Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EDT Tue Oct 23 2018

Valid 12Z Tue Oct 23 2018 - 12Z Fri Oct 26 2018


Days 1-3...

Central Rockies
---------------
A broad upper level trough in the vicinity of the Four-Corners
region on Tuesday will have a modest advection of moist 700mb
southerly inflow combined with favorable upper level dynamics. 
Although most locations are expected to get rain with this system,
the highest elevations of the San Juan, Sawatch, and Sangre de
Cristo mountains in Colorado will likely have some snow above
9000-10000 feet, with amounts approaching 4 inches for the higher
peaks.  Drier conditions return to the region for the Day 2 period
on Wednesday.


Northern New England
--------------------
The first noteworthy winter weather event of the season is
expected for much of northern New England for Tuesday night and
extending into Wednesday.  A potent shortwave disturbance tracking
across Ontario will induce surface cyclogenesis over the Gulf of
Maine Tuesday evening.  This surface low will intensify as it
lifts northeastward towards New Brunswick, Canada, and the upper
level trough acquires negative tilt.  A well defined deformation
zone northwest of the low along with low-mid level frontogenesis
is expected to produce a swath of moderate snow extending from
northern Vermont and New Hampshire to northwestern Maine. 

There is some indication in the model guidance, particularly the
00Z NAM, that a brief period of mixed precipitation will be
possible across these same areas before the transition to all
snow.  NAM model soundings suggest a modest 925-850mb inversion
above a sub-freezing boundary layer, which may result in some
light freezing rain in interior valley locations on the order of a
few hundredths of an inch. 

Many GEFS and EC ensemble members are indicating the potential for
at least 4 inches of snow from northern New Hampshire to northern
Maine, with some also indicating the potential for 8 inch amounts,
with the greatest accumulations likely north of the Canadian
border.  The WPC probabilities indicate a moderate to high
potential for greater than 4 inches from the White Mountains of
New Hampshire to Aroostook County, Maine.

The probability of significant icing (0.25 inches or more) is less
than 10 percent for all three days.

D. Hamrick