LWX has a great and awesomesauce disco this afternoon... hats off to them. Well written as well
.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
...SLIGHT TO ENHANCED WINTER STORM THREAT AS MAJOR STORM LURKS
OFFSHORE...
Overview...
The evolving large scale weather/jet stream pattern across the
U.S. as well as latest model guidance continue to suggest the
potential of a major, rapidly deepening area of low pressure
moving northward off the East Coast Friday into Saturday.
Although the brunt of the storm looks to be headed for New
England, the Mid-Atlantic region will be on the western
periphery of the rather large system resulting in an increasing
likelihood of accumulating snow. Uncertainty remains in the all
important details, and users are encouraged to focus on the
broader "low-end" and "high-end" potential snowfall amounts at
this time range, with the caveat that the situation is still
evolving, the storm has yet to form, and continued variability
is likely in the "expected" snowfall amounts as finer scale
details come into clearer focus over the next couple of days.
For the latest forecast details, please visit:
www.weather.gov/lwx/winter.
Technical summary...
A mid-level jet max and weak isentropic lift well ahead of the
large scale trough will impinge on the Allegheny Front beginning
as early as shortly after sundown Thursday. At least scattered
snow shower activity is anticipated as a result Thursday night
over favored upslope areas, with a couple of inches possible
mainly along western facing ridges through daybreak Friday.
This axis of lift will slip slowly southward and eastward during
the day Friday while the upper trough sharpens across the TN/OH
Valley. The trough will have 3 embedded shortwaves within it: a
shortwave pivoting from SE MO to E KY, a southern stream
vorticity ribbon arcing back through the ArkLaMiss to TX, and a
trailing northern stream shortwave diving into the upper Great
Lakes. It appears the trailing vorticity ribbon will become
ingested in the main shortwave trough, phasing sufficiently to
result in strong surface cyclogenesis in a PVA regime ahead of
the trough as it interacts with a baroclinic zone near the
Bahamas. Meanwhile, the trailing northern stream shortwave
likely also becomes infused into the large trough, aided in part
by very transient/weak blocking from the Arctic high retreating
off through the Canadian Maritimes. As all of these pieces
conjoin, rapidly deepening surface low pressure will track
northward off the Eastern Seaboard. The question is exactly how
far offshore the low tracks. A consensus of the latest
model/ensemble data as well as the background UA pattern suggest
a low track from about 150 miles east of the Outer Banks of NC
to just outside (SE of) the 40 N/70 W benchmark off the New
England coast.
With the lack of a more substantial downstream block over the
North Atlantic, the low is more likely to track far enough
offshore and deepen far enough to the east and north that the
heaviest band of snow would setup toward the NJ/New England
coast, with the northwestern edge setting up over southern MD
and the Eastern Shore. The infamous gradient between lighter and
heavier snowfall may end up near the I-95 corridor, and thus
bears a very close watch.
Otherwise, most of the accumulating snowfall for the majority of
the area will come from the upper trough axis itself. Snow may
mix with rain at the onset for lower elevations east of I-81 and
south of I-70 Friday afternoon due to boundary layer
temperatures progged into the mid to upper 30s. Upslope
enhancement over the higher terrain will likely result in
locally higher amounts over the Alleghenies. One thing to note,
despite an overall relatively lighter expected snowfall across
the majority of the area, the overall pattern somewhat resembles
that of a PRE (Predecessor Rain Event), where moisture from an
intense, distant (offshore) low interacts with a mid- level
frontal zone. This may result in a band of enhanced snowfall as
hinted at in some runs of higher resolution guidance, but
certainty on its magnitude and placement is low at this time.
Of greater certainty is increasing winds and plunging
temperatures as the low lifts out toward the Northeast heading
into Saturday. Dangerously cold wind chills are likely by
Saturday evening, and blowing snow is possible as winds
increase. Snow may linger in the I-95 corridor through Saturday
morning depending on the evolution of the offshore low, but dry
conditions should return areawide east of the Allegheny Front
by nightfall.
