Area forecast discussion
National Weather Service State College PA
233 PM EDT Wednesday may 22 2013
Synopsis...
scattered afternoon and evening showers and thunderstorms will produce
locally strong wind gusts over portions of central and western PA
through early tonight. A cold front trailing the low will cross
the region on Thursday...followed by a secondary cold front on
Friday. A Canadian high pressure system will usher in cooler and
drier air that will keep temperatures below normal for Memorial
Day weekend.
&&
Near term /through tonight/...
...
Severe watch #211 in effect until 1000 PM EDT for far western and
northern counties...
Early afternoon visible loop shows stratified cumulus across
the Allegheny Highlands. Laps soundings starting to destablize
with surface temperatures approaching convective temperatures...and up to 1400 j/kg
above the level of free convection. Upstream activity over WV has continued to but this
area has overall been weakening and slightly shrinking in coverage
over the past 2 hours.
Main forecast concern in the near term is convective initiation
and eventual intensity this afternoon and tonight. With the area
is on the western periphery of amplified upper ridge...and
additional shortwaves lifting through the Ohio Valley... western
PA will be in an increasingly convergent boundary layer flow as
30-40 knots 850 mb jet noses into eastern Ohio. With cin appearing to
dissipate on schedule expect to see scattered thunderstorms
develop after 18z over the higher terrain and west. With speed
shear of 25 to 40 kts and moderate cape...some of storms may be
capable of damaging wind gusts from bowing line segments and
clusters into this evening. Peak intensity likely between approx
21-03z.
Another very warm and muggy afternoon is in store with maxes
reaching well into the 80s...~10f above normal for this time of
year.
&&
Short term /Thursday through Thursday night/...
area remains in Storm Prediction Center see text outlook Thursday. Generally weak instability
is prognosticated in the pre-frontal warm sector...and will be dependent on
the extent of any prefrontal warming/partial clearing. Moderate
shear present which could promote a few stronger storms ahead of
the front Thursday afternoon...especially east.
Continued eastward progression of the expansive eastern Continental U.S. Trough is
expected this period...however it may be at slower pace as some
model data is trending slower/sharper by the end of the period.
The attendant cold front will traverse the area during the afternoon
and evening before exiting the eastern part of the County Warning Area around
midnight.
A lighter...deformation precipitation axis is prognosticated to set up over the
area Thursday night into Friday as a shortwave embedded in the larger-
scale trough digs from the Great Lakes toward WV. There are some notable
model diffs heading into day 3 surrounding the depth and speed of
the 500 mb trough crossing the middle-Atlantic region. More on this in the
long term section.
&&
Long term /Friday through Tuesday/...
the 22/00z NAM/GFS and now ecwmf are slow and sharp with the base
of the 500 mb trough crossing the mid-Atlc/northeast states Fri-sun.
These data also show a closed middle-level low rotating within the
larger-scale trough across northern Virginia Friday before pivoting northward into New
England this weekend. Wpc is still leaning away from the closed
solution however they have given a little more weight to the oprn
GFS/European model (ecmwf) trends. The implications are important especially in the
day 4 period where the guidance spread is the greatest/most
noticeable. To combat this...used a intermediate approach taking a
blend of the 00z European model (ecmwf) and GFS...with slightly more weighting
toward the GFS given wpc preference toward the gefs mean and an
overall better fit to the oprn/ensemble consensus.
Expect plenty of clouds and light rain in the deformation zone on
Friday...with temperatures falling back to 55-65f. The European model (ecmwf) solution would
linger light rain over the area /esp in the east/ on
Saturday...but did not fully buy-into this given how strong it was
with the closed 500 mb low compared to the other global guidance.
Canadian high pressure extending southward from Nunavut into the upper
Midwest will shift eastward across the Great Lakes and eventually take up
residence over the northeast by Memorial Day. Ens data continues
to show negative thermal/moisture anoms supporting an unseasonably
cool and dry weekend with the potential for frost in the northern and
western mountains Sat/sun am. The growing season is underway in this area
/as of may 20th/ and therefore will continue to mention frost in
severe weather potential statement. Elsewhere low temperatures will be chilly by late-may standards...but
not quite cold enough for frost.
Q-stnry/warm frontal zone initially centered from Iowa-WV should
slowly return northward through the Ohio Valley early next week. GFS/ec diffs
in the pattern evolution continue into the end of the medium range
particularly upstream. In general...there should be a gradual
early-middle week warming trend with heights rising as the upper trough
continues to move away from the northeast.
&&
Aviation /19z Wednesday through Monday/...
scattered rain showers/thunderstorms and rain and MVFR ceilings this afternoon and evening from west to
east. Isolated IFR ceilings/visibility and strong wind gusts to 40 kts may be
possible near any thunderstorms into this evening. MVFR ceilings
should prevail for a good part of the overnight and some fog
is possible between 10z and 13z lower susq.
Cooler and breezy for Friday into Sunday...as a strong cold
front drops southeast of PA.
Ho
outlook...
Thursday...scattered thunderstorm impacts possible...mainly in the PM.
Fri-Mon...no sig weather expected.
&&
Ctp watches/warnings/advisories...
none.
&&
$$
Synopsis...devoir
near term...devoir
short term...devoir/steinbugl
long term...steinbugl
aviation...devoir
