Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
230 PM EST Sat Jan 27 2024

Valid 00Z Sun Jan 28 2024 - 00Z Wed Jan 31 2024

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

A robust upper level trough spawning an unusually strong 850mb low
over the Ohio Valley will direct a plume of 850-500mb moisture
northeast into the Mid-Atlantic tonight. The ECMWF ENS shows the
850mb low tonight with heights <10th climatological percentile and
a warm conveyor belt of 850-700mb moisture flux wrapping around
the northwestern flank of the 850mb low. For northern Indiana,
northwest Ohio, and southern Michigan, boundary layer temps remain
very marginal and the DGZ aloft is not particularly deep, making
the DGZ aloft not particularly deep and resulting in lower SLRs.
However, the region happens to be ideally placed beneath a pair of
divergent jet streak regions this evening (right entrance via a
jet streak over southern Ontario, left exit via a jet streak in
the Mid-South) where there is also exceptional 700-300mb averaged
Q-vector convergence. The concern is that intense synoptic-scale
ascent within a conveyor belt of moisture could result in rain
transitioning to a heavy/wet snow band that produces 0.5-1"/hr
snowfall rates tonight. This is a complicated forecast that relies
upon a delicate balance of both marginal boundary layer temps and
strong dynamic cooling aloft that can make for drastic changes in
which areas witness heavy snow and heavy rain. WPC PWPF currently
features <5% odds of snowfall totals >4", but these are low
chances (10-30%) for >1" from northern Indiana to southern
Michigan through Sunday.

...Southern Appalachians...
Day 2...

Strong upslope ascent in wake of the the upper level trough will
contain plenty of 850-700mb moisture flux beginning Sunday
afternoon and continuing into Sunday night. The ECMWF situational
awareness tools showed northwest winds oriented orthogonally to
the Smokeys that were above the 90th climatological percentile,
supporting strong topographically-induced ascent. Snowfall rates
could easily top 1"/hr at elevations >3,000ft in western North
Carolina and far eastern Tennessee. WPC PWPF does depict
moderate-to-high chances (50-80%) in the Great Smokey Mountains
between Sunday afternoon and early Monday morning. The WSSI-P does
sport moderate-to-high chances (50-80%) for Minor Impacts with any
passes likely seeing some hazardous travel. 

...Northeast...
Days 1-2...

Continuing the described synoptic scale pattern mentioned in the
Lower Great Lakes section, farther east, the same duel-jet
structure will move east into the northern Mid-Atlantic tonight
and the 850mb low will track into the Upper Ohio Valley. Boundary
layer temperatures will initially be too mild to support snow
across northern Pennsylvania and the Southern Tier of New York,
but as the column aloft cools and surface-925mb winds shift out of
the ENE, boundary layer temps will cool further and precipitation
will switch over to snow from western New York to Northern Tier of
Pennsylvania early Sunday morning. Precipitation in parts of the
Catskills and Berkshires may initially start out as an icy wintry
mix. This has prompted WPC PWPF to sports low chances (10-30%) for
>0.1" in those areas. As the new coastal low begins to take shape
Sunday afternoon, a brief double-barrel low structure will form as
the primary low over the Upper Ohio Valley weakens (a Miller-B
type evolution). This will prompt the formation of a cold conveyor
belt (CCB) on the northern flank of the 850mb low over over
southern Pennsylvania and foster periods of moderate-to-heavy snow
from the Catskills and Berkshires to the Green Mountains, Monadock
of southern New Hampshire, and the Worcester Hills starting Sunday
afternoon. Given the marginal boundary layer temps, the higher
elevations >1,000ft will have the best odds of seeing heavy snow.
It is also worth mentioning, that as the 500mb trough tracks over
northern VA, 850-700mb winds out of the northwest will accelerate
and promote strong upslope flow into the central Appalachians.
Latest WPC PWPF shows moderate chances (30-60%) for >4" totals in
the windward slopes of eastern West Virginia.

By Sunday evening the coastal low will have fully developed and,
with the added help of high pressure over Quebec, lead to a surge
in colder northeasterly winds over New England. Around 06Z Monday,
most guidance has a ~992mb surface low just north of the Benchmark
(40N/70W), which is a textbook setup for heavy snow in the
Northeast. A band of heavy snow is likely to setup from the
Catskills and Berkshires to the Worcester Hills and Merrimack
Valley Sunday night into Monday morning with snowfall rates of
1-2"/hr expected. Given the relatively lower SLRs and ample
low-mid level moisture, snow will be more heavy/wet in nature,
adding to the potential for added snow load of tree limbs and
power lines. The storm will quickly move east Monday morning and
track south of Nova Scotia, leading to stronger NE-NNE winds over
the Gulf of Maine that could lead to locally heavy snow rates just
inland from the South Shore of eastern Massachusetts and
potentially the Boston metro area. Snow could linger around
through the daytime hours Monday in eastern Massachusetts before
final concluding Monday evening.

For the entire event, WPC 48-hour PWPF shows moderate-to-high
chances (50-70%) for snowfall >8" in the Catskills, Berkshires,
Greens of southern Vermont, and Monadnock of southern New
Hampshire. Similar probabilities are present in west-central New
York just west of the Finger Lakes for snowfall totals >4". Some
parts of the Catskills, Berkshires, and Green Mountains sport low
chances (10-30%) for snowfall amounts >12". For the areas
highlighted, the WSSI-P shows moderate-to-high chances (60-80%)
for Minor Impacts (even including the northern most section of
Pennsylvania along US Route 6). While this is primarily due to the
Snow Amount algorithm, there are some moderate chances (40-60%)
for Minor Impacts due to Snow Load from western New York all the
way to the western Boston suburbs. Expect hazardous travel
conditions in these affected areas through Monday with a low
chance that heavy snow load could cause very localized tree damage
and power outages. Overall, this has the feel of a late season
winter storm focusing the heaviest totals and most noteworthy
impacts across higher terrain and generally well north and west of
the I-95 corridor.



Mullinax