Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
227 PM EDT Thursday Jul 24 2014
high pressure and a refreshingly cooler and less humid airmass
will push southeast from the Great Lakes region today and stay
with US right into Saturday. A warm front will slide east out of
the Ohio Valley late Saturday followed by a wavy...slow moving
cold front for Sunday into Sunday night. Periods of showers and
thunderstorms will affect central Pennsylvania and the Allegheny
plateau Saturday night through Sunday night.
Near term /through tonight/...
cold air aloft and good solar heating generated lots of cumulus over
the entire region. The weak/moderate short wave trough passing
overhead will be pushing to the S/east soon and diurnal heating on
it/S way down soon as well. Clouds are already on the decrease at
18z as dry air has mixed well/deep. By sunset a lot of the clouds
will be gone...with the greatest coverage over the SW/SC
mountains. Expect most of the area to become clear and allow temperatures
to drop off The Cliff. Buf pointed out that a good smoke layer is
heading at US /mainly to the north of PA/ from Canada. This may serve
to keep temperatures up just slightly tonight...but will weight the
forecast only very slightly to account for this possible effect.
MOS guidance rarely pumps out record mins at any point...so when 2
of our climatology sites are guided toward rec mins...that is a strong
signal for a significant event. The set up is nearly-perfect. High
pressure moves overhead. However...the high pressures and 800 mb temperatures
are not highly anomalous - just slightly below normal. The wind
should die down tonight and the clouds dissipate. Will take temperatures
down to within a degree or two either side of record mins at many
places tonight. See the climatology section for the current record mins.
The cold temperatures are a classic scenario for fog in the northern valleys.
The air will be so dry that it will make it tough to fog in. Will
go ahead and mention patchy fog for now. Those cold temperatures
immediately around kbfd could also create some frost in the open
areas. Will also mention patchy frost but not go advisory because of
the low probability of widespread coverage and the already-
tenuous temperature forecast.
Short term / Friday through 6 am Friday/...
after the chilly start...Friday will see a good recovery in temperatures.
Scattered-broken clouds may sneak into the northwest early in the day and then the
cold air overhead will allow a few diurnal cumulus to form. Maxes
under 12-14c 800 mb temperatures works out to highs in the l70s in the
highest elevations and l80s in the eastern valleys.
Long term /Friday through Thursday/...
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 moderate to
strong 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. The 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.
A potentially stronger system /stronger than the one that ushered
in the cool and much less humid airmass overnight Wednesday night/
will affect the region for the second half of the upcoming
weekend. The trend of stronger troughing mentioned above has
lead to slower arrival and departure of the frontal system in the
daily forecasts. The European model (ecmwf) in latest run has slowed up the progression
of this upper level low...where the GFS operational and ensembles
have sped up its progression through the Great Lakes region.
A warm front will be on our western doorstep late Saturday night
and Sunday...as the ec continues to slow its progression so
expect the warm front to move in closer to midday Sunday. Have
thus adjusted pops/time.
Associated cold front does not cross the state until Monday...where
surprisingly at this point...the ec and GFS come closely in phase.
This large upper level low could become quasi stationary over
southern Canada...Great Lakes region which could lead to a
potential prolonged period of showers and thunderstorm through the
middle of next week.
Aviation /18z Thursday through Tuesday/...
diurnal cumulus being helped along by weak short wave trough pushing southeast
through the area at 18z. This feature and the heating should both
be going away by 00z...and clouds will continue to get bigger
breaks/dissipate. By or shortly after sunset most of the cumulus will
be gone. Expect VFR conditions and light winds overnight. Only the
northern valleys will see the usual valley fog form with
seasonally-cold temperatures dipping to well-below the
stream/river water temperatures. However...the air is pretty dry and have
kept mentions of fog away from all but kbfd. VFR expected for all
of Friday and into Saturday as high pressure floats overhead and
slides to the east. Humidity and scattered thunderstorms and rain will return Sat
aftn/evening...but Sunday looks worse for flying conditions with
more-widespread sh/thunderstorms and rain in advance of a cold front.
Sat...patchy am valley fog north...otherwise VFR. Scattered PM thunderstorms and rain/MVFR
Sun...shra/tsra with warm front passage
Monday...more widespread rain showers/thunderstorms and rain with cold front passage.
min temperature records Friday morning July 25th:
Bfd - 39 set in 2013 - not long ago but short period of record
jst - 45 set in 1957
aoo - 48 set in 1957
ipt - 48 set in 1953
MDT - 53 set in 1953