Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
613 am EDT Tuesday Apr 21 2015
an upper level low over the Great Lakes will slowly edge eastward
to northern New England by late in the week. A shortwave rotating
along the southern periphery of the upper low will push a cold
front through central PA on Wednesday.
Near term /until 6 PM this evening/...
cold front has pushed east of central PA as of 10z. Middle level
shortwave still producing lingering Post-frontal -shra across the
susq valley. However...much drier air is overspreading the area on
westerly flow and latest radar trends suggest the back edge of the
rain will exit the eastern counties between 11z-12z.
Infrared loop already showing some clearing developing over SW PA and
expect this clearing trend to overspread the susq valley around 12z
as west wind advects drier air into the region. Further
west...shallow layer of moisture ascending the mountains will likely
result in broken stratocu persisting over the alleghenies.
Model data indicates central PA will be in a region of lg scale
subsidence/surface ridging late this morning with dry weather anticipated.
Have reduced probability of precipitation significantly between 12z-18z. Model soundings and
upstream satellite imagery imply there will be a fair amount of sunshine
later this morning...especially east of the mountains...with ptsunny skies
across the northwest mountains diurnal heating/deepening blyr depth will
allow gusty west-southwest winds to mix to the surface during the late am and
afternoon hours. BUFKIT soundings support widespread gusts to around
25kts by afternoon.
Anomalously cold middle level temperatures will promote diurnal convection
during the afternoon...primarily over the northwest mountains lack of deep moisture
will result in light rainfall amts. However...have bumped up probability of precipitation
to between 60-70pct across the northwest mountains based on ensemble quantitative precipitation forecast output.
Per Storm Prediction Center outlook...have included the chance of a PM thunderstorms and rain over the northwest
counties...where model soundings become marginally unstable between
Ens mean 800 mb temperatures support maximum temperatures today from around 50f over the
northwest mountains to the l60s across the lower susq.
Short term /6 PM this evening through 6 PM Wednesday/...
scattered diurnal convection should dwindle after sunset across the
central mountains focus will then shift toward shortwave rotating around
southern periphery of upper low over the Great Lakes. This feature is
prognosticated by models to swing through central PA between 12z-18z Wednesday. Have
increased the chance of rain showers during this time frame based on latest
sref/gefs...which support categorical probability of precipitation across much of the
area. Although clouds should predominate Wednesday...enough early
sun/heating is poss across the lower susq valley to potentially
support an early afternoon thunderstorms and rain from kmdt S and east.
Long term /Wednesday night through Monday/...
the long term pattern will be dominated by a large upper level
low moving from the northern Great Lakes to northern new eng by
the weekend. There is good model consensus through the Middle Range
period...indicating a cold and dry pattern. Weak shortwaves
rounding base of upper trough could support scattered snow showers across
the northwest mountains Wednesday night through early Friday. Despite negative 800 mb temperature
anomalies around 2sd...chilly lakes will limit lake effect
potential. Still...weak forcing from shortwaves and upslope could
potentially result in a light coating of snow across the northwest mountains
Wednesday night and again Thursday night.
Some gradual moderation appears likely Sat-Mon...as upper low
lifts into the Canadian Maritimes and surface ridge builds into PA.
Low pressure approaching from the miss valley will likely be shunted
south of PA this weekend...but included low chance of rain showers across the
southern counties late Saturday.
Aviation /10z Tuesday through Saturday/...
just a few lingering -shra across the airspace. Aside from brief
reductions in those rain showers...expect MVFR/VFR conditions to prevail
overnight and into Tuesday. Surface winds from 250 to 300 degrees will
become gusty at 25-30kts by Tuesday morning with predominately scattered
to broken MVFR to VFR ceilings.
Wed-Sat...persistent northwest flow pattern will favor MVFR ceilings over
the northwest airspace in zob sector...with mainly VFR conditions
prevailing in zny domain. Isolated to scattered showers will be possible
across all of the airspace through the remainder of the week.