Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
142 PM EST Sun Jan 29 2023

Valid 00Z Mon Jan 30 2023 - 00Z Thu Feb 02 2023

...Southwest to the South-Central Rockies...
Days 1-2...

The broad cyclonic flow enveloping much of the CONUS today will
shift to a western U.S. longwave trough as a shortwave amplifies
into a closed low and digs down the CA coast through Monday,
driving height anomalies to -2 standard deviations below the climo
mean at 500mb and 700mb according to NAEFS ensemble tables. The
peak amplitude of this feature is expected to occur late D1,
before it becomes absorbed into the increasing westerlies and
shears into an elongated positively tilted trough moving into the
Four Corners on Tuesday. This evolution will be accompanied by
persistent and intensifying LFQ diffluence as a subtropical jet
streak arcs from near Baja through the Northeast, leaving
impressive deep layer ascent across the region with moisture
advecting onshore from the Pacific on SW flow downstream of the
primary trough axis. While moisture is still progged to be
somewhat limited, the deep layer ascent combined with a stripe of
enhanced fgen as a cold front drops southward will result in a
collocated swath of moderate to heavy snow with snowfall rates
potentially reaching 1"/hr at times as the DGZ deepens and SLRs
become more fluffy with time. Most of the snow should occur along
and ahead of the front, and be of relative short duration due to
the progressive nature of features. However, WPC probabilities for
more than 6 inches are above 50% in parts of the Great Basin and
southern Wasatch on D1, and above 80% in the CO Rockies where
locally more than 12 inches is possible in the higher terrain of
the Flat Tops and Sawatch ranges. By D2, the heaviest snow will
push south/southeast, but WPC probabilities continue to feature a
moderate risk for 6 or more inches across the San Juans and
Mogollon Rim before moisture gets shunted farther southeast by D3
bringing an end to the area snowfall.


...Great Lakes..
Days 1-3...

Periods of moderate to heavy lake effect snow (LES) are likely,
especially in the westerly flow snow belt areas, as channeled
westerly mid-level flow within weak troughing becomes reinforced
by a shortwave which will track across the Great Lakes Monday, and
another moving eastward during Wednesday. This will result in
stronger CAA causing 850mb temps to fall to as low as -22C over
Lake Superior, and around -16C for Lakes Erie and Ontario. The
entire column becomes quite cold, with some of the regional
soundings suggesting the important mixed layer will be all below
the DGZ temperatures. This will likely limit SLR to result in more
modest accumulations. However, lake temperatures are well above
average and nearing record highs for early February according to
GLERL, so steep lapse rates combined with enhanced lake moisture
should still result in periods of heavy snow rates which could
accumulate efficiently at times. WPC probabilities on D1 are high
across the Keweenaw Peninsula and near Whitefish Point for more
than 6 inches of snow, with the highest probabilities for more
than 4 inches shifting to east of Lake Ontario near the Tug Hill
Plateau both D2 and D3.



...Central Texas through the Mid-South...
Days 2-3...

Long duration overrunning precipitation event becoming more likely
across parts of the Southern Plains and Mid-South in response to
uniform SW flow aloft moving atop a stalled cold front draped from
TX into the Southeast. Cold and expansive high pressure behind
this surface front will dig down into the region producing cold
surface temperatures through the period, while at the same time
confluent flow in the mid-level helps to reinforce this high. It
is this same confluent flow, especially that arising from the
southern stream, that will advect more significant moisture
northward into the region, and the NAEFS ensemble tables suggest
periods of IVT exceeding 2 standard deviations above the climo
normal will spread across TX and into the TN VLY, especially by
D3. This significant overrunning moisture will result in waves of
precipitation, especially D2 and D3, likely falling as rain to the
south and freezing rain/sleet to the north. The guidance has
trended a bit colder today which has increased the freezing rain
and sleet threat, but there remain considerable difference among
the various deterministic and ensemble clusters. Still, the signal
is increasing for a long duration with waves of freezing rain
spreading from parts of West Texas through the Red River Valley
and northeast into the Ozarks and Tennessee Valley. On D2, WPC
probabilities for more than 0.25" are generally 5-20%, highest
across eastern AR, before shifting westward and increasing with
the next wave on D3, reaching 20-40% for 0.25" for parts of West
Texas near San Angelo and west of Dallas. Regardless of the
exactly evolution, it is becoming more likely that significant
impacts to travel, and potentially infrastructure, will occur due
to ice accretion through the middle of the upcoming week.


Weiss