Great write-up this morning by LWX:
.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/...
The upper-level trough and its associated strong cold front will
approach the Appalachians through this evening. The southern
stream system will begin to phase with the northern stream system,
causing the trough to become more neutrally tilted while surface
low pressure develops and strengthens along the front. Much of
this evening may turn out dry, but rain will quickly
overspread late tonight into early Saturday morning in
response the cold front and its associated low pressure
approaching the area.
The low will rapidly intensify as it passes through the area
Saturday morning along with the strong cold front. A strong
northwest wind will develop behind this system, bringing in much
colder air. Temperatures in the 40s and 50s ahead of the front
will fall rapidly into the 20s and 30s behind the front Saturday
morning into Saturday afternoon (teens and single digits
along the ridge tops).
The potent upper-level trough will pass through the area behind the
cold front Saturday afternoon/early evening. Given the strong
dynamics with the upper-level trough, this does cause an
anafrontal setup, where a period of precipitation is expected to
occur on the cold side of the boundary. Therefore, rain is
expected to end as a period of snow for most areas. Latest guidance
has shifted slightly east with the track of the surface low, hence
allowing the cold air to spill in quicker. With this scenario,
confidence increases for accumulating snow across most locations
Saturday morning into Saturday afternoon. Confidence is still
highest over the Mountains and west of Interstate 95, but having
that been said there is increasing confidence for snow
accumulation along and east of Interstate 95 as well.
Typically, it is difficult to get a rain changing to snow setup east
of the mountains on a northwest flow in the low-levels, especially
for this time of year. However, reasons for increasing confidence in
this occurring are that there is Arctic air being drawn into this
system behind the cold front. Also, there is stronger Fgen forcing
in the 850-700mb layer behind the cold front, and there are coupling
jet streaks in the upper-levels (right entrance of northern stream
and left exit of southern stream). The strong dynamics suggest that
a period of moderate to perhaps heavy precipitation will add to the
cooling from a dynamical standpoint while the Artic air is rushing
in.
For these reasons, we have issued a Winter Storm Warning in the
Allegheny Highlands and northern VA Blue Ridge Mountains with
advisories elsewhere late Friday night into Saturday. For locations
farther east (the Washington/Baltimore Metro areas), they certainly
may need to be added to the advisory. However, with a slightly later
start time, thinking was to re-assess with the latest information
later this morning or afternoon.
For along/west of the Allegheny Front, there may be a break
when the synoptic snow ends around midday Saturday, but
additional snow showers are expected, and some may be heavy due
to an unstable atmosphere, deep boundary layer that has
moisture, and temps in the DGZ. Near blizzard conditions are
possible. Confidence was too low for a Blizzard Warning at this
time, since it may be more localized.
Strong winds are expected behind the cold front Saturday
morning through at least Saturday evening. Strong cold
advection coupled with rapidly rising pressure behind the cold
front and a deep mixing layer suggests that 50 to 55 mph are
likely across much of the area during this time. Wind headlines
will likely be needed during this time. Dangerously low wind chills
around -10 to -20 degrees are possible along the ridges later
Saturday afternoon/Saturday night. Elsewhere, wind chills near zero
are most likely west of the Blue Ridge Mountains with single digits
and teens elsewhere.
Winds may gradually diminish later Saturday night while dry and
very cold conditions persist.
One other concern is for a strong to severe storm or even an
isolated tornado near extreme southern MD early Saturday morning.
The warm sector will be nearby with the low passing through, and
there will be plenty of low and deep layer shear. However,
confidence is low at this time since instability may not be rooted
within the boundary layer.