Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Pittsburgh PA
827 PM EST Tuesday Dec 10 2013
a cold front with Arctic origins will bring a reinforcing shot of
cold air and light snow to points near and north of I-70 and to
the higher elevations late Wednesday. Temperatures will remain
far below normal for the remainder of the week.
Near term /through Wednesday/...
a band of lake-effect snow showers has developed along the Eastern Shore of
Lake Erie amid west-southwesterly low-level flow. Flow overnight will shift to westerly
and will maintain this onshore flow...so snow showers expected to continue
primarily north of forecast area. Southern periphery of this band could graze
far northern portion of forecast area...where probability of precipitation were modestly increased.
Lake-effect snow showers expected to continue through much of Wednesday and will be
enhanced by a subtle shortwave trough. By middle-day Wednesday...guidance is
suggesting southward development of light snow coincident with the development of 500 mb
jet streak associated with the shortwave trough. This brief period of light
snow Wednesday afternoon-evening could affect evening commute...although amts will be
less than one inch.
Cold front will cross the region Wednesday evening and will rekindle lake-effect and
lake-enhanced snow showers Wednesday night. There may be a 12-hour window
where northern zones could reach adzy-level snowfall...dependent upon the
precise location of any lake snow bands. At this time...potl for adzy snow
will be mentioned in hazardous weather outlook. Next shifts will need
to evaluate the potl for headlines.
Wind chill adzy for mountains still looks possible Wednesday night as wind increases in
wake of frontal passage. Wind chill vals area-wide will drop blw 0f... with
vals in the mountains reaching -10f. Previous discussion follows...
Strong subsidence has allowed the forecast area to clear out in
all but the ridge counties...which should still see clouds scatter
for a period late this afternoon. Have adjusted sky grids
drastically to account for this and also adjusted temperatures.
Tonights low temperatures will be impacted significantly by the
amount of cloud cover that is around. Most of the model guidance
suggests that there will be enough shallow moisture to redevelop
clouds in strengthening cold advection...although the most likely
area to cloud up will be the northern zones where some lake
enhanced showers could develop. If skies remain scattered/clear
temperatures will drop like a rock where the ground is now covered
in freshly fallen snow. Adjusted temperatures down to the cooler
mex guidance but no lower to take in account this uncertainty.
As mentioned above...cold advection should allow for the
development of lake enhanced snow showers later this evening.
However...with the upper level wind expected to remain
westerly...think that any snow will generally be confined well
north of I-80. The greatest snow amounts with the band will be
seen outside of the forecast area but very light accumulations
were added to the top tier of counties.
Short term /Wednesday night through Friday/...
think that the better chance for snow accumulations...albeit
minimal...will be possible later Wednesday as a fast moving Arctic
front crosses the region. The best shot for a couple of inches of
snow looks to stay north of I-80 and along the ridges as the upper
level wind shifts northwesterly. With the window for these northwest
winds remaining quite small...anticipate accumulations won't be
enough for any headlines...at least for now.
High pressure ridge will finally begin to build northeastward late
Wednesday night and Thursday. With the reinforcing shot of cold
air...temperatures could struggle to make it into the
20s. Factor in the wind chill and temperatures may feel more like
they are in the single digits or even below zero in the higher
elevations. Will mention these cold wind chill values in the severe weather potential statement
for at least the ridge counties for now...with values forecast
close to -10 Thursday morning.
Weak warm advection and sunshine may help to bring temperatures
up Friday into the 30's but this is still 5 to 10 degrees below
Long term /Friday night through Tuesday/...
an active portion to start the period as another complex low pressure system
moves from the MS valley region into the Ohio Valley/Great Lakes on Sat. Models are in
genl agreement at this time on the initial low weakening across the lower Great Lakes
Sat/Sat night while a coastal low develops in response to a digging upper
trough. This would help lmt precipitation amts across the region...however low conf
in specific details at this time. Models have backed off on warm air intrusion
across the region...so forecast features mostly snow with some rain possible.
Still a chance of freezing rain so maintained some mention of this generally S of I
Snow shower chances will continue sun as an upper trough crosses the Great Lakes. By Monday
snow shower chances will be confined to lake effect and upslope flow areas. High
pressure builds in lt Monday before the next cold front is prognosticated to approach Tuesday.
Temperatures will continue to run well below seasonal avgs through much of the period.
Aviation /01z Wednesday through Sunday/...
as high pressure builds over the region...VFR conditions are
expected at terminals tonight except for kfkl and kduj...as MVFR
stratocu will likely occur due to westerly flow off the Lake.
A passing cold front will quickly cross the Great Lakes Wednesday
afternoon...allowing ceilings at most terminals to be at low-end
VFR or high-end MVFR. Have included the onset of snow showers in
tafs at northern terminals with more confidence in forecast.
Possible restrictions will occur at kfkl and kduj with the passage
of the front.
Outlook.../Thursday morning through Sunday/...
in the wake of the cold front...cold northwest flow with some snow
showers might cause restrictions north of pit. High pressure
slowly building from the south will allow conditions to eventually
improve through Friday night before low pressure returns and
brings back the chance for restrictions over the weekend.