Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
802 PM EDT sun Jul 5 2015
a passing upper level disturbance will bring increasing rain
chances tonight and Monday. Unsettled weather will continue into
midweek with an approaching cold front.
Near term /through Monday/...
made an update this evening to adjust probability of precipitation slightly based on
current radar trends and hi res model guidance. There seems to be
enough instability early this evening to support thunderstorm
development along the ridges but with very weak steering
flow...these storms may move very little. This is already apparent
in the cells that developed in Garrett County. With precipitable water values
expected to increase as the low approaches...will have to watch
for continued development over the same area until diurnal heating
wanes. With expansive cloud cover overnight...have made no
adjustments to temperatures at this time...as lows look to stay
elevated in the 60's.
slow-moving and well-defined upper level low currently in eastern
Kentucky will gradually fill as it trudges northeastward into the
central Appalachians through Monday. Progress will be slow until
system is kicked northeast ahead of the Tuesday front. Models are
in good agreement with the overall progress of this system.
Probability of precipitation slowly increase tonight from the south with the approach of
the upper low. 1.5+ inch precipitable water values overspread the area...but
meager support for lift will keep coverage low overnight...save
for the southeast ridges with moist upslope flow. By Monday...low
level lapse rates will steepen enough to allow for more coverage
of rain showers and thunderstorms and rain during the day. Continued southeast flow into the
ridges will enhance coverage there...thus the higher likely and
categorical probability of precipitation. Any thunderstorms will be unimpressive...with
cape below 1000 j/kg and little significant flow aloft.
Reduced diurnal range of temperatures over the next 24
hours...higher min temperatures and lower maximum temperatures...with clouds and
moisture incoming. Still close to seasonable levels.
Short term /Monday night through Wednesday/...
an active weather regime remains in place with southeastern Continental U.S.
Ridge trying to re-establish placing the upper Ohio Valley in the
corridor of the middle level steering flow.
What is left of low pressure slowly trudging northeast from
Tennessee River valley will depart Monday night. With position of
middle level trough axis...opted for precipitation probabilities to be
highest over PA...northern WV...and western Maryland Monday night. With
eastern Ohio on the western flank and anticyclonic differential
vorticity advection removed probability of precipitation for those locations.
Tuesday features main weather maker of the short term as a
baroclinic zone crosses during the late evening hours. 12z NCEP
guidance has slowed down frontal passage a few hours with it crossing after
midnight. Several convective allowing models portray a line of
showers and thunderstorms developing over Ohio and West Virginia
after 21z Tuesday then move them through our region. SBCAPE values
surpass 2.5 kjkg-1 however with model dewpoints prognosticated in the lower
70s...convective available potential energy are inflated. Shear on the other hand is not too
impressive with 0-3km winds around 35 kts. Low level lapse rates
are modest peaking out around 6.5ckm-1. With all that
information...potential is there for strong to possibly severe
storms Tuesday evening. The amount of sunshine or lack thereof will
be a deciding factor in coverage and intensity. Still too much
uncertainty to warrant an inclusion in the hazardous weather outlook
at this time. Did increase probability of precipitation as confidence is higher in at least
Boundary stalls early Wednesday morning somewhere in the vicinity of
I-70 before retreating north during the day due to anticyclone
building over the southeastern United States. This will create a
window for showers and thunderstorms mainly north of the
aforementioned Interstate demarcation. Points south could see a pop
up shower or storm due the amount of moisture present. Most of the
upper level divergence is forecast north of Lake Erie and thus will
any organized activity middle week.
Precipitable waters surpass 2.0 inches Tuesday night into Wednesday coupled with a
warm cloud depth in excess of 11kft resulting in possible downpours.
Temperatures were a blend of the existing forecast and the adjusted
European model (ecmwf) guidance.
Long term /Wednesday night through Sunday/...
model ensemble forecasts continue to show a general zonal flow through
the period...with periodic crossing shortwaves bringing occasional
shower and thunderstorm chances. Opted for wpc ensemble based
guidance to smooth out operational model differences. Main
adjustments to this guidance were for downward pop adjustments and
upward tweaks to the expected seasonally warm temperatures.
Aviation /00z Monday through Friday/...
VFR conditions for the most part through the evening hours with
lowering high clouds. Expecting any rain showers/thunderstorms and rain to be confined
mainly to the WV/Maryland ridges and kmgw area through the evening
hours. Some spotty mainly MVFR fog is possible overnight...clouds
and a slightly stirred boundary layer should keep this from
becoming too widespread.
Low pressure system approaching from the south will spread rain showers
and a few thunderstorms and rain mainly east of kzzv from midmorning Monday
on...although any location may receive a shower. Keeping low VFR
ceilings for now although tempo MVFR/IFR may need to be added pending
Outlook.../Monday through Thursday/...
periodic restrictions are possible through the period with
several crossing disturbances.