Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Thu Jan 19 2023

Valid 00Z Fri Jan 20 2023 - 00Z Mon Jan 23 2023

...Upper Midwest, Northern Great Lakes, and Northeast...
Days 1-2...

An expansive winter storm stretching from the Upper Midwest to the
Northeast will produce a variety of impactful winter precipitation
types through Friday, with typical trailing lake-effect snow
following behind the storm through Friday night.

A large vertically-stacked low will be centered over lower
Michigan to start off the period this evening. To the west,
plentiful cold air is in place for all snow across the upper Great
Lakes. However, with little in the way of environmental moisture
in place, as evidenced that most of the reflectivity on radar
appears weak, the primary areas for heavier snow will be downwind
(south) of the Lakes. As with many vertically stacked lows, the
surface low has fully occluded, with most of the significant
forcing having shifted east into the Northeast. The forcing having
shifted east is largely due to the fact that the Northeast will be
in the favorable left exit region of a strong 150 kt jet streak,
which stretches from southwest Texas along a more or less straight
line to the VA/NC border. Meanwhile the aforementioned Midwest low
is fully under the 500 mb low. Thus, by around midnight tonight,
the Midwest low will be transferring its energy to a developing
coastal low in the Gulf of Maine. Thus, the period where the Lakes
will be the primary forcing for snow downwind of them will be
limited, only lasting into Friday night, and ending from west to
east. By Friday morning, the coastal low will be the stronger of
the two, as the center of the Midwest low moves into upstate New
York. All snow over Michigan will at least be lake-enhanced by
this point, if not pure lake-effect. In contrast, the coastal low
by Friday morning, having the advantages of much better forcing
and plentiful Atlantic moisture, will develop an area of heavy
snow with potential 1-2 inch/hour snowfall rates possible.
Further, unlike across the Great Lakes, the heavy snow from
northern coastal MA through ME will have much more marginally cold
air to work with, such that dynamic cooling and wet-bulbing may be
the deciding factor that knocks the precipitation-type over to
snow with coastal temperatures at the surface ranging between
30-32.  

By Friday evening, the inland surface low will have dissipated
into a trough, with mostly lake-effect left over the lower Great
Lakes. There should still be some instability across interior New
England along with lingering atmospheric moisture for snow
showers. Meanwhile the coastal low center will be well east of
Maine, so the comma-head region snow will be diminishing. Finally
the pure lake-effect and lake-enhanced upslope snow in the
Appalachians will die out late Friday night as a surface high
moves into the Midwest.


...Central High Plains...
Days 2-3...

The winter side of the next series of storms becomes significant
as a deep positively tilted trough emerges out of the Rockies from
the northeast corner of NM through southwest KS. An upper level
low, which itself will be weakening Saturday will drive a strong
shortwave trough out and ahead of it Friday night. Lift associated
with this strong negative vorticity advection is expected to
overcome the typically unfavorable left entrance region of the jet
south of this area, as it begins to work with some Gulf moisture
being transported northward on a 30-40 kt 850 mb jet. The meeting
of this moisture with a cold air mass in place over the
aforementioned area and strong baroclinicity throughout this
region will allow heavy snow to break out in this region starting
Friday evening. It will move east overnight into Saturday,
impacting much of KS and even into northern MO before diminishing
Sat Night into Sun. The heaviest snow is expected from far
southeast CO across portions of western KS Friday night through
Saturday morning, slowly weakening thereafter resulting in less
snow further east. By Saturday evening, the upper low will have
opened into a trough, and the primary piece of energy will begin
poorly phasing with a separate disturbance diving south out of the
Dakotas. The result will be a swath of much lighter snow from
northern MO Sat evening into northern lower MI by Sun morning. The
second disturbance diving south into the MS Valley will invigorate
the atmosphere for strong storms in the Southeast Sat Night, which
will eventually use up most of the forcing, leaving little for the
winter side of the storm. Snow ratios in this area are expected to
range between 8-13:1. PWPF peak values range from a 10-20% chance
of a foot of snow across portions of southeast CO, 40-70% chance
of at least 6 inches of snow in that same area, with a 30-50%
chance of at least 6 inches of snow for portions of western KS,
and finally over 80% chance of 4 inches of snow for southeast CO
and a 50-70% chance of at least 4 inches of snow in western KS.
Meanwhile, from eastern KS through MI there's a 50% chance of 1
inch of snow from this storm, showing its forecast for weakening
with time.

...Pacific Northwest...
Days 2-3...

A deep and highly energetic trough digging southeastward out of
the Pacific and into the intermountain West will be the primary
forcing for heavy snow into the terrain of WA, ID and MT Saturday
into Sunday. The trough remains somewhat negatively tilted
throughout the weekend. Associated moisture will be lifted against
the terrain, resulting in high forecast snowfall totals from the
Washington Cascades, the Idaho side of the Bitterroots, the
Flathead and Swan Ranges in northwest MT, and the Little Belt
Mountains in central MT. Expect around a foot of snow in western
WA, with 8-12 inches of snow expected for the other three
aforementioned ranges.

-----------------

Key Messages for the Northeast Winter Storm:

-- A winter storm will produce heavy snow and some freezing rain
across parts of the Northeast through Friday.

-- Heavy snow rates of up to 1 inch per hour will create dangerous
travel tonight into Friday morning across portions of New England,
with lighter periods of snow continuing through much of Friday.
More than 6 inches of snow are expected from Vermont through
southern Maine.

-- Freezing rain will likely produce ice accumulations of up to
1/10 inch, especially in the Catskills, Berkshires and southern
Greens, creating slippery roads and hazardous travel.


Wegman