Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
439 am EDT Wednesday Jul 30 2014
the upper level low over the upper Great Lakes and eastern Canada
will persist into the weekend. While the upper low may open up
and lift a bit northward...an off shore high pressure area will
direct more Gulf and Atlantic moisture into the region over the
weekend. Temperatures will remain cooler than normals.
Near term /until 6 PM this evening/...
Middle-Lake thunderstorms continue to slide just a bit toward the
eastern end of the lake. The short wave helping those storms seems
to be just about through. A little light rain out of the anvils
will brush Warren Colorado early this morning. But the next round of
showers and thunderstorms will have to wait until middle-late
morning. What appears to be a pair of previously distinct short
wave troughs should be rotating around the base of the pesky upper
level low this morning and slide overhead through the day. This
should provide plenty of forcing to go along with the instability
downwind of the lower lakes in the heating of the day. Highest
probability of precipitation will be over the northwestern half of the area...where the cooler air
aloft resides. A few showers could sneak into the southeastern half of the
area later in the day. There will be scattered thunderstorms late
this morning and this afternoon...but convective available potential energy look meager off
RUC/NAM BUFKIT soundings...but the GFS is more robust with the
convective available potential energy. Wet bulb zero levels should be pretty low today...hanging
around 7 or 8kft. So while the convection may not grow very
tall...they will have plenty of ice in them. Therefore...have kept
mentions of small hail in the forecast for the northwest. Most recent Storm Prediction Center
day1 outlook does not mention anything specifically about the
Great Lakes...but we still expect to see some pea to dime size
hail out of the stronger cells.
Initially clear skies will grow more-cloudy through the middle-
morning in the northwest. Expansive cumulus field expected again today.
However...plenty of breaks in the sky cover will allow temperatures to
get back to the not-so-Lofty Heights of Tuesday and add a few
degrees on for most locations. Still well below normals. Try to
bank the savings you will have on the a/c this July which should
end up slightly below normal despite the 7 days 90f or better in
Harrisburg this month.
Short term /6 PM this evening through Thursday night/...
the diurnal convection should slide to the east this evening while
dying out. The cold air aloft and cyclonic flow will likely keep a
few showers around through the night in the northwest. Chance probability of precipitation for scattered
coverage should be fine there. Mins will be back closer to normals
tonight...but 40s still possible in the higher elevations.
The upper low throws one more uppercut on Thursday morning...and
scattered/numerous showers expected in the fore-noon. The afternoon might be
mainly dry...but the temperatures aloft are still cool and daytime
instability should be good enough for isolated-scattered showers in the
west/north. Highs on Thursday do have the potential to get back to
normal in the southeast...but the north/west will probably still be 8-10 degrees
below seasonal normals.
For now...the 12-18 hours around the Thursday night period seem
like the least likely time for any showers to develop. The big
short wave will help to heave the upper low a bit to the North.
Heights rise about 12dam from Thursday morning to Friday morning.
Lows Thursday night will probably be similar to the previous few with
skies trying to clear and winds light/variable.
Long term /Friday through Tuesday/...
a deep upper level trough will be present across southeastern
Canada early in the long term period...but will reload with a
significant chunk energy digging into it/S SW flank across the
middle miss and Ohio River valleys late this week and over the
upcoming weekend /as the Canadian portion shears out to the NE/.
This potent shortwave trough /and an associated well-defined right
entrance region of a 90-95 knots 300 mb jet/ will lift northeast
across the middle Atlantic region Friday through Sunday before
shifting to the southern New England coast by the start of next
week. A slow moving /and likely strengthening/ baroclinic zone...
and one or more waves of weak low pressure moving NE along it will
be located across central or eastern Penn during this 2-3 day period.
Periods of unsettled weather /showers and thunderstorms/ will occur later
Friday...Saturday and Sunday. The strength of the large scale
forcing and increasingly anomalous high precipitable water air beneath the
northward migrating jet and its thermally direct branch will bring
the potential for some locally heavy/training showers and thunderstorms and rain...
especially across the eastern half of the state/.
Model details differ slightly with respect to the timing and
location of the strongest mesoscale/larger scale forcing. But all
guidance agrees on the basic evolution and NE movement of this
trough and axis of high 40-50mm precipitable water air riding northward up along the
middle Atlantic Piedmont and spreading across the eastern half of our
County Warning Area.
As a result of these timing/location differences...probability of precipitation were
capped off in the high chance category. But as the finer details
unfold with higher res model guidance over the next few
days...probability of precipitation will undoubtedly trend higher for several periods
between late Friday and Sunday.
Gradual improvement with near normal temperatures is expected for early
Aviation /08z Wednesday through Sunday/...
some SC still across the area.
Main issue will be fog and low ceilings at spots like bfd at
Showers and thunderstorms over Lake Erie at this time.
Models show some activity moving across the region
today. Some storms could have some small hail and gusty
winds...given the cold air aloft.
A stronger 500 mb trough will rotate across the area
on Thursday...thus showers and storms with small hail will
likely be more widespread than today.
Perhaps a small break in the action for Friday.
Models show some chance of showers across the south by
late in the day. Some model spread.
With the longwave trough to the west...not seeing much
chance for any one day being dry prior to next Monday...if
Thursday...scattered thunderstorms and rain impacts possible.
Fri-sun...scattered PM thunderstorms and rain impacts possible.