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

Valid 12Z Fri Jan 19 2024 - 12Z Mon Jan 22 2024

...Great Lake Effect Snow...
Days 1-2...

Through Saturday night/Sunday, the broad cyclonic flow will remain
in place over the Great Lakes region with one strong shortwave
trough moving through early this morning which then will be
followed by another shortwave trough quickly behind it coming
through between later today and Saturday. This will usher in a
very cold air mass over the region, characterized by 850 mb temps
of -20 to -25C over the western Lakes, moderating some to near
-20C by Saturday over the eastern Lakes. This sufficiently cold
air over the relatively warmer waters will support intense lake
effect snow bands to develop for the favored areas, especially
downwind of Lake Superior and Lake Michigan where the latest WPC
snow probabilities show above 50 percent probabilities for at
least 6 inches and in the localized favored area of northwest
Indiana, the WPC snow probabilities for 12 inches is above 60
percent. The lake effect snow does begin to wind down during Day 2
(Sat-Sat night) as the deep cyclonic flow begins to move east and
more upper ridging builds in but a few inches to locally 4"+ will
be possible, especially off Lake Erie in northwest PA and
southwest NY state, supported by the moderate (40-50%) WPC
probabilities. 


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

The West will remain with an active weather pattern through the
next few days a broad upper trough currently well offshore slowly
moves inland and opens up across much of the Western U.S. by late
Saturday into Sunday. A series of low pressure systems will
approach/brush the northern areas from northern CA into the
Pacific Northwest while a very strong 250 mb jet streak (150+ kt)
sets up to put much of California in the favored left exit region.
Overall, the large scale forcing overlapping with modest amounts
of moisture spilling eastward will support widespread
precipitation not only along the coastal regions but further
inland into the Intermountain West, especially by Day 2 and Day 3.

For today/tonight, precipitation this morning will continue to be
mixed with some threat of lower elevation ice accumulation but
additional amounts should be relatively light/minor. Precipitation
then begins to move into the Sierra Nevada late in the period
(06-12Z Saturday) and should support several inches of wet snow
for the higher elevations where the WPC snow probabilities for 6
inches are slight to moderate (30-50%).

By Day 2 and Day 3, the strengthening upper jet and greater height
falls and forcing will support heavier and more widespread
precipitation across the entire region. Lower snow levels as well
will bring heavy snow to much of the Sierra Nevada as well as the
northern California mountains where several inches to locally a
foot or more will be possible. The WPC snow probabilities for at
least 12 inches are moderate (40-60%) while further east into the
Intermountain West, amounts should be relatively light and
generally under 4 inches. By Day 3, the active pattern remains in
place with additional heavy snowfall likely for the Sierra Nevada
and may total another 1-2' for the highest peaks based on the
latest WPC snow probabilities (12" probs are 50-70%). The 2-day
totals will be impressive for the Sierra Nevada with the higher
peaks likely to see 2-4 feet of accumulation.

Residual cold air seeping into the Columbia River Gorge/Basin will
keep the threat of ice accumulation through the weekend with the
latest WPC ice probabilities showing chances of at least 0.10"
reaching moderate levels (30-40%) into Day 2.


...Ohio Valley through Southern New England...
Days 1-2...

Early morning analysis showed a pair of shortwaves in the region,
one sliding eastward across Kentucky while upstream another wave
was organizing in northern Minnesota. This lead wave over Kentucky
is expected to intensify and deepen as a very impressive 150+ kt
jet streak develops over the Southeast and Mid-MS Valley,
positioning the Mid-Atlantic region in the very diffluent aloft
zone. Meanwhile, at the surface, low pressure organizing off the
North Carolina coast will track northeast and deepen quickly as it
moves toward New England. The combination of the upper shortwave
moving overhead and a modest amount of moisture wrapping into the
low pressure will squeeze out greater amounts of QPF from the Ohio
Valley toward the Mid-Atlantic. An interaction with the lingering
inverted trough over MD/PA/NJ will locally enhance QPF amounts and
help snowfall linger into the evening hours. Overall, the trend
this model cycle was for an uptick in QPF and snow amounts,
especially northern MD into southeast PA. Snow-to-liquid ratios
will also play a factor in this system, as the airmass is
relatively cold in place and should support SLRs slightly higher
than climatology, generally 12-15:1. A broad area of 1-3" is
expected while localized higher amounts between 3-5" will be
possible for far northern MD into eastern PA and central NJ tied
to longer duration forcing associated with the inverted trough
where WPC snow probabilities for at least 4 inches are 40-50
percent. The greatest snowfall accumulations are expected for the
upslope region in the Central Appalachians where early this
morning there should be light/moderate snowfall but then in the
wake of the shortwave passing, the much colder air and strong
west/northwest flow will lead to a longer duration upslope snow
event where total amounts through the weekend could reach 8-12"
(probabilities for 8 inches are locally 50-70 percent). Finally,
the strong upslope snow will even be found into the higher
elevations of the NC Blue Ridge where 4 inches probabilities are
between 30-50 percent.


Taylor