Area Forecast Discussion

FXUS61 KPBZ 151741
Area Forecast Discussion
National Weather Service Pittsburgh PA
1241 PM 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.
Mid morning update made to make only minor tweaks to the
forecast as snow is already being generated by convergence along
the terrain as a shortwave in the upper trough pivots toward the
Ohio Valley. Expecting little accumulation from this but still
thought it warranted an increase in PoPs through the afternoon.
Otherwise, shortwave trough over lower Michigan will make it's 
approach this afternoon. Little change made to the analysis 
below, so will leave this as is. 
Previous discussion...
The GOES-16 Simple Water Vapor RGB shows our next disturbance 
quite well, currently entering southern 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.
Warm air advection begins in earnest Saturday afternoon as high
pressure builds into the region. This will quell any remaining
shower activity that survives through midday. High temperatures
may still be held below average values, especially further
north where clouds will hold a little longer, but temperatures
will certainly be warmer than the previous couple of days.
High pressure will hold through Sunday morning, but another
disturbance will be knocking on our doorstep to end the weekend.
Isentropic lift along and just behind the lifting warmfront will
provide modest ascent as a weak upper-level wave moves into the
area. Will continue to carry chance PoPs for rain Sunday
evening. A little snow could mix in further north where
temperatures will be a few degrees cooler. No accumulation is
A brief spell of zonal flow aloft will give way to 
amplification once again heading into the middle of next week. 
First, a trough will dig into the central CONUS Monday night 
into Tuesday, bringing rain chances to the forecast area.
In the wake of this system, another brief spell of mostly dry 
weather is expected Wednesday night into Thursday. Thereafter,
models have consistently shown another, strong system moving
through the eastern US late next week. Strong warm advection
ahead of this system should mean precipitation at onset would be
rain. Colder air is set to return next weekend.
MVFR ceilings continue to linger many south and east of
Pittsburgh. Most locations should see a window of VFR conditions
during the afternoon. The next upper disturbance will bring snow
showers and MVFR cig/vis mainly north of PIT tonight. FKL, and
to a lesser extent DUJ, have a chance of getting occasional IFR
conditions in a couple heavier snow showers. After a lull
overnight as the disturbance departs, a fresh injection of
moisture will bring another round of snow showers and MVFR
ceilings. Flight conditions will start to slowly improve from
south to north by the end of the TAF period.
The next chance for general restrictions arrives by Sunday with
an advancing warm front.