Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Fri Dec 28 2018

Valid 00Z Sat Dec 29 2018 - 00Z Tue Jan 01 2019

Day 1...
Upper low near the Four Corners will continue to drop
southwestward towards Baja Mexico while slowly filling. Favorable
diffluence within the LFQ of an upper jet streak will begin to
weaken as this occurs while much drier air advects into the region
from the north. This weakening forcing combined with drier air
will bring a slow end to the snowfall from NE to SW, and WPC
probabilities for 4 inches or more remain below 50 percent and are
confined to the Sacramento and Guadalupe Mountainous areas.

In the Pacific Northwest, amplified mid-level ridge will weaken
and drop southward allowing moisture to funnel into Washington and
then spill across into the mountains of ID and MT during Saturday.
This moisture will be wrung out as snow in the mountains as a
shortwave rotates atop the ridge, but snow levels will rise to
above 5000 feet in WA/OR, and 4000 feet elsewhere. This system
will move quickly southeast, so total snow amounts will be modest
even in the highest terrain, and WPC probabilities are high for 4
inches only in the Washington Cascades and Northern Rockies of
ID/MT.


Day 2...
Shortwave dropping through the Pacific Northwest from Day 1 will
feature better Pacific moisture within a jet maximum moving
overhead on Sunday. Diffluence aloft combined with height falls
will spawn weak cyclogenesis moving from western Canada into the
Rockies, which will drag a cold front across the northwest as
well. These features together will produce significant snowfall
from the Washington Cascades southwest through the Northern
Rockies and into the ranges of northern WY and UT. The highest
snowfall will fall above 5000 feet in the Cascades, Bitterroots,
and Northern Rockies, and WPC probabilities are high for 8 inches
in these areas. As snow levels crash due to CAA behind the front
and lowering thicknesses, snow will accumulate as low as 1000-2000
feet, and WPC probabilities feature a slight risk for 4 inches
across most of the terrain above 4000 feet.


Day 3...
Complex split stream pattern on Day 3 produces lowered confidence
in snowfall accumulations, but a large part of the mountain and
central part of the CONUS may see accumulating snow into Monday. A
northern stream shortwave will dig from western Canada through the
Northern Plains while a 300mb jet arcs NE from the Dakotas. A weak
low pressure will develop beneath these features and lift into the
Great Lakes Monday afternoon. On the NW side of this low, a band
of deformation and mid-level frontogenesis will likely produce a
swath of snowfall from eastern MT through Minnesota. 1000-500mb RH
is high so moisture is sufficient for heavy snow, but the northern
stream impulse kicks quickly to the east so WPC probabilities are
less than 30 percent for 4 inches of snow.

At the same time, a southern stream shortwave lifting out of Texas
will induce surface low pressure over the Mississippi Valley which
will lift NE around the east coast mid-level ridge. Strengthening
low-level SW flow will drive WAA and precipitation will overspread
the Southeast and into the OH Valley. Much of this precipitation
will be liquid. However, on the NW periphery, there will be a
region where cold air dropping southward behind the cold front
draped from the northern stream low will interact with the
precipitation shield and potentially produce rain/snow/freezing
rain. Confidence is low in the interaction of these features and
WPC probabilities are low for a few inches of snow and a tenth of
an inch of freezing rain from the Central Plains northeast into
the OH Valley.

Additionally, the shortwave and associated jet maximum from day 2
will continue its progress southward towards the Four Corners and
close off by the end of the period. This will bring lowering snow
levels and modest ascent to produce a high chance for 4 inches or
more of snow in the Central and Southern Rockies, as well as the
potential for some light accumulations in the terrain of Arizona
and New Mexico.


The probability for significant freezing rain (0.25 inches or
more) is less than 10 percent.


Weiss