Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
636 am EDT Friday may 29 2015
multiple disturbances will bring unsettled weather today through
the weekend...along with warm and humid conditions.
Near term /through tonight/...
630 am...updated for sky/temperature trends. Can see plume of
moisture starting to work its way north across eastern Kentucky on
WV loop. Think western counties and/or southeast ridges will see
first rain showers in our County Warning Area...with other areas filling in early to middle
Dry air mass in place at present...but this will start changing
shortly. With h500 ridge axis pulling east and surface high
tracking into upstate New York...return flow will begin to push
dewpoints back into the 60s by midday. This happens in Ohio
first...where slight chance probability of precipitation are introduced by late morning as
the lower and middle levels quickly moisten up on BUFKIT soundings. A
middle-level impulse riding up the back of the departing ridge will
help to provide a trigger despite warm temperatures aloft keeping
cape profiles from becoming very deep. Thus...scattered showers
and thunderstorms will develop areawide during the afternoon.
Along with the relatively modest cape...shear is unimpressive...so
severe storms are not a threat. Brief heavy rain is a possibility
with precipitable water between 1.5 and 1.75 inches...but warm cloud depths
should remain below 10k feet during the day which will put a lid
on rain potential.
Wave departs by evening...and expect that storm coverage will drop
off by 06z as instability and trigger is lost. Next frontal
boundary will be making a push from the Great Lakes by 12z
though...and elected to bring chance probability of precipitation back into the northwest
counties by that time.
Temperatures nudged up slightly towards model consensus for
daytime highs. Little change to values for tonight. Cl
Short term /Saturday through Sunday/...
warm southerly flow will continue into Saturday with the cold front
still well to our west and a broad Atlantic high to the east. Strong
surface heating will bolster low-level lapse rates...combining with
the moist airmass will generate pre-frontal showers and
thunderstorms. Stronger middle- and upper-level flow remains well to
our north...closer to the deeper wave. Still...enough instability
and shear exists for thunderstorm development...especially in the
late evening hours as the boundary nears our northwest and upstream
thunderstorms can progress eastward.
Deep upper-low advances to near James Bay Canada late Saturday with
an elongated deep moisture field extending from the Mississippi
Delta through eastern Quebec. Biggest question remains how far east
that deep moisture progresses Saturday night into Sunday. 00z GFS is
fast as it has moderate perpendicular middle-level flow driving the
boundary and associated moisture through pgh by midday Sunday. The
00z NAM and European model (ecmwf) dampen the middle-level flow and stall the boundary
overhead...keeping ample moisture in the area. Thinking the latter
solution is more plausible given that the driver to the fronts
eastward progression exits well to our north and east. With this in
mind...will keep high chance and likely probability of precipitation in the forecast for
Sunday afternoon and again Monday as another upper-level wave
Temperatures will be well-above average Saturday and Saturday night
with the warm southerly flow. Sundays temperatures will be somewhat
dependent on the placement of the front...but with increased cloud
cover...should be held in check and closer to the average.
Long term /Sunday night through Thursday/...
upper-level wave approaching the Great Lakes Monday night will
effectively sweep the deeper moisture out of the area. Model
guidance a lot more optimistic than 24hrs ago with re-building the
surface high over Pennsylvania middle of next week. This leads to a
drying trend with decreased cloudiness. The near average
temperatures that start the week will increase through the latter
part of the week as the high pressure shifts east and southerly flow
Aviation /12z Friday through Tuesday/...
still expect the bulk of the next 24 hours to feature VFR
conditions. However...with moisture increasing from the
south...clouds will be on the increase this morning. Scattered-broken cumulus
deck below some higher clouds at first...then scattered rain showers/thunderstorms and rain
will develop beginning around midday as a midlevel wave arrives.
Still handling this with thunderstorms in the vicinity as any restrictions will be brief
and hard to time...but amendments will be needed once
precipitation starts to develop. Activity will wane as daytime
heating is lost this evening...and expect another mostly quiet
night with lingering middle clouds. Some rain showers may sneak close to zzv
just prior to 12z Saturday as a front tries to push closer from
Outlook.../Saturday through Tuesday/...
restrictions are likely through the weekend with a slow moving
cold front...and could continue especially S of pit into early
next week as the front stalls.