Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 PM EDT Fri Apr 14 2023

Valid 00Z Sat Apr 15 2023 - 00Z Tue Apr 18 2023

...Central Rockies...

A broad-scale trough centered over Utah Friday afternoon will
cross the Central Rockies tonight with the mid-level trough and
cold front providing ascent, with easterly low level upslope
behind the front, but the signal for that has trended weaker and
overall probabilities for 4+ inches are mainly confined to the
Colorado Front Range and some lighter snow for the Palmer Divide. 
Most of the snow should be over across this region by 12Z Saturday.

...Upper Midwest and Western Great Lakes...

The most challenging event is across the western Great Lakes/Upper
Midwest beginning Sunday morning and persisting into Monday as a
deepening surface low continues tracking northeastward, reaching
northern Lake Michigan by 12Z Monday.  This will be the same storm
system that will be exiting the Rockies Friday night.  This upper
trough will likely be acquiring negative-tilt going into Saturday
night, and an upper level low then develops over Iowa/Illinois and
southern Wisconsin on Sunday.  The overall thermal structure to
the column is marginal for snow initially on Sunday, but as the
low pulls off to the northeast, strong cold air advection
commences and results in a change-over from rain to snow on the
northwestern edge of the precipitation shield.  While the high sun
angle will likely help mitigate the threat of rapid accumulation
to some extent during the day, the models do suggest that rates
will be great enough for accumulating snow.  However, the big
concern is where a strong deformation axis develops WNW of the
850-700mb low, and the 12Z guidance suite has trended more intense
with this feature.  This will result in strong ascent via low-mid
level frontogenesis and deformation, and there is even the
potential for some convective snow rates at times if conditional
symmetric instability can develop, especially Sunday night.  There
remains some uncertainty in the ultimate placement and timing of
changeover, but some areas could get a heavy wet snow and gusty
winds from eastern Minnesota through central/northern Wisconsin
and then the Upper Peninsula of Michigan.  By Monday afternoon,
some lake enhanced snow likely develops across portions of the
Upper Peninsula as colder winds flow across Lake Superior.

...Pacific Northwest...

Shortwave ridging centered over the region through Saturday will
give way to a trough moving out ahead of a low dropping southeast
from the Gulf Alaska.  This will bring a well-defined cold front
and the return of unsettled weather into the region on Sunday. 
Western Washington and Oregon begins to get active Sunday but more
so going into Monday as a modest atmospheric river (IVT > 250
Kg/m-s) advects deep moisture inland downstream of a reloading
trough west of Washington state.  The core of the trough is
expected to remain offshore through Monday afternoon, but this
helps maintain moisture advection and spokes of vorticity
advection to cause continued ascent.  Snow levels initially around
4000 ft Sunday fall to 1500-2000 ft Monday with the passage of the
cold front, so impactful snow at the cascade pass level is
possible late in the forecast period.

The probability for icing over 0.1" is less than 5 percent Days
1-3.

Hamrick/Weiss