Area forecast discussion
National Weather Service Pittsburgh PA
328 PM EDT Monday may 20 2013
Synopsis...
the risk for afternoon and evening showers and storms will
continue through Tuesday. A better chance for widespread
activity on Wednesday evening...as an upper level disturbance
moves through.
&&
Near term /until 6 am Tuesday morning/...
cumulus development this afternoon has been widespread and fairly
prolific with even the last vestiges of this morning's stratus
over Ohio being difficult to get rid of. The combination of ample
low level moisture...decent instability...and weak steering flow
has allowed for all of this to occur through the day. As the
evening ensues...cumulus should gradually dissipate...however
convective debris clouds and reformation of some areas of stratus
seem a decent bet overnight. This will result is less than clear
skies through the night.
Low level flow...while weak...should continue to be light out of
the south overnight. Upstream dewpoint readings are generally in
the middle 60s to near 70f. With this in mind...and the likelihood at
least scattered to broken clouds overnight...lows should be fairly
range bound by the high moisture content of the air mass and poor
radiational cooling prospects. As such...lows dropping much below
the middle to upper 60s seem like a bit of stretch. Have generally
kept temperatures above guidance through the night as a result.
Fries
&&
Short term /6 am Tuesday morning through Thursday/...
another day much like today looks to be in store on Tuesday.
Although some model solutions track a weak middle-level vorticity maximum
through the northern portions of the County Warning Area early in the day...the
best chances of showers should generally again be relegated to the
afternoon. Diurnal instability should be a bit more pronounced on
Tuesday afternoon as the low level thermal ridge arches across the
area during the afternoon hours. This should allow for cape values
to eclipse 2000 j/kg per the NAM. With that in mind...shear is
once again rather unimpressive...and given the lack of large scale
forcing during the most unstable portion of the day...relatively
disorganized isolated to scattered showers/thunderstorms seem
likely during the afternoon.
With the low level thermal ridge swinging through and soundings
indicating better mixing during the afternoon Tuesday...things do
look likely to heat up nicely. Temperatures at 850 mb run up
toward +16 to +18c or so on the consensus...which suggests highs
in the upper 80s would be likely. This is quite warm for this time
of year as records are generally only in the lower 90s for
Tuesday.
As the system currently over the upper Mississippi Valley finally
wanders eastward...the weather gets more interesting heading into
Wednesday. First of all...a decent consensus on the formation of a
low level jet over western Ohio could drive dissipating nocturnal
convection into some of the area Wednesday morning. While this
would likely increase low level moisture over the
area...convective debris may make warming less efficient on
Wednesday and limit instability. That said...the upper level
system trudging eastward doesn't seem to bring the best synoptic
forcing into the area until after 00z/8 PM...which is well past
the prime daytime instability hours. However...35-40 kts of 0-6 km
shear with cape values in the 1500 j/kg range along with an
incoming shortwave trough and upper jet right entrance region do
mean widespread thunderstorm development is likely by Wednesday
evening. Probability of precipitation were transitioned a few hours later in the forecast
to suggest a later arrival of the system for Wednesday evening to
account for a bit later arrival of the main system itself.
Colder air floods into the forecast area starting Thursday with
the upper trough axis nearing. As this occurs...we again
destabilize due to more rapid middle-level cooling rather than low
level warming. Shower and thunderstorm chances will therefore
continue through Thursday. Fries
&&
Long term /Thursday night through Monday/...
a cold front moving slowly south from the Great Lakes will bring
showers and scattered thunderstorms for Thursday night. Behind the
cold front Friday the risk of showers will diminish and temperatures
will drop below normal. Dry weather can be expected for Friday night
into Sunday with temperatures falling into the 40s for overnight
lows Saturday and Sunday morning.
&&
Aviation /19z Monday through Saturday/...
broken cumulus ceilings can be expected through the afternoon with
the heating of the day. Ceilings will slowly lift from MVFR to VFR
by late afternoon. Just a slight risk of a shower or isolated
thunderstorm into this evening...but too small a risk to put into
the tafs. Another round of MVFR fog is possible late tonight into
Tuesday morning.
Outlook.../Tuesday through Friday/...
chance for shower/thunderstorms will return Tuesday afternoon and increase through Thursday evening as a
cold front aprchs and moves through. Building high pressure and VFR conds expd
Friday.
&&
Pbz watches/warnings/advisories...
Maryland...none.
Ohio...none.
PA...none.
WV...none.
&&
$$
