FXUS61 KPBZ 200039
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
839 PM EDT Mon May 19 2025

.SYNOPSIS...
Slightly below average temperatures are expected through the 
week. A Frost Advisory is in effect tonight into tomorrow 
morning for areas north of Pittsburgh. Precipitation chances 
return late Tuesday into Wednesday with the passage of a low 
pressure system.

&&

.NEAR TERM /UNTIL 8 AM TUESDAY MORNING/...
KEY MESSAGES:

- Frost Advisory in effect for counties north of Pittsburgh late
  tonight/early Tues morning
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Surface high pressure east of Michigan and rising heights aloft
will promote clear skies for a large portion of the night
alongside light wind. Coupled with seasonably cool temperature,
the risk for frost formation led to the issuance of a Frost
Advisory for Forest/Venango/Clarion/Jefferson (PA) Counties
between 2am and 8am Tuesday morning. Local valleys outside of
the advisory area where radiational cooling can be maximized may
also see frost formation, but confidence in a widespread frost
event precluded additional highlighted counties. Take time now
to protect sensitive outdoor vegetation that could be harmed by
frost.

The one component of the overnight period that should prevent
additional regions seeing frost is the increase of cirrus from
the southwest after midnight in response to upper level warm
advection. Even if it arrives early and limits nocturnal
cooling, the region is still likely to see low temperature 5 to
10 degrees below the daily average.

&&

.SHORT TERM /8 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/...
KEY MESSAGES:

- Unsettled mid-week pattern with heavy rain potential late Tuesday 
  and Wednesday.
- Temperatures remain below normal.
----------------------------------------------------------------

Dry weather will continue on Tuesday as weak surface high
pressure departs and the upper ridge axis crosses. Warm and
moist advection in southwesterly flow will ramp up by Tuesday
afternoon ahead of the next system. Warm advection will battle
increasing cloud cover, only modifying high temperatures by a
few degrees versus today.

Occluding low pressure will cross out of the Upper Mississippi 
Valley and into the Ohio Valley by Wednesday. Upper diffluence 
and falling heights in tandem with the low will support high 
confidence rain chances increasing overnight Tuesday night and 
Wednesday. Low pressure will then essentially remain parked over
our are through Friday, elongating and splitting at times. 
Meanwhile, a series of shortwaves will traverse the broad upper 
trough, bringing enhanced periods of rainfall into the weekend.

Heaviest rainfall appears to be Wednesday through Wednesday
evening with 24 hour probability of 1" or more 50% or greater
east of Pittsburgh. Probabilities drop of west of Pittsburgh
into Ohio. WPC guidance continues to highlight a Marginal Risk
for excessive rainfall for both Day 2 (Tue) and Day 3 (Wed). 
Latest CSU-MLP machine learning guidance supports this idea as 
well. It also hints at a marginal threat of a wind/hail threat 
south of the Mason-Dixon line during this time, which is now
conveyed in a Marginal Severe Risk by the Storm Prediction
Center.

&&

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

- Cooler weather Thursday, but warming again this weekend.
- Rain chances prevail through the weekend.
-------------------------------------------------------------------

Low pressure remains overhead Thursday and Friday. Wrap around 
cold advection and continued saturated low/mid levels on 
Thursday should keep low-level cloud coverage and scattered 
light showers in the forecast. Temperatures will be seasonably
cool, maxing out in the upper 50s to lower 60s.

Additional upper shortwave troughs will support continued daily
rain chances into Saturday. Ensembles continue to show slight 
differences in speed of low pressure departure, impacting shower
chances on Saturday. Building high pressure to our south may 
begin to win out on Sunday and some solutions suggest just some 
upslope driven showers, but confidence this far out is low. 
Despite some moderation, temperatures are still favored to hold 
below normal.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
Lingering northwest-flow stratocumulus will clear out this
evening as wind veers with approaching surface high pressure. A
mostly clear sky and light wind is forecast later this evening,
with some patches of cirrus.

VFR continues late tonight and past sunrise Tuesday, although 
middle and high clouds will be invading from the southwest as an
upper ridge axis passes and flow aloft turns west/southwest. 
Warm and moist advection may begin to support some light rain 
mainly southwest of PIT before 00Z Wednesday; PROB30 groups were
used at ZZV, HLG, and MGW at the end of the TAF period. Chances
of restrictions before 00Z are low, as HREF MVFR probabilities 
for both ceiling and visibility are 10 percent or less. Low-
chance thunder at ZZV at the end of the TAF period was left out
for now.

Outlook... 
Restrictions and rain overspread the region between 00Z and 06Z
Wednesday. Periods of rain and restrictions are expected to 
continue through Thursday as surface low pressure moves across 
the region. 

Potential restrictions and showers could continue Friday under 
a subsequent upper low.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Frost Advisory from 2 AM to 8 AM EDT Tuesday for PAZ008-009-
     015-016.
OH...None.
WV...None.

&&

$$

SYNOPSIS...Rackley
NEAR TERM...Frazier
SHORT TERM...Rackley
LONG TERM...Rackley/88
AVIATION...CL