Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Wed Oct 24 2018

Valid 00Z Thu Oct 25 2018 - 00Z Sun Oct 28 2018

Day 1...

...Central Rockies...
Shortwave and accompanying upper jet will move across the Mountain
West tonight and Thursday. Diffluence within the left exit region
of this jet will provide lift in an environment marginally
conducive for snow due to warm temperatures and modest available
moisture. A few inches of snow may be squeezed out in the highest
terrain of Wyoming, Montana, and Colorado, where WPC probabilities
for 4 inches of snow are only 20 percent or higher in the Big Horn
Mountains above 9000 feet.

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


Day 2...

The probability of significant snow (4 inches or more) is less
than 10 percent.

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


Day 3...

...Northern Rockies...
The eastward advance of a Pacific Jet will transport moisture into
the northern Rockies Friday night and Saturday. Diffluence aloft
combined with weak shortwave energy moving atop the mid-level
ridge to the south will provide the impetus for lift, first in the
Montana Rockies, before dropping southeast into the mountains of
Wyoming. The system is progressive, moisture is limited, and 700mb
temperatures are barely below 0C, but light snow is likely in the
terrain above 7000 feet. Heavier snow is possible above 9000 feet
in the Absaroka and Big Horn ranges, where WPC probabilities are
low for 4 inches of snow accumulation.

...Northern New England...
Impressive coastal surface low pressure will lift north to near
Long Island, NY by the end of day 3. Cold high pressure north of
this system will retreat slowly as warm advection commences,
bringing precipitation to northern New England. The column is
initially cold enough for all snow, but as temperatures above the
surface warm, forecast soundings suggest precipitation type will
transition to sleet and then freezing rain, limiting snowfall
accumulation. WPC probabilities for 4 inches of snow are low, but
have increased for an extended period of freezing rain across this
same area. WPC probabilities are high for at least 0.01 inches of
accretion, and have increased to around 20 percent for up to 0.25
inches. There remains considerable uncertainty into how far north
precipitation will get on day 3, as well as how quickly the warm
nose will advect northward, but there is increasing confidence in
at least some wintry precipitation on Saturday.

Weiss