Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1039 am EDT Tuesday Jul 22 2014
A ridge of high pressure will remain over Pennsylvania today. A
potent cold front will move southeast from the Great Lakes and
cross the commonwealth late Wednesday and Wednesday night. A
refreshingly cooler and less humid airmass will overspread the
commonwealth for the end of the week into the weekend.
Near term /until 6 PM this evening/...
Low clouds stacked up against the alleghenies are finally showing
signs of mixing out and will lead to a partly sunny afternoon. It
is warm and humid and once again today we will be dealing with
isolated buildups leading to a few showers or thunderstorms.
Mesoscale anal shows stability eroding over western PA...but also
indicates a tongue of very warm middle level air pushing into the
area. This will help suppress activity despite the expectations
of convective available potential energy cooking up into the 1200-2000j range this afternoon. The
best chance for the isolated convection should be across southern PA
and perhaps the lower susq valley where slightly cooler middle level
temperatures are forecast to persist.
After the early to middle morning low clouds and patchy fog mixes
out...today will feature noticeably warmer temperatures as an
upper ridge slides east from the Great Lakes.
Highs Monday were in the M/u80s upstream over Michigan and expect
similar readings throughout the valleys of central PA this
afternoon. The higher terrain of the Allegheny plateau will
likely only reach the l80s.
Short term /6 PM this evening through 6 PM Wednesday/...
similar /and perhaps even muggier/ conditions are expected for
tonight with a light southerly flow and 1.5 inch precipitable water air keeping
temperatures about 6-8f above normal.
Very warm middle level temperatures /averaging 9c at 700 mb over central
Penn/ will keep conditions dry tonight through much of Wednesday
morning. The arrival of a strong cold front and upper level
disturbance Wednesday afternoon and evening will create several
bands of deep uvvel...helping to break this cap and fire off one
or more broken lines of strong to potentially severe thunderstorms and rain.
Storm Prediction Center has the entire County Warning Area in their day 2 /see text/ outlook. Given
the projected high cape and weak-MDT deep layer shear type of
environment the primary storm type will be pulse and multi-cell
clusters capable of producing localized severe hail and wet
microbursts /with a few min-Bow echoes possible as well/ Wednesday
afternoon through early Wednesday evening.
Thanks to a warming/llvl...west-southwest downslope flow ahead of the
cfront...high temperatures Wednesday will be the warmest of the week...ranging
from the lower 80s across the highest terrain of the north and
west...to the middle and upper 80s in the Central Valley...and lower
90s in the greater York/Harrisburg area.
Long term /Wednesday night through Monday/...
overall scenario remains unchanged in the long term period as a
large upper ridge building over The Four Corners region is
forecast to expand north and east into The Rockies and plains
during the period. Downstream...this will lead to long wave
troffing over the eastern third of the country. Upper low to the
east of Hudson Bay will be strengthened at times by shortwave
energy in the stronger jet stream flow across Canada. Southward
extension of this low/trough will at times during the period
strengthening the troffing across the northeastern U.S./Our
region. Todays trend of medium range model guidance is for
stronger/deeper troffing downstream across the eastern
u... an almost full latitude trough extending from the
eastern Canada low southward to Florida. This will likely serve to
expand and prolong the chances for precipitation this weekend.
Guidance forecasts continue to bring a front and increased
changes for precipitation toward PA in the Wednesday/Thursday time
frame and show a second...potentially stronger system late next
weekend. The trend of stronger troughing mentioned above has lead
to slower arrival and departure of the frontal system in the daily
forecasts. A warm front will be on the doorstep of PA Sunday
morning...a bit slower than forecast yesterday. Have thus adjusted
probability of precipitation/time to suit. Associated cold front does not cross the
state until Monday at this point...leading to a potential prolonged
period of showers and thunderstorms. At this time frame
however...a lot will likely change before we reach next weekend.
Aviation /14z Tuesday through Saturday/...
Morning low clouds and fog/haze will continue to slowly mix out
and burn off. Eastern terminals will be slowest for a change given
the deeper layer moisture in place...but still expect all airports
to become VFR between 15-18z.
Warming temperatures aloft and lack of synoptic forcing suggests very
low chances of afternoon pop-up showers or thunderstorms.
No mention of thunderstorms in tafs as point probs/precipitation covering will
be very low/sparse. Diurnal cumulus should fade after dark with some
patchy fog and associated lower ceilings/visbys re-developing overnight.
Wednesday...scattered thunderstorm impacts likely associated with cold front.
Thursday...scattered thunderstorm impacts possible southeastern airfields as cold front slows as
it approaches the middle-Atlantic coast.
Fri-Sat...patchy a.M. Fog possible. Otherwise no sig weather.
near term...la corte/Lambert