Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
225 PM EDT Tuesday Apr 21 2015
unseasonably cool temperatures and gusty winds can be anticipated
through the week.
Near term /through tonight/...
large upper trough will continue to dominate the forecast
period...with the next shortwave prognosticated to cross later this
evening. In the mean time...cold air aloft and steep low level
lapse rates will support diurnal showers with daytime heating.
However...with very dry dewpoints as indicated in the surface
observation...precipitation may manifest itself more in virga and strong
wind gusts as deep mixing is expected. Instability will remain
meager...but it's placement between -10 & -30c may be enough to
support some graupel. Since the top of the layer may tap into
40kt gusts per the latest BUFKIT data...have made no changes to
the Wind Advisory at this time.
Did bump up the mention of probability of precipitation this afternoon since the hi res
models do suggest expansion of showers and with the convergence
zone indicated just along and south of Lake Erie...probability of precipitation were kept
highest in those locations.
A brief lull in the precipitation may occur around or just after sunset
as we lose heating for destabilization. But...an approaching wave
and cold front will return the chance for showers by late
tonight/early Wednesday. A strengthening upper level jet supports
greater lift for rain by Wednesday morning and all of the models
have higher probability of precipitation...thus categorical probability of precipitation were introduced. With
some weak warm air advection ahead of the front overnight...did keep low
temperatures up a bit. But...lows were still forecast below
Short term /Wednesday through Friday night/...
a fairly prounced short wave trough will continue to push into and
through the area on Wednesday. Strong Theta-E advection off the
surface will likely drive an expansive area of rain across the
southeastern two-thirds to three quarters of the area through the
early half of the day...before drier air starts to enter from the
west. As this drier air starts to enter...however...the main upper
trough will start to sag farther southeastward from Hudson back
toward eastern Ontario. This will allow for decent 850-500 mb
cooling to initiate by Wednesday night. As a result...even with a
bit drier air...substantially stronger lapse rates and increased
instability will allow for chances of showers to remain in the
northern portions of the area as well as in the upslope areas
until probably Thursday night. As the instability
increases...layer temperatures will be falling with 850 mb
temperatures below zero throughout the latter half of the week.
Precipitation type considerations will have to begin to be made
with the convective showers as a result. The net seems likely to
be a mix of rain and snow shower chances by Wednesday night until
even drier and slightly warmer air aloft enters on Friday.
While lapse rates start to stabilize by the weekend and a bit
warmer air aloft comes in...this will likely not be immediately
translated to the surface. In fact...drier air by Friday night
seems likely to mean for substantial clearing going into Saturday
morning. While many areas will challenge or break freezing in the
mornings leading up to the weekend...stabilized lapse rates and
drier conditions may make widespread freezing conditions even more
likely going into the weekend. All in all...a fairly cool period
seems on tap for the County Warning Area even as things settle down going into the
Long term /Saturday through Tuesday/...
GFS/European model (ecmwf) models have flip flopped over the last 24 hours on
whether the Saturday system will pass across the forecast area or
remain to the south. European model (ecmwf) now has the more northern track while
the GFS has the southern track. Similar to yesterday...ensembles
support the European model (ecmwf) solution...so have added some probability of precipitation back into the
forecast from Pittsburgh south. Temperatures will remain below
normal through the extended forecast...with several nights of near
Aviation /18z Tuesday through Sunday/...
very little change made to the afternoon taf issuance as coverage
of showers will continue to expand. Dry air at the low levels may
preclude measurable precipitation and also limit any restrictions
that would occur. However...steep lapse rates and some middle level
instability will support gusty winds through late afternoon. Some
graupel may also be possible but this will not be added to any taf
sites until it confidence is higher. West-southwest wind with gusts
approaching 40kts will continue until after sunset...relaxing but
remaining occasionally gusty with the approaching cold front
The best chance for restrictions will return in rain with the cold
front tomorrow morning...and MVFR visibilities and ceilings were introduced.
Outlook.../Wednesday through Saturday/...
periodic restrictions are possible much of the period mainly
north of pit as the upper trough persists over the Great Lakes.
Ohio...Wind Advisory until 7 PM EDT this evening for ohz039>041-
PA...Wind Advisory until 7 PM EDT this evening for paz007-013-014-
WV...Wind Advisory until 7 PM EDT this evening for wvz001>003.