Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
835 am EDT Monday Aug 3 2015
a large upper low centered over northern Ontario will translate
eastward over the next few days. It will drive a cold front
through the state today and early tonight. Largely confluent
northwest flow aloft over the Great Lakes during the middle week
will help high pressure settle in behind the front. However...the
front will hang up just to the south of the region and may return
northward as a warm front later this week.
Near term /until 6 PM this evening/...
earlier convection over Ohio has all but dissipated this late
morning hour...with just an area of layered middle and high clouds
drifting east and over the alleghenies of northwestern Penn.
This cloud area may act as one differential heating boundary as it
streams east into the Central Ridge and valley region during the
midday and afternoon hours...while abundant sunshine to its west
combines with an approaching cfront to trigger another line or
two of strong to potentially severe thunderstorms and rain across Northern Ohio and near the
Lake Erie breeze boundary in far northwestern Penn.
Light southwest winds at the surface will veer a bit through the day
as the cold front nears. The front then washes out and will be
tough to discriminate this afternoon and evening.
Storm Prediction Center day one outlook keeps the slight risk over almost all of
central PA. The exception is the far southeastern cos. This is presumably
due to the loss of cape as the heat of the day GOES away.
However...the mixed layer convective available potential energy of 1-2k joules and good deep layer
shear make for an environment where storms will grow tall and move
fairly quickly W-E. 0-3km bulk shear will increase to 30-35
kts...with 0-6km values about 10 kts faster. The main severe
weather threat should be several...embedded organized bows within
the few broken lines of showers/tsra...considering the nearly
Uni-directional vertical wind profile. Marginally large hail is
the secondary severe threat in this case.
Heavy rain does not seem to pose a threat today without slow-
moving storms or much potential for back-building. Sunshine with
help from a west/downslope surface wind will help the maxes touch 90
in the southeastern third of the area. Afternoon dewpoints in the upper
50s and lower 60s will keep heat-related flags on the shelf
Short term /6 PM this evening through 6 PM Tuesday/...
most likely...a broken assortment of storms will be on-going at
the start of the short-term. Some organization from multi-cell
clusters to bowing segments should emerge due to the Uni-direction wind
profile. The current crop of mesoscale models has a poor level of
continuity among them when it comes to how long convection will
last and when the all-clear will sound. The cold front currently
across the lower lakes will move into the state later today...but
gets lost in the disturbances in the p-filed caused by convective
cold pools. However...by midnight the front should start to emerge
over the eastern end of the state. Will keep probability of precipitation in the liklies
with the uncertainty of where the convection will occur.
Lows will get into the 50s in the northwest but fog may not be a problem
due to some air movement. This usually spells low clouds instead
Tuesday seems like a dry day as high pressure builds in. But...
the heating could again pop an isolated shower in the northern mountains and
perhaps The Laurels. The coverage of anything which would make
precipitation reach the ground is going to be widely scattered. Have held onto
the idea of a low pop in the far southeast as well since the front
should not move too far away. Maxes in the 70s northwest half and 80s southeast
half. Not too far off normals for late Summer.
Long term /Tuesday night through Sunday/...
the upper low over northern Quebec should continue to drift to
the northeast and zonal to slightly northwest flow is expected to
follow as a quasi-stationary boundary hangs out just to the south
of the state. Earlier runs of the GFS/ec had moved the boundary
further north but they have returned southward a bit with this
package. Uncertainty in the placement of the boundary will keep
some pop in the southern part of the state...but most locations will be
dry until Wednesday night or Thursday. Then a stacked low will slide
eastward from the middle Mississippi Valley and may make a more-or-
less constant rain instead of showery/convective precipitation. The
upper level low will finally move off next weekend which will be a
Herald for quieter weather. Temperatures should trend cooler
later in the week.
Aviation /13z Monday through Friday/...
VFR conditions are expected overnight and into Monday. Low
confidence in localized fog/br visibility restrictions over the
southeast airspace where the blyr may decouple toward daybreak.
Included low level wind shear at bfd 06-12z Monday as 40-50kt southwesterly low level jet precedes a cold
front pushing southeast from the Great Lakes. Thunderstorm complex moving southeastward
across central lower Michigan will bring some layered middle-to-high clouds
into the northwest airspace overnight.
The aforementioned cold front will continue to advance east-
southeast downwind of the lower lakes and move through the airspace
during Monday afternoon/evening hours. Scattered thunderstorms are expected
with the cfropa and a few storms may be strong to severe...with
brief IFR visibility in +ra and highly-localized westerly surface wind gusts
at or above 40kts. Will use -shra with thunderstorms in the vicinity between 18-22z to show thunderstorm
impact risk at all terminals.
Tuesday...VFR. Very isolated p.M. Rain showers possible xtrme northern airspace.
Wednesday...VFR. No sig weather.
Thu-Fri...VFR/MVFR with rain possible as low pressure tracks south of