Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1141 PM EDT Tuesday Aug 19 2014
a massive ridge will build over east-central Canada for the next
week. Another strong ridge will build over the southern United
States. Short-waves embedded in the weak flow moving under the
Canadian block will slow down and weaken as they move through our
region. This will lead to a potentially prolonged period of
unsettled weather. But it should protect US from the late season
heat building to our south.
Near term /until 6 am Wednesday morning/...
dry conditions will prevail for most or all of the overnight hours
across scent Penn and the susq valley with patchy fog forming as a
result of clear to partly cloudy skies...earlier scattered evening
showers...light and variable wind...and moderately high surface
dewpoints in the lower to middle 60s.
Across the western zones...increasing probability of precipitation were painted in as a
gradually weakening line of showers/thunderstorms and rain slides east from Ohio.
Although the best dynamic forcing /linked to a compact upper
shortwave/ heads east-northeast across southern New York...the greatest ml
instability and higher precipitable water air will be focused across The
Probability of precipitation will reach categorical 80-100 percent across a few of the northwest
counties between 03-06z before gradually decreasing further S and
east as the convection slowly weakens and fragments.
Shortwave ridging aloft over central Penn for the next several
hours will maintain dry and partly cloudy conditions...with
scattered showers and isolated thunderstorms and rain moving east into the region
Temperatures will slowly cool off in the 60s in all areas...reaching
fairly uniform lows in the lower 60s early Wednesday morning.
Short term /6 am Wednesday morning through 6 PM Wednesday/...
a surge of cape and precipitable water into western PA in the sref and
NAM...similar to older gefs/GFS imply some more organized rain
possible on Wednesday. Used a blend to get the probability of precipitation and the 12km
NAM to add some texture to convection and rain area that appears
to be poised to move west to east across the region Wednesday.
The sref probability of precipitation/pdf implies showers coming into west around 1500 UTC
and most of central two thirds of region by late evening. Sref is
kind of optimistic southeast PA could have yet another mostly
rain free day Wednesday.
The key features are the retrograding ridge over eastern Canada
which is taking the umph out of the systems cutting under this
feature. In most models the near 1sd above normal precipitable water surges weaken
as the move into PA. Thus more optimistic...sref-like in the west
than the east with rainfall and not really confident going likely.
Long term /Wednesday night through Tuesday/...
the weakening ridge over eastern Canada will allow for divergent
flow as the upper level low slowly moves eastward. Daytime
heating...decreasing pressure...moist southeasterly surface flow...and
west to northwesterly flow aloft should allow for enough
shear...for chance to likely probability of precipitation early Wednesday evening. Cape
values are low...with an average of 500-1k j/kg throughout most of
central PA. So scattered showers and thunderstorms remain possible
early Wednesday evening. These will slowly fade as the evening
Thursday looks like another unsettled day...as upper low makes
it/S closest pass to PA...tracking across upstate New York...with the
850mb low moving through PA. Cool temperatures aloft...combined with
diurnal heating...should yield numerous rain showers and thunderstorms and rain by the
afternoon. Have continued probability of precipitation in the likely category based on 12z
operational and gefs output. Due to the changing position and
timing of the low...have slightly lowered probability of precipitation along the southern
half of the County Warning Area. The southerly ll jet and plume of high precipitable waters will
have passed NE of the area Thursday...limiting the threat of any
heavy rainfall. Model blended quantitative precipitation forecast is in the quarter to half inch
range Thursday. Before the low passes...temperatures could reach up to
the low 80s. After Thursday/S numerous showers and the advection
of cooler temperatures aloft...high temperatures should drop below normal for
August...with maxes mainly in the 70s through the end of the work
week into the weekend.
Latest medium range guidance pointing toward drier weather Friday into
early next week...as subtropical high is prognosticated to edge toward
the region from the Midwest. Daily ring of fire convection on NE
periphery of this ridge will threaten western PA during the
Friday-sun time frame...while eastern PA is likely to remain dry.
Despite approach of upper ridge...no Big Warm up expected...as
large surface high over the Canadian Maritimes will circulate an
easterly flow into central PA. There may be issue with am low
clouds/fog due to the expected moistly southeasterly flow over the area.
Aviation /04z Wednesday through Sunday/...
an isolated shower will be possible through 1 am across south
Most of the activity is over Ohio and moving east. Tops were
cooling...but now storms are weakening some.
Minor changes made on the 03z package.
Otherwise the main concern will be MVFR and IFR conditions
late...given light winds...mainly clear skies...and dewpoints
higher than we have seen them for much of the Summer season so
Expect things to improve Wednesday morning...but the remains of
convection to the west and the upper level trough moving in
will result in at least some showers. Some thunderstorms
with heavy rain will be possible.
Not a lot of change in the pattern before the weekend if
Thu-Sat...morning fog chances each day. Overall VFR...but chance
of mainly afternoon rain showers/thunderstorms and rain and associated restrictions.
Sun...mainly VFR. Still chance of rain showers in the morning.