Area forecast discussion
National Weather Service Pittsburgh PA
423 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/...
expect most of the morning to be quiet with increasing high
clouds...the remnants from last nights storms over the Midwest.
Convection will likely hold off until the afternoon...when the
atmosphere rapidly destabilizes (capes of 3000-3500 j/kg)...and a
weak frontal boundary moves through. This is the first real
trigger since last week. The front itself is weak...only
noticeable by a surface wind shift. Additionally...there is not
much upper level support...with a small shortwave swinging across
the north late today. However...models are showing low and middle
level forcing increasing with the boundary and low level moisture
pooling. The upper level ridge will begin to break down this
afternoon...and if we can reach our convective temperature...in the middle
80s...not much to stop the parcels from rising. That may be the
most important question...whether we can reach the convective
temperature. With full sun...have no doubt we would go above the middle
80s...however concerned about the cirrus blow-off...from previous
Midwest convection...that will move over the area today. If the
high clouds hold together...could negate just enough sunshine to
keep our temperatures down a couple of degrees. Will continue with high
chance probability of precipitation later today...doing so with uncertainty on if widespread
convection can develop. Could see a scenario where a line of
convection develops over ec Ohio and northwest PA early this afternoon
and barrels through...reaching the southeast mts by evening.
The wind field aloft is not overly impressive with winds of 25 to
35 kts...however...with wbz levels ranging from 9500-11000 hail
would be a good bet.
So if convection does get going this afternoon...there will be
some wind gusts...but think the main concern would be large hail.
Will include the wording of gusty winds and hail with thunderstorms.
For highs today...will lean close to the latest lamp as those
numbers have worked well that past couple of days.
&&
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 /08z Tuesday through Saturday/...
light south winds will remain through the overnight. Another
round of MVFR fog is a good possibility through the middle morning.
Scattered afternoon thunderstorms are a possibility...with
slightly more coverage expected than yesterday. For now will
include thunderstorms in the vicinity from kpit north and west in terminals. Did not have
enough confidence in yet another round of MVFR fog to include in
30 hour pit taf...with thunderstorm coverage and associated
precipitation likely playing a role in this forecast.
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 passes across the region. Building
high pressure will bring VFR Friday and Saturday.
&&
Pbz watches/warnings/advisories...
Maryland...none.
Ohio...none.
PA...none.
WV...none.
&&
$$
