PBZ issues Area Forecast Discussion (AFD) at May 12, 7:41 PM EDT966
FXUS61 KPBZ 122341
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
741 PM EDT Mon May 12 2025
.SYNOPSIS...
Upper level low pressure will bring daily rain chances through
the end of the week with gradually moderating temperatures.
There is a chance for severe weather late Thursday into Friday
with a cold front. Temperatures will dip back down for the
weekend behind the front.
&&
.NEAR TERM /THROUGH TUESDAY/...
KEY MESSAGES:
- Clouds increasing from south to north.
- Showers moving in and continuing through tonight.
-------------------------------------------------------------------
Evening update: Very few tweaks made to previous forecast. Mainly
updating POPs and ingesting the latest NBM data for the current
period. Our first precipitation is just crossing our southern border
at this time and climbing on north. Latest ACARS soundings from PIT
still show a very dry SFC-700 mb layer that will struggle to
saturate from the top down as these first showers move through.
Areas may experience a period of high-based sprinkles before any
kind of steadier showers arrive behind the initial push. Portions of
the previous discussion below:
A closed upper low across the Mississippi Valley will slowly begin
to meander northeast today overspreading height falls locally as a
southeast to northwest oriented mid-level ridge slides off to the
east.
Broad ascent with height falls, lobes of mid-level vorticity, and
increased low level convergence on the nose of an 850 mb jet will
eventually support a shield of rain this evening and continuing into
the overnight hours. The best deep layer moisture through tonight is
forecast to be across western Pennsylvania and northern West
Virginia where the highest PoPs and amounts will lie while areas of
eastern Ohio are favored to see more scattered showers and even some
dry periods possible.
&&
.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
KEY MESSAGES:
- Rain continues with locally heavy amounts possible in the
ridges.
- Moderating temperatures through mid-week.
----------------------------------------------------------------
The upper low will cross through the Tennessee Valley on Tuesday
with continued deep warm and moist southerly flow pumping
precipitable water values locally up to 1.25-1.5" which is nearing
or exceeding the 90th percentile climatology. Spokes of vorticity
will support scattered showers in a very moist atmospheric setup,
and the higher amounts and steadier rates will be favored to the
east along the ridges on Tuesday in conjunction with the best jet
support. That said, HREF probability for >250 J/kg of CAPE reach up
to 80% west of Pittsburgh aided by some slightly drier mid-level
air. This could allow for convective enhancement in the afternoon
hours and locally higher rainfall rates/totals, though severe
weather is not expected given the saturated profiles and lacking
instability.
WPC has maintained the Marginal Risk (1/4) across our entire area,
but widespread flooding concerns are not expected and would only be
tied to areas that see training heavier showers or thunderstorms as
rainfall rates are not forecast to be much of a concern. Current
flash flood guidance in a 1 hour period is ~1.25-1.75" and a 6 hour
period >2.5" across most of the area. HREF probs for these amounts
are low (less than 10%), but forecast MBE vectors show values around
12 knots or lower which would point toward the potential for slow
moving showers/storms.
Wednesday, as the core of the upper low is close overhead, we may
see a decrease in overall rain totals and coverage given lack of
stronger synoptic ascent. But, mid-level temps will cool some under
the cold core low and allow for some better destabilization into the
afternoon hours. NBM mean SBCAPE values are upwards of 1000 J/kg and
could nudge even a bit higher if clouds can scatter at all, though
that's a low probability solution. What we will be lacking is
appreciable shear given the minimal flow as we're sitting very close
to the core of the low. It's likely that we'll see a diurnally
driven enhancement in showers and storms in the afternoon, but
severe weather is low probability. Will have to continue watching
for localized flooding threats, but if the CAPE profiles are deep
enough, we may not capitalize on warm rain processes which could
help mitigate heavy rainfall.
Most likely total QPF through Wednesday night is between half an
inch to an inch of an inch outside of the ridges. Over that way,
values are up toward the 1.5-3" range. The higher end of the goal
posts could put 1-1.25" in the lowlands and up to 3.5" in the ridges
tied to aforementioned convection. The lower end suggests at least a
quarter inch areawide.
&&
.LONG TERM /THURSDAY THROUGH MONDAY/...
KEY MESSAGES:
- Strong to severe thunderstorms possible Thursday night.
- Temperatures 15-20F above normal Thursday/Friday and cooling
slightly into the weekend.
-------------------------------------------------------------------
Ridging building ahead of a strong trough in Upper Mississippi River
Valley will allow for a drier and warmer setup on Thursday, though
will maintain the mention of some residual showers across the ridges
especially. 850 mb temperatures locally could warm into 14-17C range
and a 30-50% chance of highs >85F. That chance ramps up even more on
Friday to 60-80% with stronger warm advection in southerly flow, but
confidence isn't quite there yet given the dampening effects of
cloud coverage.
Models have remained relatively persistent at pushing strong warm
advection across area late Thursday night and Friday morning.
That is when the strongest moisture transport is shown to push
through Upper Ohio River Valley and a cold front drags through from
the west. Still some timing differences among the ensembles with the
upper pattern and progression of the cold front; current general
consensus is pointing toward the overnight hours but solutions on
the faster and slower end still exist with the slower one more
suggestive of a higher severe weather chance. SPC post-processing
GEFS based severe shows highest probabilities of severe staying west
of the area on Thursday and then south and east of the area on
Friday which appears reasonable given the most likely front timing.
An unsettled pattern is favored into the first half of the weekend
with rain chances continuing, but heights begin to rise on Sunday as
troughing exits suggesting a drier trend, although timing differences
in the ensembles suggest showers are still possible into
Sunday.
&&
.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
The initial wave rounding the eastern periphery of an upper
level low will bring mainly light rain as it crosses the region
through 06z, with the surface warm front stalling near I-80. A
plume of moisture will continue to work NW from the Mid-Atlantic
to create light rain through Tuesday morning, favoring western
PA and the WV higher terrain. Uncertainty remains high on
timing and coverage of cig reductions amid moist
advection/convergence and likely will be predicated on areal
coverage of rain to moisten the prior dry air mass. TAFs through
12z exhibit a blend of the less aggressive HREF probabilities
and more aggressive LAMP/MOS guidance.
Diurnal heating will be the primary driver of cig alterations
Tuesday as the airmass remains moist, if somewhat stagnant. The
expectation is greater heating/lift in OH will yield greater
improvements than in northwest PA where IFR could linger most of
the day. Broad ascent will favor scattered to numerous rain
showers, but probabilities peak at 30-40% for afternoon
thunderstorms at MGW/HLG/ZZV.
Outlook...
Restrictions and periods of showers with afternoon thunderstorms
will continue into Wednesday before the upper level low shifts
east of the region.
Brief VFR under ridging Thursday will give way to a fast-moving
low pressure system Thursday night into Friday. An active
weather pattern within turbulent upper flow should result in
periodic rain and restrictions chances into the weekend.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...MLB
NEAR TERM...MLB/AK
SHORT TERM...MLB
LONG TERM...Craven/MLB
AVIATION...Frazier
Source:
PBZ issues Area Forecast Discussion (AFD) at May 12, 7:41 PM EDT---------------
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