Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
658 am EDT Wednesday Aug 27 2014
a cold front will move across the region today and provide a
focus for thunderstorms. High pressure will arrive behind the
front and bring dry weather to the area through the end of the
week. Above normal temperatures and increasingly humid conditions
are forecast over the Labor Day Holiday weekend and into the first
week of September.
Near term /through tonight/...
patchy fog in the river/stream valleys of scentrl/southeastern PA will
erode shortly after sunrise with local visbys improving by 14z.
Clouds are increasing over the northwest mountains in advance of surface cold
front shifting southeastward from the Lake Erie shoreline.
A low amplitude upper trough over the Great Lakes region this
morning will translate eastward into the northeast states tonight and
move offshore during the d2 period. At the surface...a cold front
analyzed from the St. Lawrence Valley southwestward back across the lower lakes
will push southeastward across central PA this afternoon before shifting south of
the Mason Dixon line after 00z. The global models are well
clustered with the upper trough and associated cold front crossing
the northeast...with a non-NAM consensus solution preferred by wpc
as it appears to be stronger aloft and a tad slower with the
advance of the surface front.
The main forecast concern in the near term is the potential for
strong to locally severe storms this afternoon. Models advect rich blyr
moisture eastward from the Ohio Valley into central PA in advance of the surface
cold front...with 60-65f dewpoints prognosticated by the consensus
guidance. The NAM is likely too moist with low 70s dewpoints.
Strong heating and destabilization will lead to mu convective available potential energy in
excess of 1000 j/kg along a narrow corridor preceding the cold
front. The low-middle level background wind fields will not be
particularly strong...and the best deep shear lags the front.
However...a few cells capable of producing small to marginally
severe hail and locally strong to damaging wind gusts will be possible
given combo of "sufficient" shear and decent instability. The d1
Storm Prediction Center outlook places the eastern zones in the categorical see text with
5% severe probs but could see an isolated severe storm further west across
S-central PA to the Laurel Highlands. Convective activity should
weaken quickly with the loss of daytime heating and eventually
dissipate/shift S-southeastward after midnight.
Afternoon temperatures will approach 90f from the scntrl Ridge/Valley
region into the lower susq valley...with upper 70s to low 80s common
in the northwest/north Central High terrain.
Short term /Thursday through Thursday night/...
high pressure and much drier air with low/comfortable humidity
will overspread the area on Thursday. Ens data shows temperatures aloft
slightly blw normal over northern/northestern PA which should promote afternoon
cumulus/SC clouds. Temperatures range from around 70f in the northwest to 80f in the
southeast. Thursday night looks seasonably cool with valley fog probable under
surface ridge with M/clear skies and nearly calm winds. Warm air advection clouds may
impact min temperature forecast but for now see no reason to deviate from
Long term /Friday through Tuesday/...
medium range flow pattern across the Continental U.S. Will feature troughing
in the west and fairly high heights forecast to prevail over the east
(for a change) supported by subtropical ridge anchored over the
southeast. High pressure centered over upstate New York at the start of
the period will move off the northern New England coast on Saturday as
todays frontal boundary pivots back northward as a warm front...in
response to shortwave/surface wave forecast to track northeastward from the eastern
plains/MS valley. Over the past 12-24 hours the latest guidance has
trended northward with surface low tracks through the Great Lakes around d4/sun
and with leading frontal boundary which is now prognosticated by wpc to
stay north of PA. A more defined cold front should approach the
area by the middle of next week.
A rather moist return flow around Southeast Ridge and ahead middle
level shortwv tracking northeastward from the eastern plains/MS valley along with
interaction with associated surface wave/wavy leading frontal zone
should spell an increasing probability/opportunity for rain into
the weekend. However confidence in precipitation details (i.E. Pops) is
medium at best due to model spread/dependence on smaller scale
features. The forecast may be perceived as somewhat pessimistic with
nearly daily chance for showers/thunderstorms however precipitation will certainly not
be continuous with periods of dry weather mixed in between generally scattered
and largely disorganized/weakly forced precipitation.
There is higher confidence in a warming trend with maximum temperatures 5-10f
above normal over the weekend and into the first week of sept.
Aviation /12z Wednesday through Sunday/...
leading edge of strato cumulus and low-end Alto cumulus associated with an
axis of much higher precipitable water air /near 1.75 inches from northestern Ohio to
southwestern New York/ were drifting into the Central Ridge and valley
region at sunrise...while skies were mainly clear across the
southeastern half of the state. Light and variable...to light
southwesterly wind was noted at all airfields.
Surface cold front was sliding just southeast of the I-90 corridor from kcle
to Keri and kroc at this time. Cloud bases at kbfd and kjst should dip
into the MVFR range at times later this morning and early this
afternoon /with some occasional IFR possible at Bradford between
The cold front will drift slowly southeast and cross central PA airfields
during the late afternoon hours...before reaching the lower
Susquehanna valley /kmdt and klns/ later this evening.
Instability and moisture will be quite high ahead of the cold
front...so included thunderstorms in the vicinity for about a 4-hour period preceding the
frontal passage. Brief MVFR conditions are possible in the
Mainly VFR is expected tonight into Thursday in the wake of the
cfropa. A 6-10 hour period of northwest winds average 8-10 kts...with gusts
in the middle-upper teens is expected late today and tonight in the
moderately tight low level pressure gradient following the cold front.
Thu-Fri...late night-early morning restrictions in fog
Sat-sun...mainly VFR. Chance of afternoon showers/isolated thunderstorms.