Area forecast discussion...updated
National Weather Service Pittsburgh PA
115 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 /through today/...
quiet night across the region under strengthening ridge. Large
area of activity well to the west and another area swinging into
southern Canada.
Expect most of the morning to be quiet with increasing high
clouds...the remnants of the large area of 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 with little to no upper
level support. 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 am concerned about the cirrus blow-off...from 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 few degrees. Will continue with high chance probability of precipitation
later today...doing so with uncertainty on if we can get more
widespread convection to 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.
For highs today...will lean close to the latest lamp as those
numbers have worked well that past couple of days.
&&
Short term /tonight 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 every 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. Have decided to go
with likely probability of precipitation Wednesday due to the aforementioned factors.
Slow moving front drifts through the area on Thursday so will
continue with likely probability of precipitation. Thursday not nearly as unstable as
today and 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 /05z Tuesday through Saturday/...
diurnal cumulus dissipating as heat of the day has passed. Light south
winds will remain through the overnight. Another round of MVFR fog
is a good possibility late tonight into Tuesday morning. Much the
same scenario for Tuesday with chance probability of precipitation for scattered afternoon
thunderstorms as models do show a shortwave approaching after 21z.
For now will include thunderstorms in the vicinity from kpit north and west in terminals.
Outlook.../Tuesday night 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.
&&
$$
