Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
956 am EDT Thursday Sep 3 2015
above normal temperatures will continue into next week with
gradually decreasing humidity levels. Isolated showers and
thunderstorms are possible through the end of the week. High
pressure will ensure fair and dry weather for Labor Day weekend.
Near term /through tonight/...
locally dense fog in the process of dissipating should be gone in
the next hour or so.
Very isolated shower/thunderstorm developing over southwestern PA may impact the Laurel
Highlands through late morning as leading edge of area of
increased upward motion moves into the region.
Modest large scale lift ahead of weak middle-level perturbation
drifting east-southeastward over lower Michigan/Lake Huron...combined with abundant
low-level moisture...weak mlcinh and strong diurnal heating should
promote isolated to scattered convection by the afternoon. The
hires ncar/sseo mesoscale ensemble systems...and to a lesser
extent the global short range deterministic models...seem to be
favoring a corridor from the upper Ohio Valley east-southeastward across northern
WV/southwestern PA with the greatest risk/probability for storms later
today. However low probability of precipitation will exist across the entire County Warning Area with
slightly higher probs painted over the higher terrain of the north/west
High moisture content and forecast moderate instability suggest some
locally heavy downpours. The deep-layer flow will be rather weak
under broad upper ridging which should limit potential for
organized severe storms. However sub-severe water-loading-related wind
gusts under 50kt will be possible and may produce localized/minor
damage. Nocturnal blyr stabilization should reduce convective
coverage/intensity considerably this evening with any lingering
storms likely dissipating into tonight. Persistence and time of
year make a strong case for overnight fog/reduced visibility into
Short term /Friday through Saturday night/...
upper level ridging will remain a dominant feature into the
weekend. Short term model guidance still shows a low to medium
chance for isolated/scattered thunderstorms Friday afternoon as weaker shortwave
impulses drift southeastward within shear/instability/mstr axis from the
Great Lakes across the north-central Appalachians. A backdoor cold
front should align itself near this axis and provide enough low
level convergence to spark some convection over central and
especially southern PA per multi model consensus. Diurnal cooling
should allow any storms to dissipate through Friday evening. The boundary
should pivot to a north-northwest/south-southeast orientation on Saturday while being
shunted southwestward...as a large dome of surface high pressure moves from eastern
Quebec into New England. Risk area for storms on day 3 should be
positioned to the west of the local County Warning Area. Temperatures should cool
off a bit with noticeably less humid air arriving by Saturday
Long term /Sunday through Wednesday/...
high pressure should maintain dry weather Sunday-Monday. Lead
cold front advancing eastward from the Great Lakes/Midwest and
potential onshore flow moisture may impact the forecast by Tuesday...but
a consensus solution holds back higher probability of precipitation until Wednesday-Thursday in
association with a stronger shortwave/surface cold front. So timing
diffs will need to be resolved.
The global numerical models/ensembles are coming into better
agreement on a pattern change during the second half of next week
with a shuffling of the longwave flow allowing eastward progression of
the western trough into the central states and recent ridge over
the eastern United States off the coast into the western Atlantic. This
should eventually bring an end to the recent hot/dry spell. A
return to seasonal temperatures and more active precipitation pattern
seems probable by next weekend in the new flow regime.
Aviation /14z Thursday through Monday/...
locally dense fog has dissipated but widespread MVFR visibilities in fog
and haze remain. Expect improvements to VFR at most taf sites by
VFR conds and light winds are expected for the rest of today.
However...an area of low pressure pushing in from the Great Lakes will
produce scattered PM thunderstorms and rain across the area. A brief visible reduction could
accompany any thunderstorms and rain...with the best chance over the western half of
the state between 18z-00z.
Patchy fog will once again become the main concern tonight...with
IFR conds likely at kbfd and possible at kipt.
Friday...patchy am fog possible. Scattered PM thunderstorms and rain impacts possible.
Sat-Mon...patchy am fog possible...mainly kbfd/kipt.