Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 PM EDT Tue Mar 21 2023

Valid 00Z Wed Mar 22 2023 - 00Z Sat Mar 25 2023

...The West...
Days 1-3...

An anomalously strong closed low moving onshore over the Bay Area
will weaken/fill as it moves onshore tonight, merging into a
larger longwave trough centered over the Intermountain West, with
spokes of vorticity rotating eastward to help evolve the trough
into a positive tilt and shift into the Central Plains by
Wednesday night. Following this first wave, an intense trough will
drop rapidly southeast from the Gulf of Alaska and cross the PacNW
coast Thursday afternoon before extending low pressure over the
Northwest through this weekend.

Continued onshore flow south of the first low will provide above
normal moisture to the southern Sierra Nevada and southern CA
ranges with snow levels dipping to or just below 4000ft tonight
where there are high Day 1 snow probabilities for an additional
foot of snow with locally 2-3ft additional in the higher peaks.
Snow levels initially over 9000ft in the Four Corners states drops
below 6000ft late tonight into Wednesday with Day 1 snow
probabilities high for a foot or more in the southern UT up
through the Wasatch and Uintas and western CO ranges with moderate
to high probabilities for 8 or more inches for the highest points
along the Mogollon Rim with 4-8" most likely for Flagstaff as they
approach their seasonal snowfall record. 

A prolonged low snow level event occurs over the Pacific Northwest
Thursday into the weekend as the trough axis/cold front and
subsequent NWly Pacific jet streak surges southeast from the Gulf
of Alaska providing a long fetch of onshore flow. Snow levels
around 2500ft can be expected with the cold frontal wave of heavy
precip with levels dropping below 2000ft across the PacNW and then
below 1000ft over much of the PacNW Thursday night. Day 2.5
probabilities for 8 or more inches are moderately high over the
WA/OR Cascades and higher coastal ranges of western OR down
through the Klamath into far northern CA . Then it's nearly a
repeat for Day 3.5 with moderately high probabilities for 8 or
more inches over the same areas. There is a risk for over 2 feet
in 48hrs mainly over the western slopes of the OR Cascades above
about 2500ft. There is a Day 3 risk for 2 or more inches down to
around 750ft elevation around the Puget Sound of WA through the
Willamette Valley of OR.


...Northern Plains through the Upper Midwest...
Days 1 and 2...

A compact but amplified shortwave pushing into South Dakota this
afternoon will track northeast across northern Minnesota through
tonight before phasing with a northern stream trough dropping out
of Saskatchewan with the stronger low pressure tracking across
Ontario Wednesday. The progression of these mid-level features
combined with the approach of a strengthening SWly subtropical jet
streak reaching out from the Desert Southwest will produce
enhanced synoptic ascent over the Northern Plains, resulting in a
slowly strengthening surface low pressure system moving northeast
along a warm front tonight into Wednesday. Downstream of this low,
meridional moisture transport out of the Gulf of Mexico will draw
theta-e northward on moistening 290K isentropic upglide, noted by
mixing ratios reaching 4g/kg on the 750mb surface, and this will
try to lift cyclonically into a modest TROWAL late tonight over
northwestern Minnesota. This will support at least a period of
moderate to heavy snow across eastern North Dakota into Minnesota,
but the system will remain relatively progressive and the overlap
of moisture and ascent is modest. Still, forecast soundings are
indicative of above-climo SLR noted by a relatively cold column
with a deep isothermal layer beneath an average depth DGZ, so this
should allow for efficient accumulations, with some heavier snow
possible where a deformation band sets up NW of the surface low.
Day 1 WPC probabilities for more than 6 inches moderate for
eastern ND through NWrn MN, with locally more than 8 inches
possible mainly between the Red Lakes and Lake of the Woods. There
will also be a secondary maxima in snowfall along the Lake
Superior shoreline across the Arrowhead of Minnesota where lake
enhanced moisture and pronounced upslope flow will drive more
intense snowfall with a longer duration. Here, WPC probabilities
for more than 6 inches are 70%, with local 10 inches possible.

The next wave is the large low pressure system currently impacting
the Bay Area of California which will cross the Central Rockies
Wednesday night and lift quickly to the E/NE as it becomes
embedded in more confluent mid-level flow. This will be
accompanied by more impressive upper level diffluence as the
subtropical jet streak intensifies to 150 kt and arcs more
strongly poleward, placing impressive left exit diffluence atop
the Northern Plains, efficiently overlapping the mid-level height
falls. This will result in a wave of low pressure moving eastward
across Kansas Wednesday night along the residual baroclinic
boundary. While this system will also remain progressive and may
just weaken from its initial lee-side low though the low/mid level
fgen will intensify through the ageostrophic response of the upper
jet streak as well as the confluence between a surface high to the
north and the eastward advancing surface low. This most strongly
sloped fgen appears to focus in the 850-600mb layer, driving
intense UVVs into the saturated DGZ to support a band of heavy
snow tracking west to east from near the Neb/SD border and along
the MN/IA border. WPC probabilities are low for over 4" snow,
reaching 20 percent in south-central SD for Day 1.5 though it is
noted that most guidance has a narrow stripe of 6" with the
ECMWF/CMC guidance farther south in Neb and GFS/NAM solutions
north over southern SD.


...Northern Maine...
Days 2/3...

A positively-tilted trough over Quebec embedded within 500mb
confluent flow across southern Canada will combined with modest
left exit diffluence as a subtropical jet streak arcs across the
CONUS to produce modest deep layer ascent across Maine Thursday
into Thursday night. This lift will occur within a moist column
characterized by PWs reaching +1 to +1.5 standard deviations above
the climo mean, with robust WAA surging out of the Mid-Atlantic on
SWly 700mb flow downstream of a surface wave moving through
Ontario. The atmospheric column will feature marginal thermals for
wintry precipitation, with the warm nose >0C reaching potentially
as far north as Caribou by Thursday evening, but north of this
area some moderate snowfall accumulations are likely as
precipitation expands eastward during the day. Day 2.5 WPC
probabilities for more than 6 inches are 50-60% in Aroostook Maine.

The probability of receiving at least 0.1" accumulation of ice is
less than 10%.

Jackson