Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Wed Feb 08 2023

Valid 00Z Thu Feb 09 2023 - 00Z Sun Feb 12 2023

...Northern to Central Rockies...
Day 1...

A strong arctic cold front, associated with an amplifying
shortwave diving southeast over the northern Rockies, will move
through the central Rockies tonight. While widespread heavy snow
accumulations are not expected, sufficient low level moisture,
instability, and strong winds will continue to raise the threat
for brief but intense snowfall and reduced visibility. See the key
messages below for additional information.

...Mid Mississippi Valley to the Upper Midwest and Northeast...
Days 1-2...

A vigorous and increasingly negatively tilted shortwave trough
will lift from northern Texas this evening to lower Michigan by
Thursday evening, then quickly weaken as it passes west of Maine
Thursday night. The shortwave will support the development of a
surface low that will take the same track, following the right
entrance region of a jet that itself will be rapidly lifting
north. As such, significant upper level divergence will take the
moisture currently over AR/MO and surrounding states and move it
northeast as well. The key missing ingredient with this system
will be cold air, at least at first, as the parent longwave
positively-tilted trough only slowly drifts eastward across the
northern Plains. This longwave trough would normally supply the
cold air, which will be present further west across the Plains,
but will really struggle to move east.

Meanwhile, as the surface low deepens, the Gulf moisture wrapping
around the low will develop into a semi-classic comma shaped low.
There's good agreement that in the comma-head region north and
northwest of the low-center, banded precipitation will develop.
How strong the bands get remains uncertain (though there's rarely
certainty to that level much prior to the development of the bands
themselves). Assuming some banding develops, the combination of
some weak cold advection associated with the aforementioned trough
and dynamic cooling due to heavy precipitation will allow snow
levels to diabatically drop to the surface, resulting in
accumulating snow. For the most part, the forecast snow amounts
are little changed from the inherited forecast. However, there has
been some shifting south of where the heaviest snow totals are
most likely. There is a narrow (~50 miles wide) swath of 6-8"
storm total snow that extends from extreme northeastern Iowa
northeast across southern Wisconsin to around Green Bay, with
continued snow totals to that level across portions of lower
Michigan. These values straddle the thresholds for advisories vs.
warnings, so there remains uncertainty there.

Another area of uncertainty is how far north into the comma-head
region the warm air is able to intrude. This will largely depend
on how strong the bands get, as stronger bands will support
dynamic cooling, whereas weaker bands would allow for nearly all
rain south of Iowa. Once again, there is some uncertainty here,
but most of the high-res guidance suggests stronger bands/more
snow. Regardless, any snow that falls will have to contend with
surface temperatures at or maybe a degree or two above freezing.
This will support low snow-to-liquid ratios, perhaps as low as 6:1
at first from Missouri southward, which will gradually increase as
the low tracks north and taps into increasingly colder air.

The Northeast will be on the warm side of the storm, and as such
mixed precipitation, particularly freezing rain, is a definite
possibility from the Hudson and Mohawk Valleys of New York
northeast into southern Maine. There will be a decent push of very
warm air, which will eventually change all low-elevation areas
south of central Maine over to rain, but how long it takes for the
cold air to be scoured out or warmed up by the warming process of
freezing rain remains very uncertain. While lots of ice
accumulation appears unlikely, it does not take much to cause
major impacts. The snowfall winner with this storm will be
portions of far northern Maine, which will have the benefit of
lots of Atlantic moisture running into air that is cold enough to
remain all snow. Expect snow totals approaching a foot by the time
all is said and done Friday afternoon.

Wegman



...Key Messages for Northern and Central Rockies Snow Squall
threat tonight...

--Snow Squalls in the Central Rockies
A cold front pushing through the central Rockies this evening will
continue to cause snow squalls and scattered snow showers.

--Gusty Winds Also Expected in the Region
Widespread wind gusts in excess of 50 MPH, with gusts as high as
65 MPH, will lead to difficult travel conditions, especially to
high profile vehicles in strong crosswinds.

--Bursts of Heavy Snow and Low Visibility
Snow squalls and snow showers will produce brief bursts of heavy
snow. This will lead to sudden reductions in visibility,
especially in areas of strong wind gusts.

--Expect Rapidly Changing Conditions
Anyone traveling on roads through the central Rockies should be
prepared for rapidly changing conditions, potentially going from
sunny skies to whiteouts in a matter of minutes. Use caution if
traveling.