Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Thu Mar 09 2023

Valid 00Z Fri Mar 10 2023 - 00Z Mon Mar 13 2023

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

An impressive atmospheric river (AR) event will begin tonight as
IVT which has a high probability of exceeding 750 kg/m-s according
to the ECENS and GEFS probabilities from CW3E shifts into
California. Not only will this AR carry high IVT, but it will also
be of a longer duration of impressive moist advection, with IVT
above 500 kg/m-s persisting into early D2 /Friday night/. The
primary driver of this AR onshore will be the downstream confluent
mid-level flow south of an amplified closed low dropping along the
Pacific Northwest coast and then shifting onshore Oregon Friday
before pivoting into the Northern Rockies Saturday. South of this
feature will be impressively confluent and then zonal flow,
directing high PW anomalies onshore, with moisture further
enhanced by a persistent upper level jet streak arcing towards CA,
with core strength reaching 130 kts on Friday.

While the most impressive moist advection within the AR will occur
late D1 noted by PW anomalies surging towards +5 standard
deviations above the climo mean, the continuous jet energy
overlapping zonal mid-level flow should allow anomalous moisture
to persist across CA and the Great Basin through early next week,
with yet another robust AR approaching just beyond this forecast
period. This tropically sourced airmass with strong WAA will drive
snow levels rapidly upward today, beginning around 300-5000 ft but
then rising rapidly to as high as 10,000 ft across the southern
Sierra and southern CA, with more general 8,000ft or higher all
the way into the Great Basin and Four Corners. Of course, snow
levels will be lower farther north through the Intermountain West
and Northwest, but available moisture and ascent will be weaker up
there as well. Even so, this is likely to result in a widespread
heavy snow event, especially in the higher terrain and above many
of the passes.

The heaviest accumulations through the period are expected on D1
as the core of the impressive AR pivots onshore and forcing is
most intense beneath the upper swinging eastward. WPC
probabilities for more than 6 inches on D1 are above 80% along the
length of the Sierra and northward across much of the western
terrain through the Shasta/Trinity/Siskiyou region, the Cascades
of WA and OR, and eastward as far as the Uintas of UT and Wind
Rivers in WY. While heavy snow will be widespread, the truly
extreme snow is likely across the central and southern Sierra
where, despite very low SLR in the warm environment, many areas
will receive 2-3 feet, with locally more than 6 feet likely above
8,000 ft in elevation, and extreme impacts are forecast via the
pWSSI thanks in part to snow rates that could reach 4"/hr at
times. During D2 the heaviest snow becomes somewhat more
constricted to the Sierra, Great Basin, and Four Corners,
generally south of the upper low and still embedded within the
upper jet. WPC probabilities D2 for more than 6 inches are again
above 80% in the Sierra where locally an additional 3+ feet is
likely, as well as in the Wasatch, Uintas, CO Rockies, and San
Juans where locally 12+" are probable. By D3 the area of heavy
snow shrinks once again despite snow levels returning to more
normal levels, and WPC probabilities for 6+ inches are 30-50% in
the San Juans and CO Rockies. The Sierra remain active as the next
closed low drops southward to commence yet another AR moving into
the region late D3.

Additionally, a low pressure advecting eastward beneath the
primary mid-level close low will trail a cold front southeastward
behind it. There is an increasing signal for some convective snow
showers or snow squalls along and behind this front beneath the
cold core to steepen the lapse rates, along with increased
low-level instability and negative 0-3km theta-e lapse rates. The
snow squall parameter reaches +4, and there appears to be an
increased threat for short duration but potent snow squalls Friday
and Friday night, generally focused from east of the WA/OR
Cascades through the Northern Rockies.


...Upper Midwest, Great Lakes, & Northern Mid-Atlantic...
Days 1-2...

A negatively tilted shortwave trough diving out of the Northern
Plains will amplify into a closed low over the Great Lakes as
potent but strung out vorticity wraps around the base of the
trough to help intensify the trough, and this closed low will
continue to deepen as its shifts off the NJ coast Saturday
morning. At the surface, this will manifest as a surface low
moving south of the Great Lakes, with intensification likely in
response to the overlap of height falls/PVA and diffluence within
the RRQ of a departing upper jet streak. This primary surface low
will move quickly eastward, but secondary development is likely
off the Mid-Atlantic coast Saturday morning as the departing jet
streak and an approaching sub-tropical jet streak begin to couple
in concert with the amplification and continued deepening of the
closed low over NJ. This will result in a deepening secondary low
pressure moving east off the coast, while the primary low begins
to weaken over the interior Northeast. Between these two features,
an inverted trough is likely to continue to drive some enhanced
local convergence, which will be collocated beneath a persistent
theta-e axis, the residual from the TROWAL likely to wrap around
the primary low Friday.

For the primary low moving south of the Great Lakes, there is
likely to be a region of intense deformation and some fgen north
of the low track. Regional soundings across this area suggest the
potential for impressive UVVs into the saturated DGZ, which is
topped additionally by some negative theta-e lapse rates
suggesting the potential for CSI and 1+"/hr snowfall rates as
depicted by the WPC prototype tool. The low remains progressive,
SLRs are likely to be below climo within the WAA, and in some
areas these rates will be fighting against the increasing March
sun angle. However, there is likely to be at least a band of
heavier snow from the western shore of Lake Michigan where some
lake enhancement is likely, through southern lower Michigan to
along the Lake Erie shore, into central Pennsylvania. WPC
probabilities for 6+ inches are as high as 80% near Milwaukee, WI
and northward towards the Door Peninsula due to lake enhancement,
but are still impressive around 50% across lower MI eastward
through Lake Erie including the Chautauqua Ridge and into central
PA. Some of this snow east of the Great Lakes will be enhanced by
some LES behind the low on increasing north winds.

Farther to the east as the secondary low develops and becomes
dominant, generally early D2 /Friday night into Saturday morning/
the inverted trough beneath the theta-e ridge should pivot
southward along the Mid-Atlantic coast from near New York City to
Philadelphia. Models fluctuate greatly with the position and
intensity of this trough, and overall ascent is weakening with
time, but this could result in a subtly over-performing snowfall
somewhere near the NY/NJ/CT tri-state area or eastern PA. WPC
probabilities are modest for 4 inches except in the higher terrain
of the eastern Poconos and southern Catskills, but WSE plumes
feature a lot of spread indicating the potential for some
over-performing snow where this convergence band pivots into early
Saturday morning.

This event could also produce some "conversational" snowflakes
into the I-95 corridor as far south as Washington, D.C., either
Friday morning in the WAA or Saturday morning as the inverted
trough shifts southward, a big deal in an otherwise snow less
winter so far for those areas.


...Northern Plains to Upper Midwest...
Days 2-3...

The closed low ejecting from the Pacific Northwest D1 will deepen
as it moves along the international border with Canada Saturday
morning before diving into the eastern Dakotas Sunday morning,
eventually reaching the western Great Lakes by the end of the
forecast period. Modest LFQ upper diffluence atop the best height
falls will help deepen a wave of surface low pressure as it moves
from eastern MT through MN D2-3, with a leading warm front surging
into the Upper Midwest. Moisture will increase ahead of this low
on meridional transport from the Gulf of Mexico, and may become
impressive as mixing ratios transported northward on 290K
isentropic upglide reach 3-4 g/kg. The corridor of best moisture
transport is relatively narrow as the dry slot races eastward, but
strong WAA to enhance ascent beneath the otherwise strong synoptic
lift will likely result in a swath of heavy snow spreading
eastward from eastern MT early D2 to as far east as the Great
Lakes D3. While the WAA will generally reduce the SLR, regional
soundings across the Northern Plains indicate that the best WAA
will occur just below, or within, the DGZ to help produce a
near-isothermal layer beneath a subjectively deep DGZ in a cold
column. This suggests that SLRs may be above climo and a bit
fluffy, especially early in the event, and across the more
northern regions, which could enhance snowfall accumulations more
than locations farther south. This is reflected in WPC
probabilities that are already above 80% for 6+ inches from far
northeast MT through northern ND on D2, and extending into
northern/central MN D3, with some additional heavy snow likely
along the shore of Lake Superior late. Locally, more than 12
inches of snow is possible where the snow persists with the
greatest temporal duration.

For days 1-3, the probability of icing greater than 0.1" is less
than 10 percent.

Weiss

...Key Messages for the March 9-10 (Western U.S.)Winter Storm...

--Strong Winter Storm Lasting into Friday: A winter storm will
continue to affect the West Coast, and California in particular,
through Friday with a burst of heavy precipitation.

--Heavy Rain and High Snow Levels Likely: This will be a warm
storm system with rain falling on existing snowpack up to 8500
feet. The highest snow levels, heaviest rainfall, and highest
rainfall rates are expected in central California. Considerable
flooding impacts are possible across portions of the central coast
and San Joaquin Valley.

--Rain and Snowmelt Will Lead to Flooding: The most significant
snowmelt, and overall flooding threat, is expected below 5000 foot
elevation, in areas with shallow snowpack. Creeks and streams in
the foothills of the Sierra Nevada will be most vulnerable to
flooding from rain and snowmelt.

--Heavy, Wet Snow at Higher Elevations: Higher elevations in
northern California and in the Sierra Nevada will receive a heavy,
wet snow. This will lead to difficult travel, and combined with an
already deep snowpack, may lead to increasing impacts from the
depth and weight of the snow.


...Key Messages for the March 8-12 (Northern Plains to the Mid
Atlantic) Winter Storm...

--A winter storm is set to track through the Lower Great Lakes
tonight and into early Friday, bringing periods of heavy snow with
snow rates up to 1â€/hr at times. 

--Additional snowfall accumulations of 4-8†are expected across
southern Wisconsin, far northern Illinois, southern Michigan, and
east of Lake Erie.

--Plan for travel impacts including snow-covered roads, reduced
visibility, and difficult travel across the impacted areas.
Whiteout conditions are possible within the heaviest snow bands.

--As the storm moves east, several inches of snow are expected
across the northern Mid-Atlantic, including north-central
Pennsylvania and the Poconos which will cause some travel impacts
and disruptions.