Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
247 PM EST Fri Jan 19 2024

Valid 00Z Sat Jan 20 2024 - 00Z Tue Jan 23 2024

...Great Lakes into Central Appalachians..
Day 1...

Expansive cyclonic flow enveloping the eastern third of the
country will amplify tonight into Saturday as a potent shortwave
sharpens into a closed mid-level low and then drops southeast from
the Great Lakes through the Mid-Atlantic states and then offshore
New England by Saturday night. A surface trough beneath this
feature will progress southward across the Great Lakes, enhancing
CAA on N/NW winds in its wake. 850mb temperatures are progged to
fall to around -15C to as low as -25C, coldest north, leading to
steep lapse rates over the still above-normal lake temperatures,
resulting in SBCAPE of 500-750 J/kg. This will promote intense LES
south of the lakes, with the most impressive rates likely along
the south shore of Lake Michigan into NW IN due to the prolonged
fetch along this lake to promote a steady single band of LES.
Snowfall rates in any of the LES will likely reach 1-2"/hr, with
locally higher possible in NW IN, especially the first half of D1.
During the latter part of D1 and especially by D2, shortwave
ridging blossoms into the area bringing an end to the LES. WPC
probabilities for more than 6 inches are as high as 50-70% along
the southern and eastern shore of Lake Michigan, with locally 10+"
possible.

This evolution will also support a continuation of intense upslope
flow to enhance ascent and produce heavy snow on the upwind side
of the central Appalachians. This will be most impressive along
the Alleghenies where at least some modest connection from the
Great Lakes will provide additional moisture to be wrung out as
snow in the terrain, which is reflected by WPC probabilities for
more than 6" of additional snow reaching 30-40% in the WV terrain.


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

A closed mid-level low will lift northeast to approach the CA/OR
coast this evening, before opening into a trough as it moves
onshore across WA state and lifts into Canada. While the most
intense height falls will weaken with time into D2, this evolution
will leave a broad trough enveloping the Pacific coast and
stretching as far inland as the Four Corners/Central Rockies on
Sunday. During Monday, the flow splits across the West with a
pronounced trough beginning to amplify near the Four
Corners/Desert Southwest, while ridging intensifies over the
Pacific Northwest. Despite this split flow, the combination of
zonally oriented 700-500mb flow across the Pacific and vast areas
of mid-level divergence into the West will support waves of
precipitation continuing each day. However, much of this is likely
to be rain outside of the higher terrain due to snow levels
climbing slowly through the period, reaching 6000-8000 ft by D3.

The heaviest snow this period is likely in the Sierra where 3
consecutive days of onshore flow with enhanced ascent through
upslope will contribute to heavy snow, especially above 7000 ft.
WPC probabilities for more than 6 inches are consistently above
50-70% each of the 3 days of the forecast period for the Sierra
and portions of the Shasta/Trinity/Siskiyou region, with 3-day
snowfall reaching several feet in the higher terrain of these
ranges. Additional moderate snow accumulations are likely across
many of the mountain ranges D2-D3, including the Cascades,
Wasatch, Tetons, and San Juans.

Additionally, another round of significant freezing rain is
possible D2 across the southern Columbia Basin and Columbia Gorge
as precipitation overruns the still entrenched cold air at the
surface. As precip lifts northward Saturday night and Sunday, the
WAA aloft will enhance precipitation and surge temps above
freezing between 900-800mb. However, the Canadian high to the east
will continue to drive cold easterly flow into the Basin and
Gorge, resulting in an axis of freezing rain across areas that
have received heavy icing this past week. Any additional icing
will only enhance impacts, and WPC probabilities for 0.1" or more
of ice are 30-50%, highest along the Gorge.


...Southern Plains into the Mid-Mississippi Valley...
Day 3...

An amplifying mid-level trough over the Four Corners will produce
downstream divergence into the Southern Plains on D3, with
impressive SW 700-500mb flow directing warm and moist advection
from the Pacific into the region. Moisture will steadily increase
through Monday as SW flow aloft persists and interacts with
increasing southerly and confluent flow at 850mb out of the Gulf
of Mexico. The resultant isentropic ascent will merge with the
flow aloft to push PW anomalies to as high as +3 sigma according
to NAEFS. This pool of moisture will be acted upon by increasingly
robust ascent through height falls/divergence and the RRQ of a
poleward arcing jet streak progged to sharpen over the Central
Plains Monday. Surface wet-bulb temperatures will initially be
sub-freezing as far south as the Red River Valley of the South,
thanks to expansive high pressure centered over the Mid-Atlantic
states. As precipitation expands and overspreads the area Sunday
night, this will be accompanied by rapid warming aloft on the
intensifying WAA, and 850mb temps are progged to reach as high as
+5C to +10C. However, surface wet bulb temps will be slow to
climb, and although it is likely many areas south of Missouri will
change to rain by the end of the period (last in the higher
terrain of AR), precip will feature a prolonged period of freezing
rain leading to significant ice accumulations. Uncertainty is
still high due to model differences in timing the erosion of this
cold air, but current WPC probabilities for more than 0.1" of ice
are above 40% in a swath from the Red River Valley northeast
towards St. Louis, MO, with a 20-40% chance of more than 0.25" for
parts of the Ozarks.

Weiss