunday evening. Beneath
this trough, a potent cold front will drop southward across the
Northern Plains and then into the Central Plains, while banking
against the Central Rockies by the end of the forecast period.
Height falls and upslope flow will be the primary mechanisms for
ascent, however a modest but strengthening polar jet streak is
likely to provide some enhancement through upper diffluence as it
digs southward in tandem with the other features. Mid-level
moisture profiles indicate that Pacific moisture will be limited
and blocked by the ridge to the west, and PW anomalies weaken with
southward gain during D3. However, periods of robust ascent in at
least a modestly moist column will provide areas of heavy snow,
especially in the higher terrain. WPC probabilities on D3 are high
for 6 inches in the Absarokas, Big Horns, Wind Rivers, Black
Hills, and into the Front Range of CO. The heaviest snowfall is
likely in the Big Horns where more than 12 inches is possible.


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

Weiss