Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
915 PM EST Thursday Feb 26 2015
a disturbance will bring chances for light snow through this
evening. Dry and frigid conditions will end the week.
Near term /through Friday/...
with 915pm update...only minor changes were made to forecast. As
quickly as low level clouds began to scatter out around
sunset...low clouds have filled back in at all sites. With the
clouds extending all the way to the Lake Erie shoreline...appears
that any immediate chance of clearing skies and plummeting
temperatures is low. Observed temperature trends have stayed with
forecast temperature trends through the evening hours...and have
made no changes to temperatures with this update. Previous
With 630pm update...primary changes to forecast were to sky cover
and temperatures. Low level clouds are beginning to scatter out in
northeast Ohio and northwest Pennsylvania away from the immediate
Lake Erie shoreline...and with northwesterly flow coming over
Frozen Lake Erie...not sure there will be much of a moisture
source for clouds to reform. As a result...have made some decrease
to clouds in this area overnight...although cloud cover is still
expected to be extensive across southeastern portions of the
forecast area. Following the clearing trend...have blended some
rap temperatures into the forecast. This starts to bring overnight
lows down a few degrees where the greatest clearing will
occur...with little to no change in the forecast where clouds will
remain thickest. As previously discussed...if more scattering
occurs than currently forecast...some locations north of
Interstate 80 could easily have temperatures crater to minus 10 or
lower. At this point...do not think such an aggressive change
needs to be made to the forecast. Previous discussion follows.
Very light snow will continue through the rest of the evening as
the Arctic front crosses the area. The most recent run of the hi
res models have a brief break in the snowfall before ramping up
again as the entire column cools and falls into the -15 to -20
range. However...this window is rather small for all but the
ridges before subsidence takes place. Thus...snowfall forecast
changes were rather minimal...only adding about an inch to the
highest elevations of PA/WV/MD...and generally less than an inch
elsewhere through the first half of the night. Cold advection will
allow for temperatures to once again fall into the single digits
or just below zero overnight. At this point...stayed on the warmer
side of guidance as expect cloud cover to keep temperatures from
bottoming out. However...if clouds were to scatter out
early...temperatures could fall off much more with the snowpack
and light winds expected. Even if temperatures were to fall off more than
currently forecast...the low winds expected with building high
pressure should limit any concerns for wind chill advisories
overnight. Thus...no headlines were issued on this shift.
Cold advection and should limit high temperatures tomorrow.
Short term /Friday night through Sunday/...
with high pressure overhead Friday night/Sat morning temperatures
should drop significantly once again...with the help of strong
radiational cooling and snowpack. Temperatures should moderate on
Saturday however...as winds shift to the southwest and warm
Next system will make its approach on Sunday. As of now...the
surface low looks to move right across the forecast area...with
warm advection and 850 temperatures warm enough to bring a mix to
areas south of I 80...with mainly snow to the north. A brief
period of freezing rain is possible but not enough confidence in
the details to include at this time. Have highlighted this feature
in the severe weather potential statement as some accumulating snow may be possible...depending
on the track and warm air aloft.
Long term /Sunday night through Thursday/...
a pattern change and relative warm up then sets up through much
of next week as western Continental U.S. Troughing develops. Shortwaves are
prognosticated to eject out of the trough and move across the Ohio Valley
region in southwest flow...keeping periodic rain and snow chances
through late week. The best chance appears to be with a
Tuesday/Tuesday night wave. Model quantitative precipitation forecast amounts appear to be
overdone in the warm advection regime...but will monitor as a
decent snowpack remains.
Ensemble based wpc guidance offered a reasonable start to the
forecast...though made adjustments to the latest GFS to warm some
temperatures...with adjustments for precipitation type based on GFS critical
Aviation /02z Friday through Tuesday/...
low level northwesterly flow has advected progressively drier air
over the area early this evening. While the sun was up...even with
dry advection...some stratocumulus developed off the Great Lakes
even with the ice cover due to diurnal heating working with modest
heat/moisture flux off the few cracks that do exist in the ice. As
the evening ensues...expect low level stability to make lake
induced stratus more difficult to maintain...which should lead to
a scattering out of low clouds. That said...middle clouds are set to
increase through the majority of the night as an area of increased
middle-level moisture moves from Indiana through western
Pennsylvania. This should clear the area by morning...resulting in
VFR conditions for Friday with light west-northwest winds. Fries
Outlook.../Friday night through Tuesday/...
restrictions are likely with crossing low pressure Sunday into