Scientific Forecaster Discussion
fxus61 kctp 182152
Area forecast discussion
National Weather Service State College PA
552 PM EDT Thu Jul 18 2019
----dangerous heat and humidity possible this weekend---
We will be dealing with hazy, hot and very humid conditions
until early next week when some relief is in sight. Dangerous
heat will build Friday and Saturday, with heat indices peaking
on Saturday afternoon, easily exceeding 100 over southern and
especially southeastern areas.
Near term /until 6 am Friday morning/...
widely scattered convection will remain possible into early this
evening across the eastern part of the forecast area ahead of
mid level shortwave. Expect any showers/storms to rapidly
dissipate around sunset, as shortwave edges east and boundary
Surface ridging over the region should translate to a mostly
clear night with light wind. Plenty of low level moisture,
combined with a calm wind, should promote patchy late night
valley fog across the eastern half of the state. Over the
western counties, a light breeze may limit the fog potential.
It will be a warm and muggy night across the entire region with
latest guidance supporting min temps in the upper 60s to low
Short term /6 am Friday morning through 6 PM Friday/...
hoisted widespread heat advys and excessive heat warnings - many
of which will last into Sat. Tried to time/place them
appropriately with just a bit of a broad brush. While numbers
may not get to 100 everywhere over the alleghenies/laurels, it
will be close, and will likely get there in the valleys. The
chance for the convective cloud debris running overhead from the
northwest and some isold/sct convection could keep temps down and bust
up the heat index values. Will hold onto some low chc pops for
the nrn tier with short wave energy running in from the NW, but
it will be running into the warm air aloft.
Long term /Friday night through Thursday/...
excessive heat peaks this weekend. Maximum heat index values
reach the 105 to 110 range Saturday before lowering slightly
on Sunday. Excessive heat watch remains in effect Sat and sun
for areas from Chambersburg to Williamsport and east. Will
also continue to highlight heat risk via hwo, social media and
dss briefings in addition to the watch.
Very warm mid-level temperatures look to limit/cap T-storms
Fri-Sat. That said, models hint at the potential for mesoscale convective system
activity propagating along wavy frontal zone on the northern
periphery of summertime upper ridge/heat dome. By Sunday, that
fron begins to track slowly across County Warning Area as upper trough begins to
work in from the Great Lakes. With surface heat dome in place,
expect convection to become more numerous sun aft into Sun
Flow is fast and strong across the US and Canada border overall
for mid July, but we are south of the main westerlies. The
airmass is warmer than what we have seen lately. 1991 and 1995
featured such warm air. In this case, the warmest air is
advected toward buf late Friday. Warmest air for central PA is
Friday into Saturday. 850 mb temperatures fcst to be near plus
24 degrees at 850 mb late Friday across Lake Erie. Will be
interesting to see how fast water temperatures come up on Lake
Erie by next week.
A cool down is expected to begin next Monday along with a break
in the oppressive humidity as the mean upper level trough is
nearby. Monday could still be muggy over the southeast where scattered
tstms will continue, but overall trend will be that temperatures
of 10-20f above normal over the weekend will retreat back to
near July normals during the work week.
Aviation /22z Thursday through Tuesday/...
the showers to the S/east of MDT will slide se, and should not be
a factor after 22z at lns. The next worry would be for fog, but
only mentioning some MVFR fog for now for the overnight. Low
clouds may try to form with a slight upslope over the wrn mtns
(jst/bfd), but held off on any IFR mentions.
Rain showers/thunderstorms and rain should be nearing bfd late in the morning, so I did
add in a mention of ts late in the taf period. Otherwise, looks
VFR with light wind for Fri.
Friday night and Saturday will still have a threat of isold
rain showers/tsra, mainly in the aftn.
Sat-sun...mainly VFR with isolated to widely scattered PM
thunderstorms possible. Heat/humidity will make the density
altitude much higher - takeoffs/landings will take more
Mon...still a chance of showers and storms as cfropa occurs Mon
or Mon PM.
Tues...no sig wx.
heat advisory from noon Friday to 8 PM EDT Saturday for
Excessive heat warning from noon Friday to 8 PM EDT Saturday
Heat advisory from 8 PM Saturday to 8 PM EDT Sunday for paz028-
Excessive heat warning from noon to 8 PM EDT Saturday for
Heat advisory from noon Friday to noon EDT Saturday for paz019-