Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 AM EST Sat Jan 20 2024

Valid 12Z Sat Jan 20 2024 - 12Z Tue Jan 23 2024

...Great Lakes...
Day 1...

Broad cyclonic flow will remain in place over the Great Lakes
region and through the Mid-Atlantic/Northeast as another potent
shortwave trough slips southeastward today through this evening. A
strong 1040+ high over the plains and low pressure exiting off New
England will put the region in a favorable northwest to northerly
flow and with 850 mb temperatures remaining quite cold (-15C to
-20C over the western lakes then moderating some eastward), this
will support strong/intense lake effect snow bands at times. The
steepening lapse rates today will promote strong lift and the
latest snow band prototype tool suggests snow rates may reach
1-2"/hr at times, especially downwind of Lake Michigan across far
northwest IN and southwest MI. The WPC snow probabilities for at
least 6 inches remain high in that area as well as far northwest
PA and far southwest NY (off Lake Erie) where additional snowfall
may reach 6-8". By the end the day 1 period and through Sunday,
upper ridging quickly moves overhead and this should shut off the
lake effect regime.

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

A strong, closed mid-level low currently offshore northern
California will approach the Pacific Northwest today as it opens
up to a trough, reaching western Washington late tonight.
Meanwhile, fast zonal flow with an embedded shortwave will move
onshore California, eventually sharpening into a shortwave trough
over the Desert Southwest and Four Corners region Sunday into
Monday. Despite the split flow regime over the West, broad/large
areas of mid-level divergence and zonally oriented flow moving
over the region will support areas of precipitation each period.
However, with climbing snow levels each day, the bulk of the heavy
snow will be tied to the highest elevations, reaching above 7000
ft by late Sunday into Monday.

The heaviest snow for the forecast period will be across the
Sierra where consecutive days of onshore flow enhanced through
orthogonal upslope will bring heavy snow above 7000 ft. WPC snow
probabilities for at least 12 inches are high and the multi-day
period through Monday is likely to bring several feet of snow to
the peaks. Additional moderate snow accumulations are likely
across many of the mountain ranges Sunday into Monday, including
the Cascades, Wasatch, Tetons, and San Juans.

Additionally, another round of freezing rain is expected late
tonight through Sunday morning across the southern Columbia Basin
and Columbia Gorge as precipitation overruns the still entrenched
cold air at the surface. As precip lifts northward late tonight
into 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 between 50-60 percent, highest along
the Gorge.


...Southern Plains into the Mid-Mississippi Valley...
Days 2-3...

As the strong Arctic high pressure begins to move east of the
region, a southern stream piece of shortwave energy will begin
moving from the Four Corners region toward the Southern Plains by
late Sunday into Monday. Strong height falls and upper divergence
will create large scale forcing for ascent while in the lower to
mid levels, a persistent southwesterly to southerly flow will
advect a warmer and more moist airmass from the Texas Gulf Coast
northward into the ArkLaTex and Ozarks region. Precipitation is
expected to develop late Sunday evening through Monday morning
when boundary layer temperatures slowly recovering from the Arctic
airmass will be near the critical freezing mark. 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 this should
support a 6-hr to perhaps 12-hr period of freezing rain. As warm
air advection takes over, surface temperatures will rise, ending
the freezing rain threat southwest to northeast during the day 3
period. Overall, the latest guidance remains on track for ice
accumulations, some potentially approaching significant levels, to
affect central/eastern Oklahoma, Arkansas, southern/central
Missouri. The latest WPC ice probabilities are at least moderate
for 0.10" of ice accumulation across central/eastern OK through
the St. Louis metro area while are highest across northwest AR
into southwest/south-central Missouri where the probabilities for
at least 0.25" are up to 40-50 percent.


Taylor