Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
447 PM EDT Wednesday Oct 1 2014
the next chance for rain will come with a Friday cold front.
Near term /through Thursday/...
update was issued to adjust for rapidly diminished cloud cover as
mixing and subsidence have finally demolished the stratus layer which
plagued the upper Ohio region today.
However...little change in the boundary layer air mass has
occurred and expect fog and St to redvlp as the night
progresses...especially with eastward developing warm front/warm advection
aloft enhancing the inversion.
A quicker mix out is expected on Thursday morning given the improving
southerly flow in the boundary layer and incrsd subsidence undr shortwave ridging.
The insolation and mixing should drive temperatures well into the
70s...about 5 to 10 degrees above the avgs region wide.
Short term /Thursday night through Saturday night/...
500 mb ridge axis is perched atop the region for one last period before
we are put back into reality with cooler...more seasonal weather.
Despite southeast winds at the surface...they do not occur long
enough nor are they deep enough to promote status / drizzle for
locations that are susceptible /Jefferson Colorado PA S into eastern
Overall trend of slowing precipitation onset continues to be the course of
action. Latest 12z NCEP suite portrays the theme of quantitative precipitation forecast reaching
western fringes of County Warning Area by 15z Friday. Cat probability of precipitation are still good for
this system as all locations should see one to two tenths of rain.
Wet weather will be predominate in the afternoon and evening hours
just ahead of the baroclinic zone. Decent wind field associated
with this system with 800 mb winds of 60kts and 500 mb near 100kts...so
gusty showers and perhaps a thunderstorm is not out of the
question. The strength of the wind field and mixing heights Friday
night may yields gusts between 30-35 knots. Thermodynamic
parameters are marginal with dynamic forcing the large contributor
to any thunderstorm initiation. A chance of thunder remains
Much cooler weather is in store this weekend as 800 mb temperatures plummet
from 14c to 0c. Daytime highs Saturday will feel more like late Oct
as they top out only in the 50s. There will be a period of sunshine
as the dry conveyor belt passes...but clouds will quickly fill in
due to destructive sunshine. Substantial cooling aloft with very
unstable low and middle levels denoted by lapse rates pushing 9ckm-1
Saturday will generate scattered showers. Cyclone track keeps winds
west-southwest Sat night...which should keep temperatures in the 40s for overnight
lows. Even if temperatures were to bottom out in the middle 30s...amount of
cloud cover and pressure gradient at the surface would offset frost
Winds will stay mixed allowing for a mild night Thursday so sided with
the warmer nam12 grids...then took a blend of the forecast and bias
corrected mosguide grids for maximum/min temperatures.
Long term /Sunday through Wednesday/...
cold air aloft will help to keep showers north of I-80 Sunday with
dry and cool conditions elsewhere. Strong closed low aloft will
wobble north of Lake Huron Monday and Tuesday with shortwaves
riding under the low bringing periods of showers. By midweek upper
trough will lift north with zonal flow developing across the Ohio
Temperatures will be below seasonal averages to start the
week...moderating by middle week.
Aviation /21z Wednesday through Monday/...
genl VFR is expected through the evening as stratus layer finally has
eroded. However...condition deterioration is expected through the
night with fog and St development once again undr a strengthening
inversion. IFR is thus forecast for all sites...but improvement
aftr daybreak is expected to be more rapid than today given btr
mixing and subsidence conditions.
Outlook.../Wednesday night through Sunday/...
the next chance for widespread restrictions will come with a Friday cold front.