Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
444 am EDT Friday Oct 31 2014
low pressure will cross the region Friday and Saturday with
Near term /through tonight/...
current satellite imagery shows our area dominated by middle-level
stratocu expected to linger through the morning. Middle to high clouds are
beginning to stream in ahead of a Stout cold front currently
positioned through the lower Michigan through in/IL.
As the front approaches...mid-level moisture will increase
further...however dry air above the inversion evident at 800 mb on the
00z sounding will be slow to erode this morning. This should
ultimately delay the onset of precipitation beginning near midday.
Even as the column saturates...most of the dynamics remain well to
our west associated with vigorous upper low. Besides for a few weak
waves ahead of the main upper low...a lack of well defined ascent
Colorado-located with the moisture is noted. As a result...expecting a
high pop/low quantitative precipitation forecast scenario to play out for much of the day today.
The strong upper level vorticity dropping south through Minnesota/Iowa currently
will be aggressively amplifying the upper trough through this
afternoon. By this evening the upper low begins to cut-off...and
will shoot directly south through the Tennessee Valley well to our south-southwest. As this
occurs the coldest air will wrap in behind the low remaining south
of the County Warning Area as well. Again...the best dynamics and thus the highest
precipitation potential will stay near the upper low as it moves
into the southern Appalachians tonight. Abundant moisture but weak
forcing directly overhead will once again yield a high pop/low quantitative precipitation forecast
forecast through tonight. Temperatures will gradually drop into the middle and
upper 30s for much of the forecast area early Sat morning. Some
locations in the ridges could flirt with the freezing mark. With 850 mb
temperatures prognosticated to get to only -1 or -2c...have continued to back off
snow potential through 12z Sat morning.
Short term /Saturday through Monday night/...
anomalously strong upper low will be heading offshore of the
Carolinas on Saturday...where an offcoast low will then begin to
develop. Plenty of moisture to start across the County Warning Area...although
depth will be steadily decreasing with time during the day. With
this...plus the steady loss of forcing/uvv...allow probability of precipitation to slowly
drop off. Kept values a bit higher in the north with possible lake
influence. North-northeast flow will slowly push colder h9850 air in...with -3
to -5 readings by 00z sun. However...looks like boundary layer will
be too warm to support snow in most cases during the day. By later
in the afternoon...colder air will start to collapse in and begin
the changeover to snow...with any lingering precipitation going over
to all snow at night. Most areas will not see any accumulation. A
very light accumulation remains possible on the southeastern ridges
on grassy surfaces Saturday night as precipitation begins to wind
down. A slightly favorable north-northwest upslope component may develop
Saturday night...allowing shsn to linger into Sunday morning...but
moisture will be lacking by that point.
The deep trough pulls east Sunday morning...with skies clearing as a
midlevel ridge begins to approach. Sunday night will be mostly
clear and cold as the surface high drifts by to our south. Middle/hi
clouds will then increase Monday and Monday night...spilling over
the ridge axis.
Much below normal temperatures Sat/sun will moderate on Monday...but
will still be a bit below climatology. Cl
Long term /Tuesday through Friday/...
followed HPC guidance for this period...which becomes more dependent
on ensemble guidance with time. After the ridge departs
Tuesday...unsettled weather returns for Wednesday and Thursday with
a couple of fronts. Temperatures will be seasonable. A fresh
trough develops over the Great Lakes and northeast by the end of the
week...with dry weather returning by Friday but with slightly below
normal temperatures as well. Cl
Aviation /08z Friday through Tuesday/...
stratocu deck will continue overnight with VFR and occasional
high end MVFR ceilings. Calm to light surface winds will prevail
until the front arrives midday Friday with precipitation. Lowered
visibility at onset of rain but maintained within MVFR category
through about 00z Sat. Ceilings will stay into MVFR and could easily drop
into IFR category Sat morning as the low positions itself nearly
Outlook.../Friday night through Tuesday/...
upsloping cold advection will then maintain shower and stratocu potential
until surface high strengthens on Sunday.