Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Sun Apr 19 2020

Valid 12Z Sun Apr 19 2020 - 12Z Wed Apr 22 2020



Day 1...
The probability of 4 or more inches of snow is less than 10
percent...

...Sierra Nevada through the Four Corners...
Days 2/3...

A 700 mb wave crosses CA late Monday and into the Great Basin,
then CO/NM Tue night/Wed morning. Low level convergence near the
front with pooled moisture also near the front will drive ascent
along the path of the wave/front.
Warm advection ahead of the wave drives snow levels up, with
consistent snow generally above 8000 ft.
Timing differences develop Tue night, with the 0z GFS/21z SREF
Mean a few hours faster than the UKMET/ECMWF, and the NAM on the
slow side of the guidance distribution, showing snow lingering
longer in NV if correct. More weighting was given to the ECMWF as
the consensus solution between the slower and faster solutions.
The 12-00z ECMWF and UKMET have a slightly more southern 700 mb
wave track, producing less precip as a result in northern UT and
CO.

WPC probabilities for 4 inches are moderate to high in the CA
Sierra Nevada, Wasatch, and Uintas, San Juans to the ranges of
central CO on day 3, with lower probabilities en route in the
ranges of central to southern NV.

Northern New York and Northern New England Day 3...

The models indicate an area of low pressure moves across eastern
Canada with a cold front moving across the lower Great Lakes and
then New York and New England.  Sufficient cold air will be in
place in northern NH and Maine for the pre-frontal precipitation
to be snow.  Low-mid level warm/moisture advection produces
precipitation but may also lead to a change to mixed precip or
rain over in NH to southern Maine. After the cold front passes
northern NY and northern VT, cold advection produces upslope flow
in west-facing terrain and moisture off Lake Ontario gets advected
inland, producing snow showers in the Tug Hill to the western
Adirondacks, then in the west facing slopes of the Green Mountains.
The NAM was an outlier with the most snow due a stronger wave of
low pressure along the front.  The GFS, UKMET, ECMWF, and Canadian
cluster well with a more progressive wave with slower development
of low pressure along the front and were given more weighting.

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

Petersen