Scientific Forecaster Discussion
fxus61 kpbz 150535
Area forecast discussion
National Weather Service Pittsburgh PA
1235 am EST Fri Dec 15 2017
cold with another round of snow showers expected this evening,
mostly north of Pittsburgh. Some temperature moderation is then
expected over the weekend.
Near term /through tonight/...
latest GOES-16 nighttime microphysics channel showing clear
distinction between the thin high cirrus over much of the area,
and two areas of low clouds straddling our north and south
boundaries. The southern stratus is expected to creep back
northward through the morning, clinging to the ridges, perhaps
insulating these locations and keeping temperatures a few
degrees warmer than the rest of the area only marginally
obscured by the high cirrus.
The GOES-16 simple water vapor rgb shows our next disturbance
quite well, currently entering northern Wisconsin. This wave
will cross the western Great Lakes today, deepening the upper-
level trough and surface low under favorable upper-level jet
dynamics. Although the system lacks deep moisture, it will
acquire some low-level moisture on its journey through the
Great Lake aggregate. Steepening lapse rates under mid-level
cooling will help fuel snow showers as they enter our
northwestern zones this evening. Most of this activity will stay
north of Pittsburgh and probably north of I-80, but there will
be potential for accumulating snow showers south to the pgh Metro
area. Current timing would be after the evening rush.
A quick 1-2" is possible across the northern tier of counties,
with a marked decrease expected traveling south.
Snow showers will likely taper down after the passage of this
wave near midnight tonight. However, a developing low-level jet
and an injection of mid-level moisture will allow for some re-
invigoration of snow showers Saturday morning. This evolution is
represented well in the bevy of high-res models at our disposal.
This activity will likely stay north of the developing mid-level
warmfront, or roughly north of Pittsburgh. Another inch or so
could be the result of these showers. All told, the roughly
24-30hr snow total will likely remain under advisory levels,
with the northernmost counties the most likely to flirt with the
3" mark. Elsewhere, and inch or less is expected.
In addition to the snow, the aforementioned strengthening low-
level jet will bring wind gust to the ridges that could near
Wind Advisory levels tonight. Current forecast remains just
below the advisory threshold, although this will need to be
monitored through today. Regardless, the increased wind
tonight will bring a bite to the already cold air in place. Wind
chills will commonly be in the low teens to single digits,
colder in the higher terrain.
Short term /Saturday/...
residual snow showers may linger into Sat, but will decrease in
coverage as warmer air invades the region and heights rise.
Flow will become zonal by sun in the Great Lakes region as a
shortwave trough strengthens in the swrn Continental U.S.. ahead of this
system, warm-air advection will help restore temperature to
near-seasonal values during the weekend and into next week.
Long term /Saturday night through Thursday/...
an impulse ejected from the trough in the swrn Continental U.S. May bring
light rain to the region on Sun afternoon-evening, but it will
be a decaying system by the time it approaches the Great Lakes.
A better chance for precipitation comes mid-week as the swrn
Continental U.S. Trough phases into a nrn stream disturbance and drives a
cold front through the region.
Aviation /05z Friday through Tuesday/...
general VFR is anticipated overnight despite lingering high-
based stratocu. A drop back to MVFR is possible during the day
Friday as the next disturbance approaches. Snow is expected at
fkl and possibly at duj Friday afternoon with visibility
next chance for restrictions is late Friday into Saturday.