Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 PM EST Tue Jan 21 2020

Valid 00Z Wed Jan 22 2020 - 00Z Sat Jan 25 2020

...Pacific Northwest...

Days 1-3...
An active weather pattern continues across the Pacific Northwest
into or through next week. A post frontal onshore flow under an
upper trough keeps snow levels low (2000 to 3000ft) tonight before
rapidly rising Wednesday to the 6000 to 7000ft range as an
atmospheric river with much above normal Pacific moisture comes
ashore into WA through Thursday. Snow levels then drop to 4000 to
5000ft as precip decreases Friday below the trough that follows
the atmospheric river. Day 1 snow probs for 12 or more inches are
the highest on Day 1 with the lower snow levels and initial wave
of moisture with the atmospheric river and are over the OR/WA
Cascades and Olympics. Day 2/3 snow probs for 8 or more inches are
confined to the WA Cascades with higher probabilities on Day 2
given greater overall precipitation expected.


...Southern Plains and Midwest...

Days 1-3...
A mid-level trough shifts east from CO tonight with the initial
surface low shifting south to the TX Panhandle and a plume of
moisture ahead of this trough and behind the sprawling high over
the eastern CONUS allowing precip to break out over the southern
and central Plains this evening. Dry air must first be overcome,
but a wintry mix (freezing rain, sleet, snow) can be expected for
the first few hours north from northern KS, then as dynamical
cooling takes place, accumulating snow develops in the inner core
of the band and spreads south over the rest of KS and northeast
across southeast NE into western IA before shifting northeast on
Wednesday. Day 1 probs for 4 inches of snow or more are 10 percent
or less for northeast KS/southeast NE into western IA and Day 1
probs for a tenth of an inch of ice or more are limited to the
OK/AR border into southern MO with less than a tenth inch of ice
possible in the wintry mix area of KS/NE. The freezing rain threat
spreads northeast into southern MO Wednesday night with the Day
1.5 probabilities highest including 20 to 30 percent risk for a
quarter inch of ice in the AR Ozarks.

A secondary shortwave trough digs south along the upper trough
from the northern to central Plains Wednesday night, closing into
an upper low over KS/MO/OK/AR Thursday before drifting east and
occluding Friday. This reinvigorates snow centered over norther MO
Thursday/Thursday night before shifting east Friday. Therefore,
light to moderate snow is expected over the east-central Plains
(MO/IA) Wednesday into Friday. While the risk for 4 or more inches
remains low in any given 24hr period over the next three days, the
72hr risk for 6 or more inches is moderate over southwest IA and
northern MO.


...Mid-Atlantic...

Day 3...
A cold air damming wedge develops Thursday as high pressure over
the area Wednesday shifts offshore. The leading wave of precip
from the occluding low over the Midwest crosses the central
Appalachians Friday with a risk for light freezing rain to begin
in the CAD area late Friday. This setup favors climatologically
prone areas from the valleys of southwest VA up the Allegheny
Plateau in WV/VA/MD and into the Laurels of south-central PA and
may continue into the weekend until the system shifts east over
the area.


Jackson