Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Thu Feb 09 2023

Valid 00Z Fri Feb 10 2023 - 00Z Mon Feb 13 2023

...Great Lakes and Northeast...
Day 1...

A surface low tracking across the Chicago-land region continues to
produce a small area of wintry precipitation around the north and
west side of the low from Green Bay southwestward across a swath
of southern Wisconsin. As expected with daytime heating the
rain/snow line has pushed well north of the low, so impacts from
wintry weather are only just beginning to be felt across the
affected area. The low is being supported by a reorienting jet
east of the low, putting it in the left exit region of the 110 kt
southwesterly jet extending from TN to OH. Marginally cold air
remains the primary limiting factor for the low, as even areas
north of the primary precipitation band are above freezing. The
closest consistently subfreezing air is an entire state away on
the western side of MN/IA. Thus, there is little prospect of there
being big changes to the very compact area of winter weather
impacts. This same scenario (of minimal cold air to work with)
will be a repeating theme for much of the country over the next
few days. Probabilities of at least 4" of snow remain near 40%
over a stripe from the IL/IA/WI confluence northeast to halfway
between Madison and Green Bay, with lesser chances extending
northeast through Green Bay and the Door Peninsula.

The low tracks into Canada by 12Z Friday. By this point the low
will be able to drag colder air located north and northwest of
Lake Superior across the lakes. Thus, expect a brief period of
light lake-effect snow over first the upper Lakes, then moving
east across the lower lakes, ending by 12Z Saturday. There may be
isolated storm totals (storm + lake-effect) to 6 inches over
portions of the U.P. of Michigan and the Tug Hill Plateau of
upstate New York. Elsewhere snow totals will be lesser.

On the warm side of the low, little has changed. Expect a rapid
changeover of any frozen precipitation over to plain rain across
New York and New England for the rest of this afternoon through
tonight, with the rain/snow line eventually intruding as far north
as central Maine. The effective advance of Atlantic warm air will
limit any ice accumulations in these areas this evening before the
changeover. Further north into northern Maine, the forecast of all
snow will result in much higher total accumulations than areas
further south, with amounts generally in the 5-10 inch range
expected.

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

Upper trough south of the Gulf of Alaska will carry a cold front
into the Pac NW this evening with a modest surge in moisture on
SSW to S flow. Snow levels will lower from around 4000ft to near
3000ft as the front moves ashore and heights lower. The most
appreciable snow will be in the higher terrain of the Cascades and
Olympics. By D2, incoming trough will split, pinching off a closed
low over NorCal as snow ends over WA. QPF will be on the light
side, and snow will accumulate to only a few inches except for the
northern WA Cascades. Thus, any new atmospheric river causing feet
of snow in the Sierras is not forecast. This is certain to be a
short event. By D3, the closed low will drop southward along the
CA coast, limiting snowfall to the Sierra where only a few inches
are forecast. WPC probabilities for more than 4 inches each day
are low in CA (<40%) but high over the WA Cascades.

...Southern Appalachians...
Day 3...

A large "bowling ball" upper level low will be centered over MS to
start Day 3 Saturday evening. While the upper low will be
dynamic/strong, with multiple vorticity maxima rotating around the
low, the lack of cold air will once again make this system a
non-event for most. The upper low simply will not have much cold
air, nor be able to generate much of its own. As the low tracks
into the Carolinas, it will bring 850 mb air as warm as +10C north
along the coast, while 850 temps on the cold side barely get to
-3C. This will largely keep almost all of the precipitation
associated with the low as rain, save for maybe a couple hours of
frozen/mixed precipitation at the onset before the warmer air
makes it. As usual when mountains are involved, valleys do help to
trap what little cold air there is, which would prolong any icing,
but only briefly as the icing process itself warms the atmosphere.
Thus, the only real winter weather concerns are in the southern
Appalachians. Here, expect the plentiful Gulf/Atlantic moisture to
upslope up the southeastern side of the mountains. This will both
enhance precipitation rates, which will both promote dynamic
cooling and also cool the surrounding atmosphere from lift. Thus,
expect as much as a foot of heavy, wet, concrete-consistency snow
into far western NC. Somewhat lesser amounts of snow will be seen
further north into southwestern VA, but the upslope flow will be
less favorable here. By Sunday evening, the surface low will be
near the Hampton Roads area of VA, but the northeasterly flow over
western VA and NC will still not be advecting any cold air, as it
remains trapped over far northern New England and eastern Canada.
Thus, the most likely scenario for the southern Megalopolis down
to the Piedmont is a period of cold rain. In what would otherwise
be a great track for a big snowfall for these areas, once again
winter remains absent for much of the Mid-Atlantic.

Wegman/Fracasso