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

Valid 12Z Tue Mar 21 2023 - 12Z Fri Mar 24 2023

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

An anomalously strong closed low characterized by height anomalies
approaching -5 standard deviations from 850-500mb according to the
NAEFS ensemble tables will drop southeast along the CA coast this
morning before shifting onshore near San Francisco this evening.
This feature will weaken as it moves onshore, 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 Thursday
aftn. Following this first wave, a secondary sharp but intense
shortwave will drop rapidly southeast from British Columbia into
the Pacific Northwest by Friday morning.

This first low will be accompanied by intense LFQ diffluence as a
130-150kt Pacific jet streak surges zonally onshore Southern
California and stretches into the Great Basin/Four Corners. The
overlap of this jet streak with confluent mid-level flow
downstream of the closed low will push an atmospheric river (AR)
onshore with IVT exceeding 500 kg/m-s according to CW3E
probabilities, while also driving a potent surface low into
California this aftn. This deep layer ascent into the anomalously
high moisture will result in significant precipitation stretching
from CA eastward into the Four Corners. Additionally, a residual
stalled surface boundary draped west to east across the region
will additional serve as a confluent boundary for ascent, with
some fgen and isentropic upglide to enhance ascent also likely as
SW flow from the Pacific surges northeast overtop this front. With
the broad trough developing overhead as the primary low weakens,
any impulses moving through the flow could result in additional
waves of low pressure, and it is likely that widespread
precipitation, falling as snow in the terrain, will plague
California, the Great Basin, and the Southern Rockies D1,
expanding into the Central Rockies on D2. Snow levels during this
time will be generally around 4000 ft, but will surge as high as
6000-7000 ft ahead of the primary low and its associated cold
front from the Great Basin into the Four Corners D1 into D2. WPC
probabilities both D1 and D2 are extremely high for more than 6
inches from the Peninsular/Transverse ranges through the Sierra
and eastward across the Great Basin including the Wasatch, Kaibab
Plateau, San Juans, and CO Rockies, extending northward into the
NW WY ranges during D2. With tremendous moisture advecting into CA
and increasing upslope ascent, snowfall in excess of 4-5 feet is
likely in the higher terrain of the San Gabriels, San Bernadinos,
and Sierra, with intense snowfall rates of 3+"/hr bringing major
to extreme impacts once again according to the pWSSI. Snowfall
across the remaining terrain will also likely eclipse 3 feet
across some of the higher terrain of the Wasatch, San Juans, and
along the Kaibab Plateau.

As this larger trough weakens and advects eastward, precipitation
will begin to expand across the Pacific Northwest late D2 into D3
ahead of the next compact shortwave, and associated with a
secondary Pacific jet streak surging southeast from the Gulf of
Alaska. A surface low associated with this synoptic ascent is
progged to move into British Columbia, but the associated frontal
structure, both the leading warm front and then trailing cold
front, should move across the region bringing enhanced ascent and
more favorable mid-level trajectories for additional moisture from
the Pacific. Snow levels will rise to as high as 4000 ft with the
warm front, but then drop to 2000-2500 ft behind the cold front,
and some impactful snow may occur at pass levels during this time.
For D3, WPC probabilities for more than 6 inches are high along
the Cascades of OR and WA, as well as into the Olympics, with
moderate probabilities extending as far south as the Sierra and
east into the Blue Mountains of OR and the Sawtooth of Idaho.


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

A compact but amplified shortwave will eject from the Central
Rockies this morning and begin to develop a negative tilt as it
interacts with a northern stream impulse dropping out of
Saskatchewan. The phasing of these two features will result in a
modest longwave trough strengthening near the Great Lakes
Wednesday evening, but this should occur just far enough east to
preclude a more significant system from impacting the Upper
Midwest this week. However, the progression of these mid-level
features combined with the approach of a strengthening subtropical
jet streak arcing out of 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 through 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 Wednesday morning
across MN. This will support at least a period of moderate to
heavy snow across the Dakotas and into Minnesota tonight and
Wednesday, 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. WPC probabilities for more than 6 inches have
increased with this iteration, and now are as high as 50% from
eastern ND through north-central MN, with locally more than 8
inches possible. There is also likely to 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 above 70%, with locally
more than 12 inches possible.

This system pulls away rapidly into Ontario Wednesday night, but a
subsequent shortwave will track immediately in its wake, emerging
from the Central Rockies by Thursday morning and then lifting
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
kts and arcs more strongly poleward, placing impressive LFQ
diffluence atop the Northern Plains, efficiently overlapping the
mid-level height falls. This will result in yet another wave of
low pressure moving eastward across the area along the residual
baroclinic boundary. While this system will also remain
progressive and likely deepen only slowly, 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 SD through MN. This axis of snow
will likely be south of the first event, and SLRs will likely be
lower than that event, a swath of moderate to at times heavy snow
is likely, and WPC probabilities for more than 4 inches reach
30-50% across parts of South Dakota, including the Black Hills.


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

Strung out vorticity lobe embedded within 500mb confluent flow
across southern Canada will combined with modest LFQ diffluence as
a subtropical jet streak arcs across the CONUS to produce modest
deep layer ascent across Maine on Thursday. 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. WPC
probabilities for more than 4 inches are as high as 70-80% in far
northern Maine, with locally more than 6 inches likely as
reflected by both NBM probabilities and WSE plumes.


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

Weiss