Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 AM EDT Sat Apr 22 2023

Valid 12Z Sat Apr 22 2023 - 12Z Tue Apr 25 2023

...Central Rockies...
Day 1...

A weakening vort max moving out of WY into CO this morning will
dissipate this afternoon as it enters the base of the much larger
circulation around the Great Lakes upper low. Limited moisture and
paltry dynamics/upper support will confine appreciable snow more
than a few inches to the higher mountains, generally above
8000-9000ft, and into the Front Range down to the Sangre de
Cristos where some easterly flow in the lower levels may help to
squeeze out a few inches of snow. WPC probabilities for at least 4
inches of snow are moderate (>40%) to high (>70%) from north to
south.


...U.P. of Michigan...
Day 1...

Upper low over WI/MI will slowly rotate eastward D1 with another
smaller and weaker upper low grazing the Arrowhead tonight into
early Sunday. Cyclonic flow will continue to bring in sufficiently
cold air over the Upper Great Lakes on northerly flow across Lake
Superior. This will support additional lake enhanced snows into
the western U.P. (Porcupine Mountains), across the Keweenaw
Peninsula, and toward the Huron Mountains. Amounts will be
generally light, with the highest WPC probabilities (40-50%) for
at least 4 inches centered across the western U.P. of Michigan.


...Pacific Northwest and northern Rockies...
Days 2-3...

A compact/small upper low is forecast to come into coastal WA late
Sunday and continue southeastward into southern ID then northern
UT by early Tuesday. Precipitable water values are forecast to
only rise slightly above climo values, but the combination of
modest height falls and upper divergence on the LFQ of the upper
jet along with some frontogenetical forcing in the lower levels
will promote light to moderate snow over the Cascades D2 and the
across central ID/southwestern MT/northern UT/CO Rockies D3 as the
system continues southeastward. Some areas of southwestern MT may
see more than six inches of snow in elevations above 7000ft, where
WPC probabilities are moderate (>40%) for at least six inches
across the Bitterroots.


The probability of significant icing greater than 0.25" is less
than 10 percent for Days 1-3.


Fracasso