Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
124 PM EST Sat Jan 28 2023

Valid 00Z Sun Jan 29 2023 - 00Z Wed Feb 01 2023

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

Expansive mid-level trough will persist across much of the CONUS
this weekend before amplifying across the West Monday and Tuesday.
This amplification will be in response to an impressive 500mb low
digging southward along the CA coast Sunday night into Monday,
with height anomalies approaching -3 standard deviations at 500mb
according to the NAEFS ensemble tables Monday morning. At the same
time, a northern stream shortwave with less amplitude will be
spinning across the Northern Plains and into the Great Lakes,
resulting in a positively tilted longwave trough from the Great
lakes into the Southwest by the start of D3 /Monday night./ This
evolution will dislodge a surface cold front, pushing it southward
across nearly the entirety of the Intermountain West this period,
ending up south of the Four Corners by Tuesday evening. Waves of
low pressure along this front will help enhance lift already
expected to be significant thanks to the height falls, favorable
position of the strengthening subtropical jet streak, and
post-frontal upslope flow. Additionally, moisture will slowly
begin to increase across the region as SW flow around the
sharpening trough advects Pacific moisture eastward, especially
into the Four Corners, although PW anomalies are progged to remain
just below normal to near normal for early February.

The overlap over forcing and moisture will result in periods of
moderate to heavy snow, generally along and ahead of the southward
advancing cold front. Snow levels ahead of the front will
generally be 2000-3000 ft, and while some snow is likely behind
the boundary as snow levels crash rapidly to the surface,
additional accumulation at that point appears to be minimal, with
most accumulations occurring in the terrain and ahead of the
boundary. Still, overall transient features and modest moisture
should limit snowfall overall. For D1, the heaviest snow is likely
across the northern Sierra, northern Wasatch, and into the CO
Rockies, near the Park Range, where WPC probabilities for more
than 6 inches are 50-80%, and locally 12 inches is possible near
the Park Range. For D2, the axis of heavy snow shifts south with
WPC probabilities for more than 6 inches above 30% reaching the
San Bernadinos/San Gabriels, and extending into the southern
Wasatch, and continuing a second day across much of the CO
Rockies. By D3, the focus shifts to the Mogollon Rim where WPC
probabilities for more than 6 inches are 20-40%. Although much
lighter, as snow levels crash behind the front, some snowfall
accumulations are possible into the lower terrain and
valleys/passes, including Tejon, Cajon, and Tehachapi passes where
up to 1 inch of snow is possible.


...Upper Midwest, Great Lakes, and Northeast...
Days 1-3...

A shortwave moving across the Central Plains today will begin to
shear out an accelerate to the northeast tonight through Sunday as
it becomes entrenched in increasingly confluent flow north of a
bulging ridge over the Southeast. The associated surface low
beneath this impulse will begin to weaken as it shifts northeast
along the low-level baroclinic zone, which will also be pushing
northward downstream as WAA surges into the Mid-Atlantic and
Northeast on Sunday. Although ascent through upper ventilation in
the LFQ of an expanding jet streak will remain favorable, overall
forcing will begin to wane Sunday, with the low-level fgen driving
the heaviest snowfall also expected to collapse. The low tracking
northeast will allow the warm nose to surge as far north as lower
Michigan, Upstate NY, and Central New England, resulting in a band
of moderate to heavy snow from near Milwaukee, WI, through the St.
Lawrence Valley and Northern Adirondacks on D1, extending into
Northern Maine on D2. Modest forcing and near climo SLR should
allow for some snow rates nearing 1"/hr at times as everything
advects eastward, which should accumulate to several inches along
this max stripe of snow. WPC probabilities along the fgen band are
40-80% for more than 4 inches, with locally more than 6 inches
likely, especially in the L.P. of MI and the NW corner of ME.

A secondary elongated shortwave will move through the broad
eastern CONUS trough Monday night into Tuesday, leading to further
lowering of heights and increasing CAA on W/NW flow in its wake.
The column becomes quite cold behind this next impulse, which will
lead to steep lapse rates across the Great Lakes, albeit with
modest inversion heights. This suggests at least periods of heavy
LES in the favored W/NW snow belts, but rates may be tempered a
bit due to the cold column resulting in just modest SLRs for LES.
WPC probabilities for LES exceeding 4 inches are around 20% for
the Keweenaw Peninsula and near Whitefish Point, and 30-40% for
the Tug Hill Plateau.


...North Texas through the Mid-South...
Day 3...

A closed mid-level low moving into the Southwest from the Pacific
on Tuesday will yield downstream height falls into the Four
Corners and Southern Plains, but a downstream ridge over the
Southeast will expand through this evolution, leading to
impressively confluent mid-level flow into the MS VLY and Southern
Plains. There is quite a bit of disagreement among the
deterministic models as to how this will evolve on Tuesday, but
there appears to be an increasing threat for an overrunning
mixed-precipitation event from central TX through the Ozarks and
into the TV VLY D3 and even beyond this forecast period. While the
strong cold front is likely to surge southward, and moisture will
begin to arc northward on SW flow out of the Pacific and Gulf of
Mexico, there is uncertainty into how far the moisture will stream
to get into a region favorable for freezing rain due to
sub-freezing wet bulb surface temps with above freezing mid-level
temps along the warm nose. The GFS suggests precip will struggle
to surge far enough north for significant ice accretion, while the
EC/CMC and their ensembles are much more aggressive. Examination
of the DESI clusters indicates more support for the CMC/EC camp
than the GFS, and will lean more in that direction. With broad
overrunning likely and ascent enhanced through the strengthening
RRQ of a subtropical jet streak, periods of moderate freezing rain
appear likely which could result in around 0.25" of ice. The
highest risk for 0.1-0.25" of ice right now according to WPC
probabilities extends in a stripe from near Abilene, TX, through
Kiowa, OK, and towards Memphis TN where 0.1" probabilities are
20-50%, with some pockets of 10-20% probabilities for 0.25". There
is potential for higher amounts as reflected by WSE plumes, and
while current pWSSI probabilities for moderate impacts are only
20-30%, expect these will start to climb as the event gets closer
and model consensus, hopefully, starts to merge.


Weiss