Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EST Mon Nov 18 2019

Valid 00Z Tue Nov 19 2019 - 00Z Fri Nov 22 2019

...Northeast...
Day 1...
Two shortwaves will interact across the Northeast Tuesday to
produce a variety of p-types, with significant accumulation of
snow and ice possible.

The first of these is a shortwave lifting across Maine tonight
which will keep a surface low pressure well SE of the coast.
Despite the sfc low remaining well offshore, WAA in advance of the
shortwave will push a warm nose above 0C towards the Canadian
border, but cold high pressure will be entrenched at the surface.
As moisture lifts northward this evening, modest ascent through
height falls and frontogenesis /WAA/ will lead to freezing rain
overspreading the region from south to north. There remains
considerable spread in amounts, but WPC super-ensemble shows a
mean of near 1/4" across parts of Central ME, and with the slow
erosion of the surface wedge, expect a significant ice accretion
across parts of NH and ME. WPC probabilities indicate a moderate
risk (30-50%) for 0.25" of ice across portions of extreme northern
VT/NH, and into much of Central Maine.

As Monday night turns into Tuesday, a secondary shortwave digging
through MD will lift northeast and may briefly close off before
ejecting into Canada late Tuesday. This feature will be
accompanied by robust ascent and a secondary area of
precipitation. Additionally, the interaction of this feature with
the lead shortwave will intensify omega across NH/ME, and dynamic
cooling of the column is likely which will gradually change the
p-type over to snow. This suggests two areas of moderate snowfall
Tuesday: In the terrain of Southern NY beneath the secondary
shortwave, and more impressive snow possible in northern ME where
rates could be heavy. WPC probabilities are low for 2" in the
Catskills and Adirondacks of NY, and high for northern VT. Higher
snowfall is likely into ME, where WPC probabilities indicate a
low/moderate risk for 6 inches.


...Southwest from California into the Central Rockies...
Days 2-3...
Prolonged precipitation event stemming from multiple upper lows
will plague the region beginning late Tuesday.

An initial upper low over Baja will lift northeast Tuesday night
into Wednesday, ejecting northeast east of the Four Corners and
into the Northern Plains by day 3. As this occurs, a second, more
potent, upper low will drop across the coast of California before
re-centering near the Four Corners Thursday. As the initial low
fills and lifts northeast, snow-levels will be quite high, so
despite robust mid-level divergence overlapping modest jet-level
diffluence, and ample moisture, snowfall will likely be confined
to the terrain above 8000-9000ft from CO southward. However, a
surface cold front and associated separate shortwave digging
through the Northern High Plains will produce much lower snow
levels into WY and MT, and here snow is likely across most of the
area. The risk for 8 inches or more is high in the San Juans and
southern Wasatch on day 2, with high probabilities for 4 inches
across the other ranges including the Uintas, Sierra Nevada, and
even the San Bernadinos in Southern California.

Late on D2 and into D3 /Wed-Thu/ the California closed low will
drop into the Four Corners region. Significant moist advection
will precede this feature as Pacific Moisture gets rotated around
the base of the trough within a strengthening upper jet streak. In
the low-mid levels, flow will become southerly ahead of sfc
cyclogenesis in the Southern Great Basin, and intense ascent is
likely through frontogenesis, deformation, and upslope
enhancement. Snow levels fall quickly to around 6000 ft as well,
so significant snowfall is likely, especially where flow becomes
orthogonal to the terrain along the Mogollon Rim and into the San
Juan Range. Additional heavy snows are likely in some of the less
common Southern California coastal ranges, as well as most terrain
above 6000 ft down to the Mexican border. Widespread moderate to
high probabilities exist for 8 inches above 6500 ft from the
Mogollon Rim into the Wasatch, San Juans, Sangre De Cristos, and
even near Mt Charleston. WPC probabilities for 4 inches extend
into the Great Basin ranges, and much of the high Plains of
southern Wyoming.


...Northern Rockies into the Western Great Lakes...
Days 2/3...
Complex evolution of a series of lows will spread a swath of snow
from the northern Rockies on D2 into the western Great Lakes on
D3. While this may be due to two distinct surface low pressures,
one lifting eastward from the PacNW, and a second developing from
a shortwave ejecting from the SW, a continuous band of snow is
forecast. The heaviest snow accumulations are likely in the
Northern Rockies where WPC probabilities are moderate for 6 inches
due to upslope enhancement behind a cold front and robust
jet-level diffluence to produce ascent. The second area of heavy
snow may be within the Arrowhead of MN as the second surface low
lifts northeast from the central Plains. This feature will become
a stronger surface low, and comma head deformation is likely to
develop on the NW flank of the low. This deformation will interact
with the RRQ of a jet streak, and upslope flow is likely into the
Iron Ranges as well. In this region, WPC probabilities are high
for 4 inches. Lighter snows are likely from eastern Montana,
through ND, and into western MN where a modest band of fgen
beneath the jet streak may produce a few inches of snowfall.


Weiss