Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 PM EST Mon Feb 12 2024

Valid 00Z Tue Feb 13 2024 - 00Z Fri Feb 16 2024


...Tennessee-Ohio Valleys to the Northeast...
Day 1-2...
In the southern stream, a compact mid-to-upper level low will move
across the Mid-South this evening before transitioning to a
negatively-tilted open wave as it lifts from the Tennessee and
Ohio valleys into the Mid-Atlantic region overnight.  As the
system moves into the Ohio Valley tonight, there remains a good
signal for dynamic cooling, supporting rain changing to snow in
the associated comm ahead as it lifts across Tennessee into
central to eastern Kentucky tonight.  While widespread heavy
amounts are not expected, accumulating snow is likely with the
potential for a stripe of mesoscale-driven, locally heavy amounts.

As the system lifts to the northeast and a surface low pressure
develops along the Mid-Atlantic coast, a coupled upper jet, along
with low-to-mid level frontogenesis, will contribute to heavy
precipitation, with a tight, heavy rain to snow gradient expected
on the northwest side of the low.  While a swath of heavy snow
extending from portions of the central Appalachians into the
Northeast remains likely, its placement remains far from certain. 
Even with heavy precipitation expected to unfold within the next
12-hours, this remains a low-confidence forecast.  The overall
model trend continued to shift south, bringing heavy snow further
south and out of areas where heavy accumulations once appeared
likely.  While probabilities for heavy snow (greater than 6
inches) remain high across portions of eastern Pennsylvania,
northern New Jersey, the lower Hudson Valley and the NYC metro,
and southern New England (Connecticut, Rhode Island, and southeast
Mass), probabilities have dropped notably further to the north. 

What remains more certain is that will be a fast-moving system,
impacting areas from the central Appalachians to southern New
England beginning overnight.  The heaviest amounts overnight are
expected to fall along the northern West Virginia, far western
Maryland, and southwestern Pennsylvania mountains overnight. 
After daybreak, the focus will likely be over northern New Jersey
to southern New England, but will quickly move east, with snow
exiting most areas by the evening hours.

...Northwest and northern California to the northern Rockies...
Days 1-3...
The Northwest will be caught in the middle of a series of
shortwave troughs that will produce multiple rounds of heavy
snowfall through the middle of the week. The first round of
snowfall occurs today as a weak shortwave trough over Washington
heads east over the Northern Rockies. A stream of 700-500mb
moisture aloft within westerly flow within the 850-300mb layer
allows for some enhanced snowfall rates via upslope flow in
orographically-favored mountain ranges. As that disturbance races
east Tuesday morning, another one diving south from British
Columbia will be accompanied by a frontal boundary inching south
along the northern Rockies.  It is the next approaching Pacific
storm system, courtesy of a negatively-tilted 500 mb trough, that
will direct the next surge of 850-500mb moisture into the Pacific
Northwest on Wednesday, spilling into the Northern Rockies by
Wednesday night. This time around, the surge in moisture will
bring about heavy snow into the Cascade Range, the Olympics, and
as far south as the Trinity/Shasta and northern Sierra Nevada of
northern California.  Areas impacted through the period include
the Cascades, the Trinity/Shasta region, the northern Sierra
Nevada, the central and southern Idaho, northwestern Montana, and
northwestern Wyoming ranges.  The WPC PWPF shows at least locally
high probabilities for a foot or more across these areas through
late Thursday.

Pereira/Mullinax


***Key Messages for Feb. 12-13 Winter Storm***

--Norâ€easter rapidly develops tonight; Uncertainty Lingers
Low pressure over the Tennessee Valley rapidly develops as it
crosses the central Mid-Atlantic tonight and moves south of Long
Island Tuesday. A tight gradient in heavy rain to snow is expected
northwest of this low through the densely populated Northeast
urban corridor with uncertainty persisting in this rain/snow line.
Intense snow bands with rates to 2†per hour remain likely
expected in the core of the snow swath from the central
Appalachians through southern New England.

--Significant Travel Impacts in the Northeast
This fast-moving Norâ€easter is still forecast to produce a swath
of snow exceeding 6†from the central Appalachians to at least
the southern New England coast. Strong winds with gusts to 40 MPH
and the high rates of heavy/wet snow should damage trees, power
lines, and disrupt travel.

--Moderate Coastal Flooding
Moderate coastal flooding is forecast during high tide Tuesday
along the northern Mid-Atlantic oceanic coasts and possibly
portions of the New England coast.