Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Wed Apr 06 2022

Valid 12Z Wed Apr 06 2022 - 12Z Sat Apr 09 2022

...Pacific Northwest...
Day 3...

Very late Day 2 into Day 3, confluent mid-level flow and warm air
advection will increase moisture into the northeast Pacific
(narrow axis back toward Hawai'i) and eventually into
Washington/Oregon. Precipitable water values will briefly surge to
+1 to +2.5 sigma as a surface cold front moves into the region
beneath modest upper diffluence as the jet streak weakens Friday.
Snow levels will be high to start (6000-7000ft) at precip onset
early Friday but will fall to about 3000ft by the evening.
Progressive flow and modest moisture/upslope will limit amounts
but higher elevations above pass level will still see more than a
foot of snow. With lowering snow levels during the event, some of
the passes will see rain changing to snow and accumulating a few
inches.


...Northern Plains/Upper Midwest/Western Great Lakes...
Days 1-3...

Deep closed low over Minnesota at the start of the period (500mb
heights about -2 to -3 sigma) will wobble eastward through the
period as infused vort maxes from Canada elongate the circulation
into a multi-spoke entity. Though not completely cut off from the
westerlies, it will be a slow-mover (typical for April). On Day 1,
tapped Gulf moisture will surge northward on the WCB (40+ kt 850mb
LLJ) into a TROWAL as the occlusion grows in length across the
Great Lakes, focusing snowfall over southern Ontario and into the
Arrowhead. To the northwest of the low, deformation axis will be a
second focus of modest snowfall over northwestern MN. Third area
will be over the Black Hills as another mid-level vort swings
through with favorable upslope.

By Day 2, the system will overturn as the parent circulation
essentially detaches from the eastern baroclinic zone as the jet
strengthens into the Mid-Atlantic and moisture axis focuses into
southeastern Canada. However, leftover moisture over the Upper
Midwest/Great Lakes will be sufficient for numerous snow showers
with local enhancement especially over the Coteau des Prairies and
Pembina Escarpment. Lake effect/enhanced snow will be favored over
the Gogebic Range into northern WI and the western U.P. as the
flow around the surface low converges across Lake Superior. By Day
3, system will continue to slowly weaken and unwind but the cold
pool aloft will drive plenty of snow showers farther southeast,
reaching through the Midwest and into the southern/central
Appalachians where a couple inches of snow are possible.

WPC probabilities for at least four inches of snow are moderate to
high over the aforementioned favored areas in MN into the Black
Hills Day 1, then into northern WI and the western U.P. of
Michigan Day 2. By Day 3, probabilities decrease over the U.P.
while far northwestern Maine has a low chance of at least four
inches Day 2.5. For the three-day period, low chances of four
inches of snow extends through much of Minnesota into western
ND/SD.


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

Fracasso