Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 PM EST Fri Feb 16 2024

Valid 00Z Sat Feb 17 2024 - 00Z Tue Feb 20 2024

...North-Central Rockies...
Day 1/2...

WV satellite depiction shows a progressive shortwave trough diving
southeast through WY with light precip focused in-of the northern
Rockies and Inter-Mountain West. Heavier snowfall will develop
across the central Rockies overnight with the latest D1 PWPF
around 40-60% with a max of 80% for at least 6" mainly situated
within the higher peaks in north-central CO. 


...Great Lakes...
Days 1-3...

No major changes to the overall synoptic and mesoscale evolution
of the forecast Lake Effect Snow across the Great Lakes. Lake
Effect Snow (LES) on WNWly flow into the U.P. continues today in
conjunction with height falls out of Ontario. This flow and
associated LES expands over the eastern Great Lakes tonight in the
wake of low pressure shifting east over the Mid-Atlantic and
continues until turning Wly Saturday night. Single band LES into
the Tug Hill is likely on Lake Ontario Sunday where Day 3 probs
are 40-60% for >4".


...Mid-MS Valley through Central Appalachians/Mid-Atlantic...
Day 1...

A fairly energetic mid-level shortwave will continue advancing
eastward out of the Mid-Mississippi Valley this afternoon, exiting
through the OH Valley and Mid-Atlantic by the end of the period.
Current reports of heavy precip and elevated instability
triggering thunder across IL indicates a more prominent
disturbance with favorable mid and upper level dynamics at play as
it advances into the east-central CONUS. Current UA analysis
indicates a strong upper jet of 180kts with expected strengthening
to ~200kts by the time it crosses the Appalachian front. This will
allow for areas within the LER of the jet streak to see primed
ascent with a swath of moderate to heavy precip falling within a
cold sector in wake of a cold front that passed through the past
12-24 hrs. Upslope component across the Appalachians within
southwest PA/western MD/WV will allow for favorable ascent pattern
with a stripe of snowfall >6" now up to 50-80% within the latest
PWPF over the course of tonight through tomorrow morning. Further
to the east, recent guidance has become more bullish with the
prospects of robust 7H and 85H frontogen situated over the Central
Mid Atlantic with a dual-banding structural component likely to
materialize within the forcing components. Latest ensemble means
and probabilistic data have increased to favor widespread
prospects of high-end Advisory level snowfall with low-end Warning
criteria now favored within the corridor of south-central PA to
just north of Rt 50. Despite the quick forward propagation of the
system in question, favorable dynamical output will induce a spell
of heavy precip within a favorable environment for enhanced SLRs
generally focused between 10-15:1. SLRs will be a touch lower
within the southern end of the precip field over NoVA to southern
DE, and higher over the terrain of MD/PA/WV and across southern PA
within the best 7H frontogen. PWPF of >4" has bumped up to 50-70%
east of the Blue Ridge with 20-40% now favored for >6", a
significant increase from previous forecast. Further north,
mid-level frontogen will shift eastward with favored impacts for
southeast PA and central NJ, including the PHI metro. A general
4-6" is expected within that zone with less further north into
NYC.


...West Coast...
Days 2/3...

The Omega high breaks down/drifts inland over western Canada
tonight through Saturday as the next Pacific trough approaches the
Pacific Northwest and directs an Atmospheric River into
California. Strong southerly flow will raise snow levels to around
6000ft as precip moves inland Saturday afternoon/night before
diminishing into the ridge with a weakening mid-level shortwave
carrying light snow into the northern Great Basin into Idaho,
western Wyoming and northern Utah. The next low then approaches
the Oregon/California border on Sunday. Day 2 PWPF for >6" remain
40-80% for the Sierra Nevada, Shasta/Siskiyou, and higher OR
Cascades with Day 3 similar for the California ranges as well as
30-40% probs for selected ranges of ID into WY.

Resident cold air over the Columbia Basin/Gorge does allow for
some risk for light freezing rain in the surrounding Cascades and
perhaps the northern Oregon Coast Ranges Saturday/Saturday night
where Day 2 PWPF for >0.1" are 10-20%.


Jackson/Kleebauer