Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
617 PM EDT Monday Oct 20 2014
rainy and cool through middle-week when upper low pressure system
shifts eastward. Warmer and drier weather is expected towards the
end of the week.
Near term /through Tuesday/...
early evening update to add Cat probability of precipitation farther south tonight based
on radar trends showing bands of showers developing well into southern
Ohio. Also speed up timing for showers into ridges early tonight.
Modest quantitative precipitation forecast added. No changes to temperatures or winds. Previous
SW trough and attendant vorticity maximum evident on WV imagery currently
swinging through eastern Indiana south of the surface nearing Lake Huron.
Middle-level deformation producing high clouds ahead of trough axis
over WV and SW PA will push east tonight. Those clouds will be
replaced by lower clouds this evening keeping temperatures moderated.
Radar returns have been quite numerous today...but little more
than virga has been reported up to this point as dry air from the
middle levels down are keeping any rain showers from reaching the
ground. Rain showers associated with the SW trough are showing up
over western Ohio and are timed to reach our western periphery by about
21z. By 00z all of the near term models have the SW trough axis
through eastern Ohio with rain showers overspreading the area from the
west-northwest. Deeper moisture availability and better forcing will remain
closer to the low near Lake Erie...thus will continue the
categorical probability of precipitation there. Although the middle to upper level moisture
begins to outrun the trough axis across our south...enough moisture
is available to produce showers across northern WV. Will not deviate
from the likely probability of precipitation.
Tonight the surface low is pulled south away from the Great Lakes as
the upper low begins to cut-off. As this happens...abundant low level
moisture will shift over the area keeping clouds and rain in the
forecast. As the upper low slowly passes Tuesday...the surface-800 mb low core
shifts east bringing an upslope component to the low level wind field.
Will keep categorical probability of precipitation in for the southeast ridges as a result. Driest
location in the County Warning Area Tuesday looks to be over eastern Ohio where drier air
aloft begins feeding into the upper low which may work to suppress
Short term /Tuesday night through Thursday/...
cold and relatively unstable northwest to northerly flow continues
over the area from Tuesday night into Wednesday night. While the
middle-level cold pool begins to depart to the east by Wednesday and
Wednesday night...amply boundary layer moisture...low level
instability...and upslope flow should maintain low end chances of
showers in the lower elevations and fairly good chances of showers
in the ridges. As shear increases after this time...mid-level
ridging starts to impinge...and drier boundary layer air finally
starts to work into the region...shower chances dwindle to less
than mentionable beyond Wednesday night.
The latter half of the week now looks rather dry...with
temperatures moderating toward normal as sunshine should finally
make a return to the upper Ohio Valley. Fries
Long term /Thursday night through Monday/...
eastward progress of the middle-week upper low with expanding ridge over the
central Continental U.S. Should ensure a generally dry and seasonably cool close to
the week. Tweaked wpc numbers were used to represent the long term
Aviation /22z Monday through Saturday/...
VFR conditions are expected with increasing clouds through this
evening ahead of an approaching shortwave. This shortwave and
associated coldfront is expected to bring MVFR conditions and
increasing shower chances tonight. As the upper level low pivots
and drifts south Tuesday low clouds and rain chances are expected to linger.
MVFR ceilings much of Tuesday...but could see IFR in any rain showers
Aviation /Tuesday through Friday/...
periodic restrictions are possible through middle week as an upper low
drifts across the region.