Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Tue Feb 05 2019

Valid 12Z Tue Feb 05 2019 - 12Z Fri Feb 08 2019

...Western States...
Days 1-2...

Upper low diving southward along the Pacific Coast will slowly
fill and drift southeast towards the Four Corners before lifting
northeast through day 2. An arctic front beneath this feature will
dive southward in conjunction, driving snow levels to less than
2000 feet as far south as southern California. Height falls, jet
level diffluence, and moist advection on W/SW 700mb flow beneath
the diving trough will produce snowfall across much of the west on
Day 1, shifting to just the Central Rockies on Day 2 as drier air
advects from the north. Significant snowfall is likely above 2000
feet from the Sierra Nevada, eastward into the southern Wasatch
and ranges of Colorado where WPC probabilities are high for 8
inches and moderate for 12 inches during D1/D2. As snow levels
continue to fall, snow will spread across even the valleys of
NV/UT and AZ from the Mogollon Rim northward. Light snow
accumulations are also possible in the transverse ranges of
southern California, and low probabilities for 2 inches exist in
the terrain around San Francisco as well.


...Northern Plains and Great Lakes...
Days 1-3...

Two distinct systems will bring widespread snow and ice from the
Northern Plains through the Great Lakes.

The first of these will lift across the region today and tonight
as a mid-level impulse and associated 300mb jet max eject to the
ENE. Although any surface reflection/trough is weak, strong WAA
ahead of the mid-level trough will spawn precipitation from
eastern Nebraska through the Great Lakes. The far northern portion
of this precipitation shield, mainly MN/WI and the U.P. of MI,
will see a quick burst of snow where snow rates of 1/2" to 1" per
hour are possible, and WPC probabilities for 4 inches are high
across WI, with low probabilities for 6 inches. Potentially more
impactful will be a stripe of heavy freezing rain which should
develop from extreme NE MO through SE IA, northern IL, into
southern MI. The guidance has trended subtly warmer/further north
this morning, but are in good agreement for a quick thump of
potentially significant freezing rain. Forecast soundings in the
area suggest a warm nose approaching +8C while surface temps
remain locked just below freezing. Lift into the columns becomes
intense as noted by strong omega just below the DGZ, and supported
by MUCape values approaching 200 J/kg. This suggests an unstable
environment capable of heavy rainfall rates, which despite not
accreting efficiently due to runoff and the self limiting
processes inherent to freezing rain in the absence of low-level
cold/dry advection, still supports warning criteria freezing rain
in a thin west-east stripe just north of the Chicago area. It is
in this ribbon where WPC probabilities for 0.25 inches are above
50%. There will be a sharp cutoff both to the north and south, the
former due to cold air and snow/sleet, the latter due to above
critical 1000-850mb partial thicknesses and rain. The major
metropolitan areas of Chicago and Detroit should see modest
freezing rain accretions, but probabilities for 0.25 inches are
minimal.

This impulse will lift away early Wednesday, but a deep mid-level
trough following quickly on its heels will spawn cyclogenesis
Thursday along a stalled cold front. This low will deepen and lift
northeast from Missouri to Lake Huron, spreading wintry precip to
its north and west. Another round of freezing rain is likely
across southern WI into MI, subtly further north than the
accretion on D1. The amounts are likely to be a bit less however,
but WPC probabilities are high for 0.1 inches. As the low deepens,
more classic mesoscale forcing NW of the low in the form of
intensifying frontogenesis and 700mb deformation will produce
heavy snowfall from eastern SD, through MN, and into the U.P. of
Michigan. Widespread high probabilities for 4 inches exist, with
moderate probabilities for 8 inches confined to the U.P. of
Michigan.


...Upstate New York and New England...
Day 2...

The same storm system impacting the Midwest on Day 1 will drive a
warm front northward into New York and New England Wednesday.
Along this boundary, a surface low will skirt south of New
England, and the combination of initially cold surface
temperatures within the Canadian high, and the isallobaric wind
into the developing low, will allow for surface temps to remain
below freezing even as 850mb temps climb well above 0C. This will
create widespread freezing rain across the region, with low-level
partial thicknesses supportive of freezing rain down the coast
initially before the cold air erodes. In the terrain from the
Worcester Hills and Berkshires, and points north and west,
moderate rainfall rates and moderate QPF will create the potential
for significant freezing rain, and many areas above 1000 feet have
high probabilities for 0.1 inches, with the chance for 0.25 inches
of accretion highest in the Berkshires and southern Green
Mountains.


Weiss