Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 PM EDT Wed Apr 14 2021

Valid 00Z Thu Apr 15 2021 - 00Z Sun Apr 18 2021

...Northeast...
Days 2-3...

A late season Nor'easter will bring rain changing to heavy snow
across primarily interior portions of the Northeast and New
England Thursday night and continuing into Friday night.

A closed upper low over the upper Great Lakes will further develop
as it shifts east-southeast to southern New England through Friday
before lingering off Maine into Saturday. Surface low development
begins along the southern Mid-Atlantic coast late Wednesday night
and tracks north to southern New England through Thursday night
where it becomes vertically stacked and occluded.

Rain develops across the Northeast ahead of the low tonight
through Thursday as Gulf-sourced moisture is lifted up the Eastern
Seaboard. Rapid height falls immediately ahead of the low allow a
changeover to snow for higher elevations Thursday evening. Later
Thursday night, the warm conveyor belt wraps around the low as a
TROWAL over northeast NY and interior New England with snow levels
decreasing to valley level by Friday morning. Localized dynamic
banding will bring heavy snow to the any elevation particularly
late Thursday night/Friday morning and allow for localized
enhanced snowfall. Heavy snow is most likely in the terrain of the
Catskills, Adirondacks, and Whites into Maine (Day 2 snow
probabilities are moderate for 6 or more inches) and most
impressively in the Berkshires, Greens, and Presidential Range of
the NH Whites (Day 2 snow probabilities are high for 6 or more
inches in the Greens to northern Berkshires and the Presidential
Range - moderate for 12 or more inches in the southern Greens)
where the longest duration of heavy precip and cold temperatures
continue to be predicted.

Lower elevation heavy snow should mostly be limited to mesoscale
bands. There is still uncertainty with where the occluded low
tracks Friday night with potential for snow bands in eastern Maine
as indicated by the 12Z ECMWF/NAM or more interior as the 12Z
GFS/CMC depict.


...Great Basin to Central High Plains...
Days 1-3...

Long duration heavy snowfall progresses south from the WY Rockies
into Friday as a deep closed low currently over northern NV opens
as it slowly drifts east to CO through Thursday with a
positively-tilted trough lingering through at least Saturday. Deep
layer ascent, ahead of this low draws tropical Pacific and western
Gulf moisture up the High Plains with ample cold Canadian air
converging across the WY and CO Rockies and High Plains Thursday
into Friday. Higher QPF is progged with 12Z guidance with a
changeover from rain to snow expected on the High Plains Thursday.
Snowfall on Day 1 will be focused in the terrain with high Day 1
snow probabilities for six or more inches for the Wind Rivers,
southern Absarokas, Tetons, Laramie Range and Front Range. The
precipitation focus shifts to the High Plains and eastern Rockies
slopes Thursday with Day 1.5 snow probabilities 50 to 70 percent
for 6 or more inches across the Neb Panhandle, southeast WY and
northeast CO along with additional heavy snow for the Front Range.

The upper low/trough tracks east of the Rockies Friday, directing
moisture and precip farther east in the warmer/lower elevations of
the Plains with High Plains Snow quickly tapering off. A surface
ridge edging down the High Plains allows upslope flow for the
southern Rockies with Day 2.5 snow probabilities moderate for 6 or
more inches for the central CO ranges. A reinforcing trough
approaches the Four Corners on Saturday, likely intensifying snow
for the San Juans of CO with moderately high snow probabilities
for 6 or more inches there on Day 3.


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

Jackson