Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Sat Apr 15 2023

Valid 00Z Sun Apr 16 2023 - 00Z Wed Apr 19 2023


...Upper Midwest and Western Great Lakes...
Days 1-3...

A robust system will start to develop overnight into Sunday over
the Plains and lift through the Upper Midwest/Great Lakes through
Monday with an area of heavy snow and some mixed precipitation
near the Canadian border. Though current temperatures in the Upper
Midwest were in the 60s/70s Saturday afternoon, a strong frontal
boundary will move eastward and bring in some cooler air tonight
and Sunday. In the mid-levels, a sharp trough is forecast to close
off with a negative tilt in the regional pattern early Sunday as
the jet sharpens across the Midwest and southern Canada through
the day, promoting further cyclogenesis over southeastern WI. Rain
will change to snow into the evening hours from west to east as
CAA on the NW side of the low. Concurrently, WAA aloft will
increase from the south and strengthen the temperature gradient on
the nose of the WCB, favoring an area of sleet/freezing rain over
the Arrowhead of MN. Snow will start to accumulate despite the
warm ground/antecedent conditions when rates increase >0.5-1"/hr
and also after dark over MN initially, then into WI as the low
wraps up. CAM guidance shows the deformation band and local min in
temperatures (near and below 32F into Monday) with strong UVV into
the DGZ which will drive 1-2"/hr rates. Orientation will be N-S
but the guidance still shows appreciable differences in how the
surface low (and upwards) evolves and wobbles a bit over eastern
WI, which results in ~100 mile difference in the heavy snow axis
from closer to the MN/WI border vs into central WI (and northward
to the western U.P.). By Monday afternoon, the better dynamics
will move into Canada with less QPF over the western Great Lakes,
but with plenty of wind around the circulation and cold
temperatures (favoring some lake-enhanced snow across the U.P.).
Lighter snow will spread eastward as colder air follows behind the
front across Lower MI and then into western NY/PA and the central
Appalachians.

WPC probabilities for at least 4 inches of snow D1 are greater
than 30% over much of central and southern MN which will expand
and increase eastward into D2 across much of western/central WI
northward across the western U.P. into the Arrowhead of MN. In
this N-S axis, probabilities for at least 8 inches of snow over
the two-day period exceed 50% and are generally >30% for at least
12 inches of snow. Low probabilities (<30%) for at least 4 inches
of snow exist over western Lower Michigan on Monday as the upper
low rotates through.


...Pacific Northwest and Northern California...
Days 1-3...

A multi-spoked upper low over the Gulf of Alaska will wobble
southeastward over the next couple of days, sending waves of
precipitation into the Pac NW/northern CA and northern
Rockies/Great Basin. Lead front will come inland on Sunday and
will push past the Divide on Monday. Brief surge in moisture ahead
of the front (PW anomalies just above climo 0.50-0.75") will
support modest QPF for the region with typical terrain enhancement
over the Olympics, coastal ranges, and Cascades. Snow levels
around 4000-5000ft ahead of the front will lower below pass level
into Monday with several inches of snow possible. Next system will
come into the coast mid-day Monday amid a cooler air mass and
lower snow levels around 2000-3000ft. Jet will dip a bit farther
south across NorCal, spreading snow through the OR Cascades into
the Shasta-Siskyous/Klamath mountains and eventually the northern
Sierra by early Tuesday. Height falls will progress eastward
across the Great Basin/northern Rockies with a rather wide expanse
of snow above about 5000ft with lowering snow levels behind the
cold front. Favored areas may include the central Idaho ranges
into southwestern Montana as the nose of the jet moves through the
region and lower-level convergence coupled with upslope
enhancement promotes some moderate snow totals over 6 inches.
Multi-day totals will likely exceed 1-2ft over much of the
Cascades, Olympics, and Klamath mountains.

For Days 1-3, the probability of significant icing greater than
0.25" is less than 10 percent.

Fracasso