Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EDT Mon Mar 20 2023

Valid 00Z Tue Mar 21 2023 - 00Z Fri Mar 24 2023

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

Troughing will persist over the West as another Atmospheric River
Event pushes through central/southern California on Tuesday. This
will move through the Interior West/Four Corners area on Wednesday
as the trailing mid-level trough follows behind the surface front.
By Thursday, much of the Southwest will see generally light snow
for the terrain as another system from the Northeast Pacific heads
into Washington and Oregon.

For Tuesday, a closed low well west of OR/CA will turn the corner
and head eastward into Northern California by the afternoon near
the Bay Area beneath a strong 130+ kt jet over SoCal. Surface cold
front will promote southwesterly flow into the central/southern
Sierra and coastal ranges outside L.A. (e.g., San Bernadinos/San
Gabriels) as precipitable water values (+3 to +4 sigma and >99th
percentile) near 0.50" (Sierra) and 0.75"+ (closer to the coast)
on IVT values of 300 kg/m/s (Sierra) to 600 kg/m/s amid higher
snow levels 5000-6000ft in the pre-frontal WAA air mass.
Post-FROPA, snow levels will drop to near 4000ft as the QPF slowly
wanes, but combination of broad lift/upper dynamics, upslope
enhancement into the terrain, and anomalous moisture will yield
another modest to heavy snowfall for much of the higher terrain in
central/southern California. Eastward, most anomalous moisture
will generally be limited to southern NV and AZ as the moisture
plume weakens and is disrupted into the Rockies. However,
development of surface low pressure through the Great Basin along
the occluded/cold front along with the stronger height falls and
PVA on SW flow will promote modest snowfall totals across the NV
terrain and into the Wasatch southward to the Mogollon Rim later
Tuesday into Wednesday as the jet strengthens overhead. Several
feet of snow are likely in the higher terrain over CA with
major/extreme impacts while areas east of the Sierra have a high
chance (>70%) of at least 8 inches of snow over many of the
mountains ridges/peaks with several feet likely above 6000ft. To
the north, weakening system moving through the northern Great
Basin will act to promote several inches of snow over central
Idaho tonight into Tuesday.

Wednesday into Thursday, progression of the surface front eastward
will carry the heaviest snowfall across the CO Rockies and
northern NM as the upper trough trails behind, weakening as it
moves through the Great Basin. Heavy snow, driven by upslope
enhancement, into the San Juans will likely yield another foot+ of
snow (higher above 8000ft). By Thursday, upper trough will only
favor light snow over the Great Basin/Four Corners region with WPC
probabilities for at least 4 inches of snow generally <50% over
CO. Higher probabilities lie to the northwest over northern Utah
into eastern ID/western WY as the weak area of low pressure moves
through and finally exits the region. Over the Pacific Northwest,
another system will dive southeastward out of the Gulf of Alaska
with a modest surge in moisture to the Cascades/Olympics. There,
WPC probabilities for at least 4 inches of snow are moderate
(>40%) to high (>70%) over the highest peaks.


...Northern Plains through the Upper Midwest...
Days 1-3...

The first of a pair of shortwaves forecast to impact the region
mid to late week is currently centered over the Intermountain
West.  This system is expected to move east into the northern
Plains by late Tuesday, where it will begin to interact with a
northern stream trough digging across the Canadian Prairies.  A
relatively modest surface low is expected to develop over the
eastern Dakotas late Tuesday, with precipitation developing
northwest of the low.  Increasing low level moisture advection
interacting with favorable upper jet forcing and low-to-mid level
frontogenesis will support a stripe of moderate snow shifting
northeast from eastern North Dakota into northern Minnesota. 
Overall, this still appears to be a fairly modest event, with its
progressive nature expected to limit the threat for widespread
heavy amounts.  However, heavier snow rates of 1-2"/hr developing
along an axis of strong low-to-mid level frontogenesis may produce
some higher accumulations.  WPC guidance reflects this with an
increase in probabilities for accumulations of 4 inches or more
from southeastern South Dakota to north-central Minnesota. 
Locally heavier amounts of 6 inches or more can be expected along
this axis.  It remains likely that a secondary maxima will occur
along the the North Shore of Lake Superior, where lake-enhancement
and upslope flow are likely to contribute to accumulations of 6
inches or more.  Farther to the south, a wintry mix consisting of
freezing rain is expected to produce some light ice accumulations
across portions of central Nebraska and central to eastern South
Dakota Tuesday morning.

As this leading system lifts into Ontario, it will be quickly
followed by a second wave forecast to lift from the Southwest on
Wednesday into the central Plains Wednesday night.  Light to
moderate precipitation is expected to develop on the north side of
the associated surface and it tracks northeast along the leading
system's trailing cold front.  The axis of heavier amount
associated with this second wave are expected to fall further
south than the those produced by the leading system, with
favorable upper forcing and low-to-mid level frontogenesis
expected to contribute to the development of at least some
light-to-moderate snows centered across South Dakota into southern
Minnesota Wednesday night into early Thursday.

...Maine...
Day 3...

Warm advection precipitation ahead of the system lifting into
Ontario late Wednesday is expected to spread across the Northeast
Wednesday night into Thursday, with precipitation expected to fall
as mostly snow across northern Maine through at least early
Thursday.  At least a few inches of snow are likely before
precipitation is more likely to mix or change over to rain later
in the day.   


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


Fracasso/Pereira