Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EST Wed Nov 28 2018

Valid 00Z Thu Nov 29 2018 - 00Z Sun Dec 02 2018


...Western U.S....
Days 1-3...

A potent shortwave and associated Pacific jet max will drop into
California tonight, serving as the onset for an Atmospheric River
(AR) event which will plague the west through the forecast period.
Anomalously high PWAT of over +3 sigma will move into California
tonight, and then spill across into the interior west during Day
2. A reinforcing shortwave and accompanying jet energy will drop
again into California on Day 3, bringing renewed snowfall after a
subtle decrease Friday.

The combination of height falls, positive vorticity advection, and
left exit region diffluence will produce intense synoptic ascent,
which will be enhanced by upslope flow early into the Sierras, and
eventually into the rest of the mountain west. The tremendous
moisture and intense lift will produce exceedingly heavy snowfall
in the Sierras of California, where more than 2 feet of
accumulation is likely on day 1, with more than 4 feet possible
during the 3 days when including the second shortwave on Saturday.
Significant snow is likely into the other ranges of northern
California, Nevada, Utah, and Colorado, where 3 day totals of 1-2
feet are likely. WPC probabilities are high for more than 12
inches in the terrain above 6000 feet across all of these ranges,
as well as the Cascades of Oregon. Elsewhere, including the
Cascades of Washington, the mountains of Arizona, and other
terrain down to as low as 3000 feet have a risk for 4 inches of
snow or more. The timing will be such that California and Oregon
will see their heaviest snow on D1 and again on D3, with the
interior mountains of the west likely experiencing the heaviest
snow late on D1 through D2 when the best moisture and strongest
lift spill across the area.


...Northern Plains...
Day 3...

A significant low pressure system will develop beneath a
negatively tilted/closed upper low, and move northeast from
Oklahoma to Iowa Friday night and Saturday /Day 3./ Guidance has
come into slightly better agreement in timing and placement of the
surface low, with the NAM still a fast outlier, the CMC displaced
north, and the FV3/ECMWF in decent positional agreement. The
intensity varies quite a bit as well, but a slower and slightly
stronger surface low is preferred due to strong height falls and
persistent shortwave ridging downstream, and a model blend near
the FV3 was used for day 3.

As the upper low deepens and cuts off, it will drive the deepening
surface low to the northeast. Initially, surface temperatures are
below freezing but a subtle warm nose exists in regional
soundings. As WAA intensifies, precipitation will overspread the
region and a period of sleet/freezing rain is possible the first
few hours after onset. WPC probabilities show a slight risk for
0.1 inches of accretion, focused near the MN/IA/SD border.
Locations along the surface low track will turn to rain, but just
NW will transition to snow as dynamic cooling occurs in response
to robust ascent within the diffluent region northeast of the
closing mid-level circulation. Increasing 700mb deformation and a
developing TROWAL as the WCB rotates cyclonically around the
system will produce intense snow rates, and a stripe of more than
6 inches of snow is becoming more likely just NW of the surface
low. While uncertainty still exists into the exact placement of
the surface low track and heaviest snow, WPC probabilities have
increased, and show a moderate risk for 4 inches, with low
probabilities now existing for 8 inches, of snow accumulation
during Day 3.


Weiss