Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Thu Apr 20 2023

Valid 12Z Thu Apr 20 2023 - 12Z Sun Apr 23 2023

...Pacific Northwest and Northern/Central Rockies...
Days 1-2...

An upper low west of British Columbia this morning will move
southeastward and shear into the northern Great Basin early Friday
and then across the central Rockies overnight into early Saturday
as it weakens into the southwest side of the more dominant Upper
Midwest upper low. A 125kt jet streak into coastal WA will support
modest snows for the Olympics and Cascades as the front and
associated moisture plume (precipitable water values up to +1
sigma) push into and through the region rather quickly, limiting
some accumulation. Snow levels will rise to around 3000-4000ft
which will still support pass level snowfall across the WA
Cascades. As the weakening vort slips through the central Rockies
D2, the higher terrain will likely see several inches of snow from
the western WY ranges into the CO Rockies. WPC probabilities for
at least 6 inches of snow are near 50% around and above 5000ft in
the PacNW but above 7000ft into the Rockies.


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

Upper low over eastern MT and western ND will get pulled
southeastward and yield to a strengthening vort max rounding its
south/southeast base today. Coincident with a strengthening upper
jet across the Corn Belt, the shortwave will close off into an
upper low over MN as the old parent upper low gets wrapped into
its circulation and dumbbells around each other on Friday. At the
surface, the more favorable dynamics will promote cyclogenesis
toward Wisconsin with a defined moisture plume out of the western
Gulf into Michigan. As the system wraps up and occludes, TROWAL
will develop as the WCB lifts up and over the warm front with a
still cold supply of air from the northeast. Tilted
frontogenetical axis to the northwest will favor a period of
1-2"/hr snowfall rates along with gust winds. Trend has been for a
bit westward shift in the heavier snow axis into the Red River
Valley (of the north) as the system lifts through the Corn Belt
then loops over MN/WI Friday. Warm nose aloft will support a mix
of sleet and freezing rain over northeastern MN until the system
moves eastward and colder air floods across the entire Upper Great
Lakes region late Friday into Saturday. WPC probabilities for at
least 6 inches of snow D1 are high (>70%) over northeastern ND and
much of northwestern and north central MN. Snowfall will decrease
overnight into early Friday, but light snow is likely through much
of the day across ND/MN where D2 WPC probabilities for at least 4
inches are around 10-30% as the surface low weakens but only
slowly moves eastward. By D3, upper low will move through MI where
northerly flow will favor lake-enhanced snowfall into the U.P. of
Michigan. WPC probabilities of at least 4 inches of snow D3 are
generally under 50%.

Fracasso