Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 PM EDT Sat Apr 09 2022

Valid 00Z Sun Apr 10 2022 - 00Z Wed Apr 13 2022

...Western U.S. to the Northern Rockies...
Days 1-3...
Anomalous cold from deep northwesterly flow expands across the
Northwest through the northern High Plains through Sunday under
reinforcing upper troughing. Through Sunday, embedded energy aloft
will support showery and unseasonably cold weather across the
Northwest and the northern Rockies. Already low snow levels will
dip further tonight, dropping below 500ft across much of
Washington, eastern Oregon, and the northern Rockies by Sunday
morning where Day 1 probabilities for over 4" snow are generally
moderate (high for the WA to central OR Cascades where there are
moderate probabilities for over 12").

12Z guidance is in better agreement for a shortwave trough axis
currently near the AK Panhandle amplifying as it shifts south off
the BC through Sunday before closing into an upper low over
western WA/OR Sunday night. Upper forcing from this developing low
(including left jet exit lift) along with ample Pacific moisture
rounding the wave will make for particularly heavy precipitation
along with low snow levels, a rare pairing for the Northwest
(particularly for April) with a focus on western OR Sunday night
and northern CA Monday. Snow levels in the heavier precip look to
be around 3000ft, lowering back down to 2000ft after the trough
passage. Day 2 snow probabilities are moderate for over 8" from
the southern WA Cascades through the northern Sierra and high for
the OR Cascades (where there are moderate probabilities for over
18").

The low opens into a wave again (with a neutral tilt) and shifts
inland over the northern Great Basin late Monday, reaching the
northern Rockies Tuesday. A focused wave of precip shifts east
ahead of this trough axis with snow levels over the northern Great
Basin generally 4000 to 5000ft in the warm air advection through
Monday evening. Late Monday night, the heavy snow focus shifts to
the lee-side as rapid surface cyclogenesis occurs over
northeastern CO.  Day 2.5 snow probabilities are moderate for 8 or
more inches across north-central OR terrain (including the Blue
Mtns in Northeast OR), central ID terrain into southeast MT, as
well as the Wasatch of northern UT.

...Northern Rockies to northern Great Plains...
Day 3...
The aforementioned lee-side cyclogenesis over northeast CO Monday
night combined with a 1040mb surface high over northern Alberta
sets up intense low level convergence with an upslope component
from very strong northeasterly flow over the northern High Plains
and eastern slopes of the northern Rockies late Monday night
through Tuesday before shifting farther east on the northern
Plains Tuesday night. The focus of the convergent axis of precip
has shifted over the past few days, with the latest trend to focus
a bit farther south over central/eastern MT to near the ND/SD
border. 12Z deterministic consensus is in good agreement with each
other. The 12Z ECMWF is a bit farther south than the rest of the
consensus, but the threat for a blizzard remains strong. Day 3
probabilities for over 12" are moderate across south-central MT
with moderately high probabilities for over 8" from the eastern
slopes of the MT Rockies (south of Glacier NP) east to the Dakota
border.

Further reinforcement of the trough over the northern High Plains
from the Pacific Northwest Tuesday night leads to a negative tilt
and pivot of the surface low track that likely brings it up the
eastern side of the northern Plains, maintaining a threat for
windy/heavy snow over the Dakotas through Wednesday.


...Northern Minnesota...
Days 1/2...
A shortwave trough crossing the northern Rockies this evening is a
reinforcing trough of a notable cold outbreak that overspreads the
western/northern CONUS through the middle of next week. This
trough axis assumes a negative tilt as it tracks from the Dakotas
and across Minnesota Sunday night. This dynamic system is expected
to support rain changing to snow in embedded bands over
northwestern Minnesota Sunday evening, expanding over the
Arrowhead late Sunday night. The risk for briefly heavy snow is
next over north-central MN where Day 1.5 snow probabilities for
over 4" are around 20%. 


For Days 1-3, the probability of at least 0.1" icing is less than
10 percent.

Jackson