Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Tue Apr 13 2021

Valid 12Z Tue Apr 13 2021 - 12Z Fri Apr 16 2021

...Northern Plains...
Day 1...

A closed low positioned over northern MN will spin nearly in place
on D1 before finally ejecting to the E/SE D2. Beneath this
feature, a surface low will retrograde slightly near the Arrowhead
of MN before kicking to the east by Wednesday morning. Spokes of
vorticity rotating westward around the upper low will drop south
across the Northern Plains, coincident with a surface trough and
aligned with an axis of mid-level deformation and fgen. Together,
these features will drive deep layer ascent and periods of
moderate snowfall will continue from MT through ND and into
western MN. The overlap of moisture and forcing is expected to be
modest, so much of the snowfall intensity should be of the light
to moderate variety, and this is reflected by WPC snowband
probabilities of 0.5"/hr or less. However, some subtly stronger
banding is possible beneath the S/SW moving deformation axis.
Should this occur locally higher accumulations are possible
through this evening, but otherwise WPC probabilities for 4 inches
are generally 20-50% in pockets across the region. As the low
pulls away and weakens this evening and tonight, forcing will wane
and snow should shut off late D1.

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

Complex and long-lasting snow event is likely to bring periods of
heavy snow from the Sierra to the Central High Plains much of the
week.

A sharpening trough digging through the Pacific Northwest this
morning will drop southward and amplify into a closed low over the
Great Basin Wednesday morning and then drift nearly in place
through Thursday as the mid-level pattern across the CONUS gets
blocked. This feature will eventually fill and eject eastward
towards the Southern plains on D3. As the closed low pivots across
the Great Basin, a subtropical jet streak arcing west to east will
strengthen from Southern CA into the Texas, placing favorable LFQ
diffluence across the area. This will combine with persistent WAA
and mid-level divergence to drive deep layer ascent. Meanwhile
700mb flow will orographically ascend some of the terrain,
enhancing lift already impressive through height falls. While
moisture on D1 may be somewhat limited outside of the CO Rockies,
by D2 Gulf of Mexico moisture begins to lift northwestward into
the region, driving PW anomalies of +1.5 standard deviations into
CO/WY. As this occurs 700mb omega becomes intense across parts of
WY through both mesoscale and synoptic ascent, and while guidance
continues to feature quite a bit of spread in position of heaviest
snow, there has been some trend towards consensus this morning for
the heaviest snow focused in WY D2. By D3 the closed low begins to
open and shift eastward, but continued warm/moist advection,
height falls, upslope, and jet diffluence will maintain periods of
heavy snow on D3.

The heaviest snow D1 is expected in the CO Rockies including the
Front Range where height falls and upslope flow will help wring
out slowly increasing column moisture. WPC probabilities for 6
inches are high across this area, with locally more than 8 inches
possible. The heaviest snow develops D2 across central and
northern WY where persistent low-level convergence in a region of
high column moisture will produce heavy snowfall, and rates are
likely to exceed 1"/hr. The heaviest accumulations are likely on
the upslope side of the Wind Rivers, Absarokas, Tetons, Big Horns,
and south into the Uintas, where WPC probabilities for 6 inches
are high, and locally more than 12 inches is likely. Further
south, a pronounced dry slot will likely limit accumulations
towards Cheyenne and into Colorado. By D3, forcing becomes more
spread out to the south and east. This should allow snowfall to
spread east into the High Plains of NE and SD, as well as the
Front Range of CO and back into the Laramie/Snowy ranges of WY,
while continuing in the western WY terrain. WPC probabilities on
D3 are moderate for 6 inches in these areas. 3-day snowfall may
reach 18 inches in parts of the highest terrain of WY and CO.


...Northeast...
Day 3...

A deep closed low of -2 standard deviations at 500mb and in the
bottom 10th percentile for mid-April according to NAEFS ensemble
tables will move from Michigan southeast to be positioned over
Long Island by the end of D3. Rapid height falls accompanying this
feature will combine with LFQ diffluence of a strengthening jet
streak to the south to drive pressure falls and a surface low is
likely to develop off the VA coast Thursday morning. This low is
then progged to lift northeast to off Southern New England while
strengthening. WAA ahead of the low will spread precipitation
northward, some of which could be quite heavy, but initially will
be all rain due to warm low-level thermal structure. However, as
the low deepens and the core of the upper cold shifts southeast,
the combination of CAA and intense deformation will rapidly cool
the column causing a p-type transition from rain to snow. Guidance
still features a wide spread in placement of the best forcing and
subsequent accumulations, but WPC ensemble means have trended
upward this morning. Despite it being mid-April, overnight
snowfall Thursday into Friday combined with what could be intense
snow rates of >1"/hr should accumulate, especially in the terrain
of the Catskills, Berkshires, and Greens. WPC probabilities
currently indicate a 20-30% for 4 inches in these areas.
 

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

Weiss