Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
511 PM EDT Mon Apr 08 2019

Valid 00Z Tue Apr 09 2019 - 00Z Fri Apr 12 2019


...Northeast...
Days 1-2...

Weak clipper type low pressure developing along a stationary front
draped over the Northeast will track east across northern New
England this evening, followed by a secondary low moving across
nearly the same area on Wednesday. While much of this
precipitation is likely to be snow, surges of warm southerly flow
will at times transition precip over to a mix, especially in
VT/NH, but much of ME should remain all snow through the event.
The highest snowfall is likely in the mountains of NW ME, where
WPC probabilities are moderate for 4 inches both day 1 and 2, with
2-day totals of 4-8 inches likely. Elsewhere, lighter snowfall is
expected in the high terrain of VT and NH, as well as the northern
Adirondacks of NY. Additionally, low probabilities exist for 0.1
inches of ice accretion through Tuesday night, focused in the
sheltered elevated valleys of VT and NH.



...The West across the northern Plains to the Great Lakes...
Days 1-3...

A broad trough currently allowing an atmospheric river to push
into the Pac NW with particularly high snow elevations digs and
amplifies as it reaches the West Coast tonight. Snow elevations
decrease with the trough as does the influx of Pacific moisture
into the Pac NW, so heavy snow on Day 1 is limited to the highest
elevations of the Cascades, and expand southward into the Sierra
and eastward towards ID/MT where low probabilities exist for 12
inches through Tuesday night.

A more significant low pressure is then likely to develop in the
lee of the Rockies early on Wednesday in response to coupled jet
forcing and height falls leading to intense synoptic ascent. This
low will drift across eastern CO early on Wednesday until robust
southern stream jet energy rotates around the trough and kicks the
low to the east. This occurs in conjunction with rapid deepening
into Thursday. At the same time, a strong southerly LLJ will
provide ample moisture into an already saturated environment
characterized by Pacific moisture and PWAT anomalies of +1.5
standard deviations. While the airmass east of the surface low
will be quite warm, north and west of the low cold air will be in
place, and will intensify as high pressure to the north and the
deepening surface low combine to drag cold air southward into the
Plains, and then further towards the Great Lakes on day 3. As the
low deepens, the parent upper trough will close off, and a stripe
of potent deformation and intensifying 850-600 mb frontogenesis
will drive mesoscale ascent, and heavy snow is likely north and
west of the low track.

On the west side, WPC probabilities are moderate to high for 12
inches from eastern WY into central NE and southern SD. It is
across this area where blizzard conditions are possible as this
intense lift occurs atop a deep isothermal layer in conjunction
with -EPV near the DGZ to produce the potential for thunder-snow
and 1-2"/hr snowfall rates, along with increasing winds. North of
the low track, theta-e advection wrapping cyclonically around the
low within a robust TROWAL will produce a prolonged period of
heavy snowfall, and WPC probabilities are high for 12 inches, and
even moderate for 18 inches, from northeast NE across southeast SD
and into southwest MN. Despite relatively lowered SLRs due to high
liquid-water content snow, tremendous QPF and heavy snowfall rates
suggest the potential for more than 20 inches in some locations.
There will likely be a very sharp gradient between heavy snow and
rain, and uncertainty remains in where this axis will settle.
Further east into central MN and WI, continued strong mid-level
fgen along the path of the finally weakening low will produce
heavy snow late on day 3 and beyond into the medium range period.

Further south, a brief period of freezing rain over MN/IA is
likely where intense frontogenesis drives lift and surface
temperatures cool beneath the southerly LLJ induced warm nose.
There remains considerable uncertainty into the placement of this
feature with the NAM being the most northerly and showing higher
freezing rain accretion than the rest of the global suite. At this
time a lighter freezing rain event is preferred as isallobaric
wind should cool the column quickly in this region to change
precip from rain to snow moreso than freezing rain. However,
moderate probabilities for 0.1 inches persist on day 3.

Yet another trough paired with Pacific jet energy will rotate onto
the Pac NW coast on day 3, bringing renewed snow to the elevations
above 4000 ft from the Oregon Cascades eastward into the Northern
Rockies.

Weiss