Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
448 PM EST Mon Dec 31 2018

Valid 00Z Tue Jan 1 2019 - 00Z Fri Jan 4 2019

...Central and Southern Rockies through Tuesday night...

An amplifying upper trough and closed low within the southern
stream flow and an increasing jet max over New Mexico will lead to
height falls and upper level divergence.  An inverted surface
trough and surface low over the Four Corners region will enhance
low level surface convergence, and this convergence axis will
slowly be shunted southward through Wednesday.  This atmospheric
set-up, in combination with an arctic airmass surging southward
from the northern plains, will support another significant snow
event for eastern Arizona, central and northern New Mexico, and
southern Colorado.  Southeasterly upslope flow east of the
inverted trough will lead to the highest snowfall totals along and
east of the mountain ridges, with the San Juan and Sangre De
Cristo ranges expected to receive on the order of 8 to 16 inches
of snow.  The Gila and White Mountains of eastern Arizona are also
forecast to have similar snowfall totals.  Valley locations should
mainly be in the 4 to 8 inch range, and there are widespread high
WPC probabilities for 4+ inches from east central Arizona to
northern New Mexico and south-central Colorado.


...Texas to Missouri for mid-week...

The same upper low responsible for the snow over the southern
Rockies is forecast to eject over the Plains by Thursday morning. 
The juxtaposition of southwesterly mid-upper level flow over a
shallow arctic airmass over central/northern Texas and the central
plains is expected to result in some light freezing rain from
central Texas to southern Missouri on Wednesday and Wednesday
night.  Based on model forecast soundings, the depth of the
subfreezing layer is below 925mb for most locations affected with
a pronounced warm layer between 925-750mb.  There are some
differences regarding the surface temperatures for these areas,
and if the readings are lower than currently expected, accretion
could be closer to a quarter inch in some areas.  Low WPC
probabilities of a quarter inch of ice accretion remain in place.

By Thursday, the core of the upper low moves over Oklahoma and the
lower troposphere cools enough to support a swath of light to
moderate snow in association with a deformation zone that will
likely develop.  There are multiple ensemble members that are now
supporting 4+ inches across central Oklahoma and extending over
southeast Kansas and southwest Missouri.  Although WPC
probabilities are currently slight for 4+ inches, this could
certainly increase going forward.


...Great Lakes and Northern New England for Day 1...

A strong surface low will continue tracking across the Great Lakes
Monday night and then New England by early Tuesday afternoon. 
Across northern lower Michigan, temperatures are expected to be
cold enough along with isentropic lift to support moderate snow
Monday night, and some lake-enhanced snow is likely across the
U.P. of Michigan after the surface low departs.  WPC probabilities
are high for 4+ inches, and a slight risk for 8 inches for the
northern lower Michigan where the best combination of forcing and
thermal profiles will exist.

Farther east across New England, strong warm advection and low
level frontogenesis will support deep layer ascent.  The lower
troposphere will initially be cold enough for all snow for much of
central New England at the start of the event, with a transition
to a wintry mix and then all rain for many of these areas.  There
will likely be a period of freezing rain for interior valley
locations during the transition, but is expected to remain under
two tenths of an inch.  The exception will be northern Maine which
may remain snow for the entire event.  Heavy snow accumulations
are likely and WPC probabilities are moderate to high for 8 inches
in northern Maine. 


...Pacific Northwest for Day 3...

The next Pacific storm system is progged to reach the Washington
coast by the beginning of the Day 3 period Wednesday night. 
Although the heaviest precipitation is expected to be over British
Columbia, a plume of moisture associated with a quasi-stationary
frontal boundary accompanied by strong onshore flow will likely
produce heavy snow for the northern Washington Cascades, and the
higher summits on the Olympic Peninsula and the volcanic peaks
closer to the Oregon border.  Given the warm air advection pattern
and rising snow levels through the Day 3 period, the best snowfall
prospects will be early in the event, with many lower elevations
changing to rain by Thursday afternoon.


D. Hamrick