Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
332 PM EDT Monday Aug 3 2015
a large upper low centered over northern Ontario will translate
eastward over the next few days. A cold front will push southeast
and through the state late today and early tonight...triggering a
few areas of showers and thunderstorms. 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 am Tuesday morning/...
waiting game is on for convection to begin firing during the next
few hours in the moderately unstable/moderately sheared environment
over the susq valley...and later on across the northwest mountains and Laurel
Highlands of Penn.
The earlier area of convective debris/layered middle and high clouds
has largely dissolved. A distinct pre...cold frontal trough slid
east across the alleghenies and central mountains during the midday
hours...and low level ridging and several degree f of dewpoint drying in
its wake has greatly negated the chance for convection for the next
several hours across the central and western mountains
Last few runs of the hrrr including the most recent 17z version
continues to first develop rain showers/thunderstorms and rain across the middle susq valley
along the aforementioned Lee trough...and then with a surge of
higher precipitable water air beneath increasing dynamics/re region of an 80 knots
Although the Storm Prediction Center day one outlook keeps the slight risk over
almost all of central PA...will make some modifications to the
areal distribution and timing of convection into tonight.
Mixed layer convective available potential energy of 1-2k joules and good deep layer shear will
create an environment where storms will grow moderately 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. In addition...downdraft cape
per Storm Prediction Center mesoscale analysis Page has increased to 1000-1200 j/kg across
roughly the southeastern half of the County Warning Area at 16z.
Marginally large hail is the secondary severe threat in this
Heavy rain does not seem to pose much of a threat late
today/tonight based on the amount of shear noted above...and lack of
back building. Mesoscale Beta element vectors via BUFKIT hourly
soundings imply much more forward propagation of storms from the
Temperatures will climb another 2-4 degree f through 21z reaching maxes in
the u70s northwest...l-M 80s central zones and u80s to around 90f across
the lower susq valley.
Several clusters of rain showers/thunderstorms and rain will develop over/move east into the
County Warning Area this evening with the cfront pushing southeast through the state
overnight. Coverage of the convection is mainly scattered...but a
few locations...especially over the middle susq valley warrant low
likely probability of precipitation to cover developing convection there and the
associated transition between the two...12 hour periods this
Lows early Tuesday will get into the 50s in the northwest but fog may
not be a problem due to some air movement. This usually spells
areas of low clouds in contrast to fog.
Short term /6 am Tuesday morning through 6 PM Tuesday/...
a rather strong upper trough and secondary cfront will be diving
southeast from the upper glakes region. Although the low level environment
will be drier and slightly cooler in the wake of Monday night/S
cfropa...strong forcing beneath the l.E. Region of a 95 knots jet will
likely spark at least isolated to scattered thunderstorms and rain across the
central and northern mountains
Maximum temperatures will vary from the 70s across the northwest half of the
County Warning Area...to the 80s in the southeast half. Close to normal for late Summer.
Long term /Tuesday night through Monday/...
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. Uncertainty remains in the placement of the
boundary so will keep some pop in the southern part of the state...but
most locations will be dry until Thursday into Thursday night.
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 late next weekend which will be a Herald for quieter weather.
Temperatures should trend cooler later in the week before warming
up into next week.
Aviation /19z Monday through Saturday/...
an approaching cold front and preceding surface/Lee trough over the
susq valley will act as two areas of focus for late
afternoon/evening rain showers thunderstorms and rain with scattered/brief MVFR ceilings and visibilities.
A few of the storms may be strong to severe...with very brief IFR
visibility in +ra and highly-localized westerly surface wind gusts at or above
40kts. Will continue to use -shra with thunderstorms in the vicinity between 20-22z to show
thunderstorm impact risk at all terminals.
Tuesday...VFR. Isolated-scattered p.M. Rain showers/thunderstorms and rain central and northern Penn.
Wednesday...VFR. No sig weather.
Thu-Fri...VFR/MVFR with rain possible as low pressure tracks south of
Sat...VFR with a low chance of MVFR in isolated afternoon rain showers.