Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
317 am EDT Tuesday Jun 30 2015
a warm front will move northeast across the state overnight
bringing showers to primarily the northwest half of the state.
Warmer and more unstable air will encompass the region Tuesday
helping to trigger numerous showers and scattered afternoon
thunderstorms as a trough of low pressure at the surface and aloft
approaches from the west. Expect an unsettled weather pattern to
persist into the upcoming Holiday weekend as a broad but weakening
upper level trough remains positioned over the lower 48 east of
the Continental Divide.
Near term /until 8 am this morning/...
middle level shortwave lifting NE through the east Great Lakes early this
morning. Southerly ll jet and associated warm air advection/isentropic lift in advance
of this feature is producing periods of rain across
western/central PA as of 06z. Latest radar trends and hrrr output
support raising overnight probability of precipitation to between 80-90 percent across the
Allegheny plateau...while chance of rain looks much lower across the
lower susq valley...further from shortwave and associated lg scale
A few lightning strikes noted at nose of ll jet across SW PA at
06z and model soundings show some marginal elevated instability
across much of central PA early this am...as ll jet sweeps
eastward. Therefore...will maintain the slight chance of thunders
across the central mountains and Allegheny plateau early this am.
Temperatures on track to bottom out early this am from the u50s across
the north mountains...to the m60s across the southeast counties.
Short term /8 am this morning through 6 PM Wednesday/...
the lead shortwave is forecast to weaken as it lifts through the east Great
Lakes later today. Latest hrrr/NAM sim reflectivity indicate
a dwindling batch of showers associated with this feature will lift
through the NE counties between 12z-15z. At the surface...warm front will be
pushing NE across the region during the late am and early
afternoon...bringing brightening skies early in the day across the
alleghenies...and by early afternoon across the susq valley.
Diabatic heating results in moderate model convective available potential energy of around 1000
j/kg by afternoon across central PA...with somewhat greater instability
noted across the susq valley...where trailing ll jet/plume of
higher precipitable waters lie in associated with weakening shortwave over the east Great
Lakes. Have thus...placed the highest /likely/ chance of PM thunderstorms and rain across
the eastern counties. Model 0-6km shear of around 40kts supports the
possibility of a few supercells and a slight risk of damaging
wgusts from the central mountains eastward this afternoon. Weaker middle level
winds imply less chance of severe weather across the northwest mountains
Other area of concern will be the possibility of localized flooding...due
to wet ground and expectation of isolated rainfall amts of around 3
inches based on output from convection-allowing models. Will mention
in severe weather potential statement for now...but a flash Flood Advisory could be considered for the susq
valley...where plume of highest moisture will lie.
Ptsunny skies should push temperatures into the m70s over the alleghenies
this afternoon...and the l80s across the susq valley.
Long term /Wednesday night through Monday/...
the large upper level low over northern Canada that will be
dominating the weather pattern over the next week will bring a
deepening trough that will continue to advect warm moist
southwesterly flow into the region Tuesday night into Wednesday.
Middle level perturbations and short wave troughs should enhance
lapse rates. Day 2 outlook is currently marginal for almost all of
Pennsylvania. So expect any thunderstorms that form Tuesday afternoon into
evening to linger Tuesday night into possibly Wednesday.
As the main trough lifts and exits the region Wednesday the
convective environment shifts the best instability with larger
vorticies due to a short wave that should allow for low level lapse
rates to be stronger through along a frontal boundary.
Models continuing to trend more zonal flow late Wednesday night
into Thursday. With an amplifying trough swinging through Thursday
afternoon. This trend should allow for more a more summertime
regime Thursday into the weekend. In the wake of Thursday/S trough
the GFS/gefs/ec begin to diverge in solutions. However with drier
air advecting in and the consensus, though the timing differs,
that a ridge should build over PA through the first half of the
weekend. This could limit any convection to along the Mason Dixon
line and only a slight chance for precipitation currently Sunday
So despite a series of middle- level disturbances moving downstream
from the Upper/Middle Mississippi Valley through the middle Atlantic
region...500mb heights should be in recovery Mode which should
allow daytime temperatures to trend warmer into the July 4th
Aviation /07z Tuesday through Saturday/...
VFR conditions dominating the region early this morning. Light
showers are increasing in coverage and moving SW to NE in across
roughly the western two thirds of the state as a well- defined
short wave trough helps to lift a warm front northward. May see an
isolated thunderstorm in far western counties...but very unlikely through
12z. Low level SW jet of around 30 kts swings in from the SW...and
will bring marginal low level wind shear to kjst-kaoo along with MVFR ceilings for
mainly western higher terrain. Surface winds will remain light
MVFR conditions will remain likely for western higher terrain
through late morning Tuesday /with brief MVFR ceilings poss central mtns/.
An upper level disturbance off to our west will bring a couple
rounds of showers and scattered afternoon thunderstorms and rain.
An unsettled pattern with a series of fronts will be affecting
the region much of this week. The best chances of restrictions
will be Tuesday into Wednesday.
Tuesday...MVFR ceilings likely west. Otherwise mainly VFR. Local
restrictions in rain showers/scattered thunderstorms and rain. Isolated thunderstorms may be strong in the
Wednesday...MVFR ceilings likely west. Otherwise mainly VFR. Local
restrictions in scattered shra/tsra.
Thursday...mainly VFR. Isolated thunderstorms and rain mainly south of I-80.
Friday...mainly VFR. Isolated thunderstorms and rain south.
Sat...mainly VFR. Chance shra/tsra.
June 2015 has been a wet month across much of central PA. Through
6/28...our two lcd climate sites /Harrisburg and Williamsport/ are
ranked 8th and 6th respectively for the wettest June on record.