Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
105 am EDT Thursday Aug 21 2014
an upper low over the Great Lakes will drift slowly southeast
across upstate New York and New England by the weekend. A large area of
high pressure over the Canadian Maritimes will ridge southward
along the east-side of the Appalachians into early next week.
Near term /until 7 am this morning/...
water vapor loop at 05z showing shortwave over western PA rotating
through base of upper low. This feature is sustaining lingering
convection across the central mountains at 05z and latest rap/hrrr
output suggest dwindling rain showers and a few thunderstorms and rain will push into the
eastern counties before exiting the area around dawn. Based on radar
trends and rap/hrrr data...will maintain highest /chc/ probability of precipitation across
the middle susq valley and mountains north of ipt during the pre-dawn hours.
Wet ground...light wind and clearing skies behind shortwave should
promote areas of fog early this morning. Dewpoints indicate min temperatures
will range from the l/m60s across the region.
Short term /7 am this morning through 6 PM Friday/...
the hrrr is pretty positive on little or no convection in the
early morning. The convection and wave it has in the Midwest is
forecast to drive into the Ohio Valley.
Larger scale models show our precipitable water values are relatively low in the
morning and are forecast to slowly rise during the day and into
the evening. The cape in most models comes up into the 600 to
1800jkg-1 range second half of the day.
Best chance of showers and thunderstorms would be in the
afternoon. Most guidance still tends to support this.
Older sref and other models seem to imply this with more members
with rain later in day. Implied lull is in the 09 to 18z
Long term /Friday night through Wednesday/...
current Middle Range model guidance puts US in a ring of fire pattern
late this week and into the weekend...with afternoon convection
possible on Friday and Saturday. Strong flow aloft should enhance
this possibility though lack of moisture of Friday could inhibit
convection. Gefs is showing precipitable water anomalies on Saturday as
subtropical high is prognosticated to edge toward the region from the
Midwest. Current set up focuses threat on western and central
PA...where eastern PA is likely to remain dry. Despite approach of
upper ridge...no big warmup expected...as large surface high over the
Canadian Maritimes will circulate an easterly flow into central
PA. Temperatures this weekend look to be below normal. There may be
issues with am low clouds/fog due to the expected moistly southeasterly
flow over the area. Drier weather looks to be in store into early
next week as ridge builds over the region.
Aviation /06z Thursday through Monday/...
not a lot of change to 03z tafs.
Earlier discussion below.
Did not change a lot on the 00z taf package from the earlier
Main concern is convection across the northwest near bfd. Other
widely scattered showers and storms could move east into
the rest of the area overnight.
Main issue for AVN concern overnight will be fog and low
clouds with light/vrb winds and high blyr moisture/surface dewpoints.
For this...went close to earlier forecast. Did lower ceilings in
some cases. A blend of guidance and timing from last night.
For Thursday...expect less shower and thunderstorm activity...as
the wind shift line and dynamics shift eastward.
Fri-Sat...am low clouds/fog followed by periods
of showers/thunderstorms and brief MVFR/IFR restrictions.