Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EST Tue Feb 11 2020

Valid 00Z Wed Feb 12 2020 - 00Z Sat Feb 15 2020



...Day 1 in the Southern Plains to Mid MS Valley and northern Ohio
Valley...

The northern and southern stream troughs across the Plains
gradually merge and phase on Wed, allowing confluent flow and
resultant frontogenesis and moisture to overlap across southeast
MN and the TX panhandle and then progress northeast to eastern KS,
MO, and then IL/IN.  A band of light snow is expected along the
axis of dilatation where is it cold enough for snow.  The
uncertainties involve lower QPF north/west of the aforementioned
axis and temperatures warming south/east of the axis. Snow
probabilities for 4 or more inches are less than 10 percent in
this swath. Light freezing drizzle is possible in portions of the
TX panhandle.

...Days 2-3 in the Midwest and Northeast...

On Wed night through Thu, the mid level confluent flow with pooled
moisture coinciding with a low level warm advection event
progresses out of the Ohio Valley across the lower Great Lakes,
NY, and New England. 
With a progressive system, the burst of precip last several hours,
but lack of a long duration event puts a cap on potential amounts.
 The models show a general axis of a quarter to half inch liquid
equivalent in the form of snow from northeast Ohio across western
and central NY, and portions of southern VT/NH/downeast Maine.
This results in potential for band of 4-6 inches of snow.  A
stripe of percent probabilities for 6 or more inches are on Day 2
from northern IL across Lake Erie to western NY, the Adirondacks,
and the Green/White Mtns.

The warm advection across PA, southern NY, and southern New
England results in mixed precip types further south, and less
snow.  A brief transitional change from snow to sleet and freezing
rain is possible across these areas, with light icing from central
OH across the Laurel highlands of PA, Poconos of northeast PA,
Catskills, Berkshires of MA, and Monadnocks of southern NH.

On Day 3, synoptic lift departs as the wave leaves New England,
with light lake effect snow downstream from the lower lakes where
cross lake trajectories add heat and moisture to the airmass
crossing the lakes.  The models show only a few inches as drier
air aloft advects downstream from the approaching ridge.

...Day 1 in Montana/Northern WY ...

The models show a mid-upper level trough progressing east from MT
across the northern Plains tonight.  The trough produces a period
of lift, followed by confluent flow 700-300 mb, which leads to
northwest flow.  For the duration of moist conditions, areas where
northwest flow turns upslope in the ranges of central MT to
northern WY (i.e. the Bighorns) result in continuing snow tonight.
Several additional inches are possible the ranges of central to
south central MT and adjacent WY, including the Little Belt Range
and mountains in the Custer Gallatin National Forest. The event
should wind down tomorrow night as drier air aloft advects into
the region.

...Day 3 in Pacific Northwest/Northern Rockies...

A pair of 700 mb shortwaves with associated bands of enhanced
moisture and lift move progressively across the Pacific northwest
and northern Rockies Thu night and Fri, continuing on to the
northern Plains afterward. 
The models indicate cluster of half to one inch liquid equivalent
precip in the form of snow in favored upslope regions of the WA
Cascades to the OR Blue Mountains and ranges of ID and northwest
MT.  Several inches of snow are expected as a result, with low
probabilities of a foot in the favored windward locations in the
WA Cascades.

The probability of significant icing (0.25 inches) is less than 10
percent Days 1-3.

Petersen