Area forecast discussion
National Weather Service Pittsburgh PA
951 PM EDT Tuesday may 21 2013
Synopsis...
thunderstorm chances will increase Wednesday and Thursday as a
cold front approaches and moves across the region. High pressure
will return Friday.
&&
Near term /until 6 am Wednesday morning/...
isolated thunderstorms remain from near Pittsburgh south along
I-79 and are quickly dissipating. Other showers and storms across
central West Virginia are weakening as they move northeast with
loss of daytime heating. What is left of these showers may brush
Morgantown area before ending. Low chance probability of precipitation between Pittsburgh
and Morgantown until midnight. Otherwise partly cloudy with middle
and high clouds crossing the region. No changes to overnight
temperatures.
&&
Short term /6 am Wednesday morning through Friday/...
a very pronounced low level jet is evident on all guidance by
Wednesday morning nosing into southwestern Ohio ahead of an
incoming upper level wave. Guidance continues to be a bit varied
on its placement and evolution going into the later morning
hours...however enough guidance members bring it into central Ohio
by late morning that thunderstorms may well develop along the nose
of it. In conjunction...several consecutive NAM runs have depicted
a compact shortwave embedded into the southwesterly flow to track
over western and central Ohio from late morning through early
afternoon. As this occurs...convection is likely to break out. In
addition...with cape values around 1500 j/kg and stronger 0-6 km
shear likely over Ohio than points farther east...better chances
of thunderstorm development exist in Ohio than the rest of the
forecast area through most of the daytime on Wednesday. With
decent middle-level support and shear/instability...damaging winds
will be possible with any multi-cellular development that does
occur from late morning Onward in Ohio...with a lesser chance
farther east.
As the upper level wave tracks farther east Wednesday
evening...the right entrance region of the upper jet tracks
directly over the County Warning Area. With is in concert with the main synoptic
wave passage roughly 06z/2am through Pittsburgh Wednesday night.
Both NAM and European model (ecmwf) solutions are suggestive of linear development
along the frontal boundary as it passes through the area late
Wednesday evening/night...resulting in the possibility of a squall
line managing to persist well past the normal diurnally favored
hours owing to large scale support for ascent. Chances of severe
thunderstorms with damaging winds were thus allowed to spread
across the rest of the County Warning Area from roughly 8 PM to 3 am as this upper
wave passes through.
Colder air floods into the area on Thursday...however with the
middle-levels cooling much quicker than the low levels...showers and
thunderstorms will again by possible across the area with surface
temperatures moderating toward 70f.
The dump of much cold air is held back for a secondary frontal
passage on Friday with a wide spread in guidance for afternoon
temperatures. European model (ecmwf) and GFS 850 mb temperatures in the afternoon
hours straddle 0c...which would suggest temperatures would
struggle to reach 50f if sunshine failed to materialize. At the
moment...it would seem more reasonable to expect at least some sun
on Friday...so high temperatures were generally adjusted for the
middle 50s to middle 60s across the area. As conditions clear by the
afternoon and evening Friday...cold air advecting into the area
will likely all for a very rapid cool off going into the evening
and possibly frost by the overnight. Fries
&&
Long term /Friday night through Tuesday/...
the models are in good agreement on the overall upper level pattern
through the extended with an upper level ridging of the central
Continental U.S. And troughing in both the west and east. While this would
generally lead to a much less than confident forecast in the
shortwaves within the flow...this doesn't seem to be the case as
both the GFS and European model (ecmwf) suggest that much of the precipitation will
be suppressed south of the region through much of the period.
Thus...opted to take a blend of the models and HPC
guidance. Also...hedged a bit cooler with temperatures with
northwest flow present especially for the first half of the
period. This kept temperatures below seasonal averages Sat/sun
before moderating closer to normal for the rest of the period.
&&
Aviation /02z Wednesday through Sunday/...
VFR conds expd into the night. Isolated thunderstorms in central Ohio...but upper and
middle level ridge and capping limiting activity. Will monitor thunderstorms but
not planning on including in tafs at this time. A period of MVFR visible in br
expd toward sunrise then VFR returns through Erly afternoon. Scattered thunderstorms experimental
later in the afternoon as a cold front begins to aprch so included thunderstorms in the vicinity.
Outlook.../Wednesday night through Sunday/...
occasional restrns likely Wednesday evening into Thursday night with showers/thunderstorms as a
cold front aprchs and moves across the region. An upper level disturbance will
bring possible restrns in rain showers Friday before high pressure and VFR conds
return Friday night through sun.
&&
Pbz watches/warnings/advisories...
Maryland...none.
Ohio...none.
PA...none.
WV...none.
&&
$$
