Area forecast discussion
National Weather Service Pittsburgh PA
1134 am EDT Tuesday may 21 2013
Synopsis...
periodic shower and thunderstorm chances will return today...
with increasing chances Wednesday and Thursday as a cold front
approaches and moves through. High pressure will return Friday.
&&
Near term /until 6 PM this evening/...
late morning trends are indicating dramatic temperature rises
across western Pennsylvania in particular. New Castle was already
83f at 11 am...with near 80f common across the Pittsburgh area.
Temperatures were nudged upward for this afternoon in the
aforementioned area...and maintained elsewhere.
Previous discussion follows...convective debris clouds are
currently in transit across the forecast area and concurrently
dissipating. Temperature readings are several degrees higher than
this time yesterday as well. With little in the way to prevent
abundant insolation across much of the area...and the fact that
the low level thermal tongue will be traversing The Heart of the
County Warning Area at maximum heating...with 850 mb temperatures nearing +18c around
21z over Pittsburgh with decent boundary layer mixing...it would
seem temperatures should surpass those reached yesterday. As
such...highs were increased into the upper 80s for the southern
two-thirds of the County Warning Area...and the middle 80s up along I-80.
With increased heating and similar dewpoints as compared to
yesterday in the upper 60s...cape values look to run up toward
3000 j/kg. Given a weakly sheared environment and no significant
synoptic or mesoscale disturbance to harness the cape...widespread
thunderstorm development is not anticipated. However...weak local
forcing seems likely to allow for scattered showers and
thunderstorms to develop across much of the area by
afternoon...particularly in areas west of Pittsburgh as well as in
the ridges. Probability of precipitation were increased in these areas. With weak shear
and weak momentum in the column...as well as a rather warm
profile...updrafts to sustain hail development will be difficult
to maintain...however a wind gust or two may be possible out of a
few of the cells. Fries
&&
Short term /6 PM this evening through Thursday/...
boundary washes out this evening...and with the loss of daytime
heating...convection should dissipate. As the ridge continues to
weaken...waves of energy...breaking away from the main Midwest
system...will ride over top of what is left of the ridge through
northern PA. Will go with low chance probability of precipitation across the north and schc
in the south. Atmosphere stabilizes overnight and any noticeable
forcing is sparse.
Expect convection to get going again late Wednesday as the entire
system creeps ever closer to the upper Ohio Valley. Atmosphere
will again destabilize and the upper level support improves as the
ridge continues to break down. Wind field aloft strengthens
pulling more deep moisture into the region and shear values
increase. Have decided to go with likely probability of precipitation Wednesday due to the
aforementioned factors. Storm Prediction Center continues to keep the area in a slight
risk for Wednesday...so will include wording of possible severe and
mention in severe weather potential statement.
Slow moving front drifts through the area on Thursday...so will
continue with likely probability of precipitation. Thursdays not nearly as unstable as
today or Wednesday...so amount and strength of convection still in
question.
&&
Long term /Thursday night through Monday/...
front will clear the area Thursday night as a shortwave trough swings
through on its heels. This will keep in the chance for showers
Thursday night into early Friday. Behind the cold front Friday
temperatures will drop below normal. Dry weather can be expected
for the second half of Friday into Sunday with temperatures
falling into the 40s for overnight lows Saturday and Sunday
morning.
&&
Aviation /16z Tuesday through Saturday/...
VFR conditions should prevail for much of the day. However since
we remain in the warm sector...plenty of instability will be
present for the development of some thunderstorms especially if we
reach convective temperatures today...although coverage remains in
question. For now...with confidence that any one thunderstorm will
impact a taf site remaining low...continued thunderstorms in the vicinity mention. There
could be temporary restrictions to visibility in any showers that
do develop...but also opted to keep this out at this time.
Overnight fog could once again develop as the low levels remain
moist.
S-SW wind will continue at 5-10kts today and overnight.
Outlook.../Wednesday through Saturday/...
showers and thunderstorms will increase in coverage with possible
restrictions from Wednesday into Thursday evening as a cold front
and upper trough approaches and crosses the region. Building high
pressure will bring VFR Friday and Saturday.
&&
Pbz watches/warnings/advisories...
Maryland...none.
Ohio...none.
PA...none.
WV...none.
&&
$$
