Scientific Forecaster Discussion
fxus61 kctp 261057
Area forecast discussion
National Weather Service State College PA
657 am EDT Fri may 26 2017
an upper low is sliding off to the northeast and leaving a cool
northwest flow in it's wake. Weak ridging will move in but many
chances for showers and a few thunderstorms are seen through the
Holiday weekend. A cold front will pass through on Monday.
Near term /until 6 PM this evening/...
rain on radar is aloft in some places, as it is to the east of
the office, but to the west it is hitting the ground. A big
clear slot has developed over the eastern counties. No sweat,
the clouds are headed that way, and it may just help develop
some instability to generate the isolated showers in the
forecast. Have tweaked pops in the next few hours for recent
radar and meso model trends.
a short wave over Ohio will continue to drop southeast and generate
showers for the next few hours. As the forcing wanes/moves away,
the showers there will break up and dissipate until more-
cellular showers develop in the day time heating. Cool air aloft
will both make it unstable and keep temps down through the day.
Clouds from the upper forcing and llvl cold advection are also
going to contribute to the keeping Max temps about 10f cooler
than normal today. A confluent flow aloft should keep the
showers from growing too tall. Will keep pops in the scattered range
due to the expected areal coverage at any time. But, a good
forecast will be that most places will have at least a sprinkle.
The showers will be moving along and, again, not tall. Thus,
the chance for hvy rain is very low.
Short term /6 PM this evening through Saturday night/...
as the sun sets Friday, the showers should dissipate nicely
except perhaps in the NE cos. However, a ridge runner may drop
out of the upper Great Lakes into the state per the GFS. The
other operational models are also generating what looks to be an
mesoscale convective system upstream. The 06z NAM has some convection just scooting to
the south of the area, and it is mainly on Saturday. The ec is
also placing a high quantitative precipitation forecast over the upper Ohio Valley and into the
mid-Atlantic on Sat morning. There is a little better consensus
on the timing and placement of convection for the short term, so
we will nudge the pops up a bit in the SW for the late tonight
and Sat time frame.
Long term /Sunday through Thursday/...
a lower-amplitude but moderately strong westerly (zonal) mid
level flow will set up in the wake of the departing upper trough
with modest height rises/weak will ridging aloft into the
weekend. A warm front is progged to extend east from surface low
in the western Ohio Valley to the mid-Atlantic coast on
Central PA should reside on the North/East fringe of high
instability convective corridor/mesoscale convective system type pattern expected to
evolve from the central and Southern Plains to the southern mid-
Atlantic coast into Saturday night. The greatest risk for
scattered thunderstorms remains over the SW 1/3 of the County Warning Area
which is consistent with previous fcsts. Storm Prediction Center has introduced a
marginal risk into this area for d3. The remainder of the area
should see a mainly dry start to the Holiday weekend but will
maintain slight chance pops.
The large scale pattern will evolve into a broadly cyclonic
flow regime by Monday with upper trough parked over the Great
Lakes and southeast Canada into midweek. The aforementioned warm
front will lift north across the area on Sunday followed by a
series of weaker cold front associated with shortwave trough
rotating around parent upper low centered to the south of Hudson
Bay. Ensemble blend still supports Max pops on Sunday/Sunday
night into Monday with some additional shower opportunities into
midweek as series of shortwaves rotate around mean upper
trough. Temps should average near normal/seasonal climo to
close out the month of may.
Aviation /12z Friday through Tuesday/...
relatively cool/moist westerly flow pattern behind low pressure
moving up the New England coast will spell MVFR/IFR ceilings
for the western 1/2 of the airspace. MVFR to VFR conditions will
be common over the eastern 1/2 thanks to downslope flow in the
Lee of the alleghenies. Expect isolated to widely scattered rain
showers through the afternoon but coverage and probs are too low
to mention in tafs. Anticipate a gradual improving/decreasing
cloud trend with most sites VFR heading into tonight.
Remnant meso/convectively enhanced wave tracking east across the
Ohio Valley should thicken clouds back up by Saturday morning.
Confidence remains low on the evolution of this system given
limited predictability and continued above avg. Spread in the
model guidance including high-res cams. Models are usually too
far north with mesoscale convective system type activity so it looks like the best
chance for light showers would be over the far SW airspace
Sat...chance of rain with sub-VFR possible.
Sun...showers likely with sub-VFR especially Sunday night.
Mon-Tue...scattered showers and a few tstorms possible.