Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 PM EST Tue Nov 05 2019

Valid 00Z Wed Nov 06 2019 - 00Z Sat Nov 09 2019


Day 1...

...Montana...
A shortwave trough diving south through western Canada is expected
to drive a cold front farther southwest across Montana Tuesday
night.  Post-frontal upslope flow along with low-to-mid level
frontogenesis is forecast to support light precipitation Tuesday
night into Wednesday, resulting in widespread light snowfall
accumulations across the state with some locally heavier totals in
the mountains.

...Midwest...
A broad upper trough currently extends from the northern Plains to
the Northeast.  As a low-amplitude shortwave moves through the
base of the trough, models continue to show a weak surface wave
moving east from Nebraska into Iowa early Wednesday.  Modest upper
forcing along with low-to-mid level frontogenesis north of the low
is expected to support snow moving east from eastern South Dakota
to central Michigan Tuesday night into Wednesday.  The overall
trend of the 12Z guidance has been wetter - suggesting that strong
low-to-mid level frontogenesis may support a narrow axis of heavy
snowfall, resulting in locally heavy accumulations for portions of
the region.  WPC Day 1 probabilities (ending 00Z Thu) are now
greater than 40 percent for accumulations of 4-inches or more
across portions of southeastern Minnesota and southern Wisconsin. 
  


Days 2-3...

...Northeast...
Light snow is expected to spread east of the Great Lakes into
Upstate New York and New England, but with only minor
accumulations expected through early Thursday.  Meanwhile,
upstream energy diving into the base of the trough is forecast to
sharpen the trough over the Great Lakes - supporting redeveloping
snow that is expected to move east from the Great Lakes into the
Northeast Thursday afternoon and evening.  This will mark the
start of what is expected to become the first significant snowfall
for the interior Northeast this season.  As the upper trough
continues to amplify and assume a negative tilt - a deepening
surface cyclone is forecast to track northeast across southern New
England late Thursday.  This will mean rain for the major cites of
the Northeast, but potentially heavy snowfall for the Adirondacks
and northern New England.  The GFS remains the general outlier
with the system, with a solution that is faster with the upper
trough - resulting in a surface low that moves more quickly
offshore and produces lighter snowfall amounts.  WPC's preference
was for a solution more in line with the deterministic ECMWF and
its ensemble mean.

For Days 1-3, the probability of significant icing (0.25-inch) is
less than 10 percent.

Pereira