Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Sun Apr 16 2023

Valid 00Z Mon Apr 17 2023 - 00Z Thu Apr 20 2023

..Upper Midwest and Western Great Lakes...
Days 1-2...

A late-season winter storm will continue to impact the region
overnight into Monday, with heavy snow accumulations likely for
portions of western Wisconsin, eastern Minnesota, and the western
Upper Peninsula of Michigan.

Rapidly deepening system moving into the western Great Lakes this
evening will be energized by a short wavelength/curvy S-shaped jet
and a closed mid-level low that will become vertically stacked by
early Monday. Snow will expand eastward across much of Wisconsin
overnight as the surface low meanders over eastern WI/Lake
Michigan as the upper low rotates toward the surface low position.
The WCB will continue to wrap in from the south into a TROWAL as
CAA continues on the back side, helping to form and maintain a
deformation band across western WI overnight into early Monday
that may linger for several hours as the system becomes stacked.
To the north, low-level cold air will be overrun by above freezing
temperatures above the surface, favoring a wintry mix over the
Arrowhead of MN where some sleet and freezing rain accumulation is
likely, though generally under a tenth of an inch or so. Snowfall
rates may exceed 1-2"/hr per the 12Z CAM guidance and WPC Snowband
Probability Tool with increasing wind around the low pressure.
Areas close to southwestern Lake Superior (e.g., Bayfield
Peninsula toward the Porcupine Mountains) will also capitalize on
lake enhancement on northeast to northerly flow through Monday,
adding to the already significant accumulations. Through Monday
into the evening hours, the low will drift eastward as the upper
dynamics separate into the downstream triple point low over
Canada, with decreasing snow through the day but increasing
snow-to-liquid ratios in the colder column. Additional lake
enhancement is likely into western Lower Michigan into the
overnight hours tomorrow. Light snow will follow behind the cold
front farther east into western NY/PA (as well as east of Lake
Ontario and into the central Appalachians) but will generally be
light (<2").

WPC probabilities indicate that storm total amounts of 4 inches or
more are likely from near the near the MS River (IA/MN/WI area)
and southwestern Wisconsin northward to the Arrowhead of
Minnesota. Some areas of west central WI and along the shores of
northwestern Wisconsin and the western U.P. could see a foot or
more, where WPC probabilities are generally greater than 50%.

...Pacific Northwest and Northern California to the Northern
Rockies...
Days 1-3...

Upper low with multiple vort maxes around its circulation south of
the Gulf of Alaska will slowly move southeastward and send in
several wave of precipitation into the Pacific Northwest into the
Northern Rockies and southward into Northern California over the
next few days. Lead front will continue to move into WA/OR this
afternoon/evening into the start of D1 with modest precipitation
as precipitable water values are near normal for mid-April
(~0.50"). Next more defined smaller upper low rotating out of the
larger upper low will move into the NW Monday afternoon as the
upper jet dips farther south into NorCal, bringing snow farther
south through the OR Cascades into the Klamath Mountains,
Shasta-Siskyous, and Trinity ranges. Height falls and PVA will
progress into ID where upslope enhancement will favor higher
totals over the Blue Mountains into the central ID ranges and
several inches over western WY as this cold front moves through
the region D2. Into D3, another shortwave will eject out of the
southern edge of the parent upper low still west of British
Columbia, with generally light to modest snows for the
Cascades/Olympics. Snow levels will be fairly low as 700mb
temperatures continue to fall through the period to -10 to -15C
which is about 1-2 sigma below normal. This will likely bring
significant snow into many pass levels where WPC probabilities for
at least 8 inches are greater than 50% above about 3000ft across
the Cascades.


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


Fracasso