Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1237 PM EST Wednesday Dec 11 2013
a cold front will bring another shot of Arctic air and snow
showers this afternoon. High pressure will end the snow showers
Thursday but the cold air will remain in place through Friday.
Near term /until 6 PM this evening/...
update to lower temperatures a couple more degrees and to adjust
sky grids as clouds are quickly moving east from Ohio with front.
Previous discussion follows...
Previous disc...another front will cross the region this
afternoon. This front will be pushed southward by a strong
shortwave trough...which will swing through the north this
evening. Expect snow showers to accompany the front...reaching
areas near pit by late afternoon. Higher snow shower accumulations
with the front will likely be confined to the far north in the
area of best upper level support. There will be some lake
enhancement to the snow late this afternoon with the cold air advection and
strong westerly flow aloft.
Temperatures again today will be well below normal.
Short term /6 PM this evening through Friday/...
Lake enhanced/effect snow showers look likely tonight...as Arctic
air crosses the lakes and winds become northwesterly behind
exiting shortwave. The limiting factor will be the strong high
pressure building in from the west and also lowering inversion
heights. Latest model soundings are showing inversions dropping
quickly after 06z...and winds aloft will begin to have more of a
westerly component. In addition to the concentration on lake snows
across the north...will also need to focus on upslope snows in the
ridges. At this point...this does not appear to be an optimal
setup for upsloping because of the rapid intrusion of the surface
high. However...model soundings and cross sections are showing
lower atmospheric levels saturating around 04z...at the same time
the dendritic snow growth zone organizes and falls. This should
allow for some accumulations across the ridges as well.
Gusty winds and cold air will provide wind chills below zero
across much of the area late tonight.
Cold air will stubbornly drift northward Thursday afternoon...as
the surface high moves over the lower Ohio Valley. Expect snow
shower activity to continue across the north until the cold pool
aloft exits. The cloud cover should also slowly erode from
southwest to northeast once the warm air advection gets going.
Thursdays highs will be some 15 to 20 degrees below normal. With
gusty winds...wind chills will be in single digits to near zero
through much of the day.
Strong warm air advection aloft...ahead of saturday's system...will highlight
our weather on Friday. This will allow for some sun on Friday with
a 10 to 15 degree moderation in temperatures.
Long term /Friday night through Tuesday/...
so many questions for the weekend storm. Big differences in model
solutions with placement of the lows...how far north the warm air
will push...the inclusion of a dry slot and how fast energy and
moisture transition to what will eventually be the main coastal
low. Additionally...the timing of the initial snowfall continues
to very between the models and between model runs. Couple things
that generally occur when a dual low situation sets up. If the
inland low moves to our west...the push of warm air tends to be
stronger then models depict...especially this far out.
Additionally...there is almost always a dry slot that develops
between the two surface systems. The problem is where will that
dry slot be...an how far north will it make it. Some similarities
that the models are currently showing...highest quantitative precipitation forecast will remain
south and east of area. Also a rain snow scenario is being
depicted...which I will go with for current forecast. There will
always be the risk for some icing in these situations...but trying
to Pin Point these details this far out is extremely difficult.
The warm air advection will end sometime Sat evening/night and the coastal low will
take over. This will mean a change back to snow at all locations.
High pressure builds in Monday...with another cold front expected
Aviation /18z Wednesday through Monday/...
VFR conds are expd to dcr to MVFR from north-S through this evening as a cold front
moves across the region. Could be some brief IFR for fkl and duj through lt
afternoon as well where more favorable moisture exits for shsn. Behind
the fnt experimental MVFR stratocu ceilings to linger for a few hours until
lifting to VFR overnight. The exception will be fkl/duj where some lake
effect clouds/shsn should help keep ceilings MVFR. Experimental a wind shift with frontal passage
from SW-northwest with gusts to 20kt.
Outlook.../Thursday night through Monday/...
VFR conds under high pressure expd through Friday evening before a low pressure
system brings rstrns overnight Friday into Erly sun.