948
FXUS61 KPBZ 201735
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
135 PM EDT Fri Sep 20 2024

.SYNOPSIS...
Seasonably hot weather is expected through the weekend with a
marginal chance of severe storms Saturday, mainly east of I-79.
A wetter pattern is becoming increasingly likely next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:

- Seasonably hot weather; record breaking heat potential for New
  Philadelphia and DuBois.
-------------------------------------------------------------------

Main change for the mid-morning update was to add a small chance
of showers to the higher terrain of eastern Tucker County, where
weak low-level convergence and the terrain may be able to work
with marginal moisture/instability. Otherwise the (quite warm)
forecast for today remains on track.

Previous discussion...

Temperatures are expected to climb rapidly into the day today
with upper ridging and surface southerly flow under high
pressure. With how warm conditions have been, it is difficult to
believe the average high for Pittsburgh this time of year is
74. With temperatures forecast to climb into the 80s (and even
90s for eastern Ohio), highs will be around 10 to 15 degrees
above average. New Philadelphia has a shot at beating their
record high of 89F (2002), and DuBois may approach their record
high of 84F (1965).

Temperatures will cool under clear skies tomorrow night, but
should be limited by dew points and light winds. This will keep
lows slightly above average. Fog appears less likely overnight
save insulated valleys with larger dew point depressions and
some winds.

&&

.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
KEY MESSAGES:

- Marginal severe storm potential, mainly east of I-79 with the
  primary threat being damaging winds.
---------------------------------------------------------------------

Guidance still remains optimistic on precipitation chances
Saturday for western Pennsylvania and eastern Ohio as a
shortwave overtops and flattens an eastern ridge. This will act
to cool upper levels and keep daytime highs slightly cooler
than the day prior, but the main question remains severe
potential.

First, the environment looks favorable for low-end severe
threats. Probabilities of at least 1000 J/kg of SBCAPE are up to
50% to 75%, while chances of 30kts to 40kts of shear in the
0-6km layer remain elevated, though most of the shear is
confined to the mid-levels. Mid-level dry air in shortwave
induced subsidence is also likely to supply as much as 800-1000
J/kg DCAPE. This suggests winds remain the primary threat. So
the main question for development is not the environment, but
rather the forcing.

There is still spread in the intensity and timing of the mid-
level vort max. Passage could be as reasonable early as 14Z and
as late as 22Z. The later the passage time, the more the wave
will be able to utilize instability, and thus, the higher chance
of severe weather. The second uncertainty is intensity. The
forcing will need to be strong enough to overcome mid-level dry
air. A number of CAMs suggest this, but roughly an equal number
rely on orographic help, leaving the primary threat east of the
forecast area. These uncertainties have justified the
maintenance of a marginal risk for severe weather.

Chances of any rain with this wave range from 50% in eastern
Ohio to 90% east of Pittsburgh. Despite this, chances of 0.25"
of rain only maximize at 50% to 60% in western Pennsylvania.
This will do little in the ways of drought relief.

Rain chances taper overnight as the wave passes to the east and
subsidence fills in behind. Fog is possible Saturday night and
Sunday morning given calm, clear conditions and recent rainfall.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
KEY MESSAGES:

- Hot and dry Sunday.
- Precipitation chances encroaching for the rest of the week
  with moderating temperatures.
-------------------------------------------------------------------

There remains high confidence in brief ridging Sunday which
will allow high temperatures to remain well above average.
Chances of topping 90F are 20% to 40% in eastern Ohio, with 80s
forecast elsewhere.

By Monday, there is more agreement that the ridge axis flattens
to quasi-zonal flow, but uncertainties in the 500mb flow
snowball by mid-week. All clustered guidance indicates some
troughing, however, roughly 40% of global ensemble membership
has a positively tilted Midwest trough, while 60% has a neutral
eastern trough by the middle of next week. A Midwest trough
would be more conducive to low tracks over our area and more
precipitation, while a eastern trough still allows precipitation
chances, but the main QPF axis would be toward the eastern
seaboard.

However you cut the cards, this is the best shot of notable
rain this month, with chances of greater than an inch of QPF 50%
for eastern Ohio and northern West Virginia and 60% to 70% in
western Pennsylvania.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
VFR conditions will prevail for the next 24 hours under high
pressure.

A passing disturbance after 15Z Saturday may impact terminals
with surrounding thunderstorms/showers. Gusty winds and small
hail may be observed for terminals east of PIT, especially near
LBE/MGW between 18Z to 23Z.

.Outlook...
The potential for fog on Sunday morning will correlate to where
precipitation forms late Saturday afternoon/evening and where
clearing persists. At the moment, Hi-Res guidance suggests
impacts to areas north and west of PIT.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.

&&

$$

SYNOPSIS...Milcarek
NEAR TERM...CL
SHORT TERM...Milcarek
LONG TERM...Milcarek
AVIATION...Hefferan