Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1005 am EDT Sat Sep 20 2014
dry and warm for one more day until a cold front brings brief
rounds of showers and thunderstorms Sunday. Much cooler
Monday...however a warming trend will ensue each day next week
with high pressure camped out over the region.
Near term /until 6 PM this evening/...
one last warm day before a cold front approaching from the west
takes the weather and temperatures downhill. The latest 12z pit radiosonde observation
illustrates lots of dry air from 700 mb through h4 which will mix out
most of the low level moisture noted near the surface. Nonetheless
still expect cumulus development during the afternoon as
temperatures climb into the middle 70s. The main weather story this
afternoon will be gusty winds. Pressure gradient increases with
low pressure encroaching from the Midwest and a fairly deep mixing
layer. Afternoon mixing heights will be around 5-6kft equating to
gusts around 30 miles per hour. NAM is generating isolated storms over the
higher terrain this afternoon...however due to the inflated low level
moisture that its predicting...ignored it and continue with a dry
forecast area wide.
Short term /6 PM this evening through Monday/...
atmospheric moisture will be increasing Saturday night ahead of a
cold front that will slowly move through the Midwest. Pre-frontal
shortwave trough will swing through Saturday night. The upper
level energy weakens as it moves through and it will be
encountering a rather stable atmosphere. Will keep probability of precipitation low
Saturday night as shortwave may provide more clouds than
Front reaches western zones by late Sunday morning. Models
continue to show a destabilizing atmosphere and an impressive wind
field ahead of the surface boundary...but continue to see a lack
of upper level support. The cloud cover from Saturday nights
shortwave will limit the amount of sunshine Sunday making it
difficult to reach convective temperatures. A significant line of low
level forcing develops along the boundary Sunday afternoon as
moisture convergence in the lowest layers increases. 0-6 km bulk
shear values are still impressive...but much will depend on the
cloud cover and how this limits surface temperatures. Additionally...any
showers that fall with the pre-frontal shortwave will cool the
atmosphere...further decreasing the threat for stronger storms.
Sunday's front will need to be watched as the potential is there
for a stronger event...but so are the limiting factors. If a line
of stronger storms develops it will be directly ahead of the
Front picks up speed Sunday evening as strong shortwave digs into
the upper Ohio Valley. Strong cold air advection and northwest flow will follow
the shortwave. Would expect a large stratocu deck to move over the
region late Sunday night and through a large part of Monday. Its
possible that a couple of rounds of showers will develop...Sunday
night with the shortwave...and again Monday afternoon with the
main cold pool.
Will go below guidance for temperatures on Monday with the cold air advection and cloud
Long term /Monday night through Friday/...
dry pattern is expected for the remainder of the long term period.
Aviation /14z Saturday through Wednesday/...
VFR conditions conditions today with gusty southwesterly winds
developing by late this morning as stronger winds aloft mix down to
Outlook...Saturday night through Wednesday...
general restrictions and gusty winds are likely Sunday as a cold
front impacts the region with a line of thunderstorms capable of
producing gusty winds.