Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
811 am EDT Friday Aug 22 2014
a weak upper low over the Catskill region of New York will drift
to the southeast as an extensive upper ridge builds from the
Mississippi Valley into eastern Canada. The effect of these
weather features will be to create enough ridging at the surface
to push a backdoor front gradually westward to the Allegheny
Front by Saturday morning. Scattered showers and thunderstorms
will over the region today into tonight. After a cloudy and cool
start to the weekend with areas of drizzle and fog...conditions
will dry out Sunday and early next week.
Near term /until 6 PM this evening/...
isolated light showers tracking towards the northwest and west
central mountains early this morning as an elongated upper shear
axis is approaching from Ontario and Lake Erie.
Probability of precipitation will be fairly low through about 14-15z. Afterward...the
fairly juicy...approx 1.5 inch precipitable water air will combine with
developing breaks of sunshine during the midday hours to quickly
increase surface-based convective available potential energy to between 1000-1500 j/kg this afternoon.
Best jet dynamics and low level Theta-E convergence appears to first
target the western mountains /particularly the Laurel Highlands during the
late morning and early afternoon/...before spreading east and
becoming focused across scent Penn and the susq valley later this
afternoon and early evening.
Basin average rainfall should be around 3 or 4 tenths of an inch
over the upcoming 15 hours...though sharp differences will
obviously occur in vicinity of small clusters of rain showers/thunderstorms and rain that are
expected to track east-southeast across the County Warning Area. Highest probability of precipitation through 00z Sat
will be in the likely category and extend from near kjst...NE to
kaoo...kipt and kseg. Lowest probability of precipitation of around 30 percent will be
across the far northwest zones...to the west of kbfd.
Maximum temperatures today will vary from the middle 70s across the northwest...to the
lower 80s throughout the lower susq valley. These readings will be
close to normal for late August.
Short term /6 PM this evening through 6 PM Saturday/...
scattered to numerous showers will be found mainly across our central
and southwestern zones early tonight. As the aforementioned backdoor
pushes westward to the Allegheny Front...convective precipitation will wane
with stratus thickening up/spreading west later tonight. Areas of
drizzle and ridge-shrouding fog are likely late tonight through
Saturday morning...with any low probability for thunder limited to the
area west of Route 219.
Low temperatures tonight under widespread clouds and a light/moist
easterly low level flow will be 60-65f.
The moist/anomalous easterly surface-850 mb flow ascending the
Appalachians should lead to widespread low clouds and patchy
drizzle/mountain top fog persisting through most or all of Saturday.
This will help to keep temperatures well below normal /perhaps by a
solid 8-10f during the day Saturday/. Portions of the central/northern
mountains may struggle to climb out of the upper 60s...with locations
across the susq valley...and west of rt 219 likely reaching the
lower to middle 70s.
Long term /Saturday night through Thursday/...
a weak upper low moving underneath the ridge should allow for a
slight chance for showers Saturday night. Afterward...the building
ridge will be the primary driver of our weather through most of the
Medium range guidance indicates high pressure and associated dry air mass will
drift south late this weekend...bringing fair and brighter weather to
central PA. Ptsunny skies should help temperatures rebound a bit on
Sunday...but still remain blw normal.
Large scale subsidence and dry air should persist...as the Middle
Range models are in fair agreement of a building ridge over the
region through early next week. By middle next week the forecast
confidence begins to lower. By the middle of next week...when medium
range models begin to diverge. Gefs keeps ridge and fair weather going
through Wednesday...while European model (ecmwf) ensemble data indicates a weak cold front
could push southeast into the region...the models seem to be off with
their timing. Have kept a low chance of thunderstorms and rain Wednesday into Thursday. What
appears certain is that the cool temperatures of this weekend will give
way to a slow warming trend through the first half of next week...as
cool air mass modifies under still strong August sun. Gefs and ec
ens 850 temperatures support high returning to above normal by Wednesday.
Aviation /12z Friday through Tuesday/...
low ceilings and 3-6sm br will erode/improve through late morning
with VFR conds by the afternoon. Confidence remains low concerning the
thunderstorm outlook and convective trends...however brief thunderstorm impacts
are possible. Made only minor changes to previous tafs with the
12z issuance and will allow the 15z intermediate update/18z
scheduled issuance address convective development and evolution.
Any lingering storms should decrease in coverage and intensity
A large area of high pressure will build southwestward from eastern Canada into the
northeast and down the east side of the applchns tonight through the
weekend. Easterly flow should bring low clouds/ceilings into the forecast
with some light rain/dz early Saturday with a few showers possible
later in the day. Look for improving conds sun through Tuesday.
Sat...MVFR/IFR in the morning with patchy fog/br/-dz/-ra...becoming
VFR/MVFR in the afternoon with isolated-scattered rain showers.
Sun...low ceilings possible early then becoming VFR.
Mon-Tue...VFR no sig weather.