Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 PM EST Tue Feb 13 2024

Valid 00Z Wed Feb 14 2024 - 00Z Sat Feb 17 2024

...Northwest and northern California to the northern Rockies...
Days 1-3...

An omega blocking upper level pattern over the northeast Pacific
will become a rex block in an unusual way, as the lows that make
up either end of the rex block merge. The low on the eastern side
of the omega block will retrograde southwestward, while the low on
the west side of the omega block is driven northeastward. A cold
air mass is already in place across the Pacific Northwest, so
these 2 areas of energy merging just off the Washington and Oregon
coasts will have plenty of cold air to work with as their
associated moisture drives into the coast. The result will be a
prolonged period of snow starting as soon as late tonight into the
northern California and southern Oregon mountains, then spreading
east across all of the Pacific Northwest and intermountain west
through Friday. The surface low out over the Pacific will move
into southwest Washington Thursday, but the upper level energy
will persist into Friday as the resulting rex block low over
Washington on Thursday shears apart by Friday.  Snow totals from
the Cascades through Idaho, Montana, and Wyoming around
Yellowstone will easily surpass a foot by the time the event is
over, with the long-duration nature of the snowfall proving the
factor most responsible for the highest snowfall totals. There are
high chances (around 70%) for over 6 inches of snow through those
areas. An advancing polar high moving into the high Plains of
Montana along with steady weakening over the upper level energy by
Friday should mark the end of the most significant snowfall.


...Central Rockies & central Great Plains...
Days 2-3...

Mid-level shortwave moving through the Great Basin early Thu will
continue eastward into the CO Rockies Thu afternoon and into the
central Plains by early Fri. Modest jet will help drag in slightly
above normal moisture (PW anomalies up to +1 sigma) across the
Great Basin to the Rockies where westerly flow will help maximize
snow into the western CO through Friday. There, WPC probabilities
for at least six inches of snow are highest in the Park Range.
East of the Rockies, combination of a front over the western High
Plains and divergence aloft at the entrance region of the northern
jet will promote an expanding area of snow across eastern
WY/western SD into NE and eventually the Corn Belt beneath a some
potentially potent FGEN. However, models/ensembles show a fairly
large amount of uncertainty in the resultant QPF and southward
extend of colder air, but probabilities of at least 4 inches of
snow are still moderate (40-70%) from around the Black Hills ESE
to west central IA. Will refine the forecast going forward, which
will likely yield higher totals and more focused probabilities
given that 90th percentile values are around closer to 10 inches.


...Central Great Plains, Great Lakes, & Interior Northeast...
Days 2-3...

Within the broad west-to-east flow over the CONUS late Wed, a
mid-level shortwave over the central Plains and incoming shortwave
moving southeastward out of southern Manitoba will merge over the
Great Lakes on Thursday as the attendant areas of surface low
pressure consolidate into western NY that evening. Narrow axis of
moisture will precede the cold front moving through the
Midwest/Mid-South though the 0.50" line may only make it as far
north as about the rain/snow line somewhere along I-80 or so.
Height falls and pre-frontal convergence tied to the northern vort
will promote a somewhat narrow axis of snow from southern MN
eastward into Wisconsin before the southern vort center
invigorates the southern low. There, WPC probabilities for at
least 4 inches of snow are rather low (<30%) with 1-3" or so more
likely.

Into Lower Michigan, WAA ahead of the warm front and sharpening
height falls will help expand snowfall eastward as the low deepens
into western NY. WPC probabilities for at least 4 inches of snow
are higher into central/northern Lower Michigan -- generally
30-60%. Low pressure over western NY Thu evening will quickly move
eastward and continue to deepen as the upper jet strengthens to
the south (axis of >150kts from the Ozarks to Bermuda as the
northern and southern branches converge). Despite the quick
movement of the system, favorable profiles of the DGZ could
coincide with modest omega for a quick-hitting moderate snowfall
over northern NY into western New England per the current
consensus as the brief negative tilt aloft helps to slip the
system into the Gulf of Maine with a weakness trying to linger
into the Adirondacks. For now, WPC probabilities for at least 4
inches of snow are highest (>70%) over much of northern NY (Tug
Hill/Adirondacks/North Country) into the Green Mountains. A
broader footprint of at least 10% probs exist from near the NY/PA
border into central New England but leaving out much of
central/northern Maine. Some lake effect snow will follow behind
the system into Friday as a Canadian cold front moves through.


Fracasso/Wegman