PBZ issues Area Forecast Discussion (AFD) at Aug 3, 1:13 AM EDT149
FXUS61 KPBZ 030513
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
113 AM EDT Sat Aug 3 2024
.SYNOPSIS...
Periodic showers and thunderstorms continue through Saturday
with a slow moving trough of low pressure. Drier weather builds
in on Sunday and continues into the beginning of next week with
above average temperatures.
&&
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
KEY MESSAGES:
- Scattered showers and isolated thunderstorms into the
overnight hours
- Warm and muggy conditions continue.
-------------------------------------------------------------------
Update...
Convection has weakened across the region with diminishing
instability. MU CAPE is expected to range from 500-750 j/kg
overnight, which could produce isolated thunderstorms. The main
forcing should be to our west overnight with an approaching
upper trough. Locally heavy downpours will be possible in any
thunderstorm, though it appears the severe weather and flooding
potential is over, or minimal.
Previous discussion...
Storm intensity to the south of Pittsburgh has backed off with
sunset. Earlier, a few storms managed to create strong enough
updrafts for likely sub-severe downburst wind, and a cell merger
in Monroe County, OH prompted a Severe Thunderstorm Warning.
Any further severe risk is waning quickly with low-level
stabilization.
Of more concern overnight is the potential for ongoing heavy
rainfall potential. Outflow from earlier lake breeze activity is
still present north of Pittsburgh, and outflow from the storms
to the south plus activity now entering Coshocton/Muskingum
Counties will add to the potential for overnight interactions.
Abundant moisture is still present, with 1.6 to 1.8 inch
precipitable water lingering tonight. CAMs still offer varying
solutions, but most have little overnight coverage. Think that
there is potential for at least scattered showers/a few
thunderstorms containing heavy rainfall overnight given the
potential for local low-level convergence, and slow cell motion
plus deep warm-cloud depths point to efficient rainfall
production. Cannot rule out an isolated instance or two of
flooding, especially in urban areas. A warm and muggy air mass
will linger through the night.
&&
.SHORT TERM /6 AM THIS MORNING THROUGH SUNDAY NIGHT/...
KEY MESSAGES:
- Showers and thunderstorms continue Saturday with a localized heavy
rainfall threat.
- Turning dry and warmer Sunday, save for the higher elevations.
-------------------------------------------------------------------
The wet pattern continues into Saturday as the low slowly progresses
east and the upper trough slides overhead with its forward progress
slowed as it runs into a downstream ridge. Coverage of showers and
thunderstorms will increase into the afternoon hours aided by
diurnal instability. Despite cloud coverage around, ensemble
probability for >1000 J/kg of CAPE is 70+% as the 500 mb thermal
trough brings in cooler air aloft. 15-16kft warm cloud depths with
skinny CAPE and weak deep layer shear will provide the opportunity
for additional heavy rain threat. Not expecting any severe potential
with less mid-level dry air available, and SPC has removed the
Marginal Risk from our area entirely. Rain should begin to wrap up
from northwest to southeast in the evening as daytime heating is
lost and a surface trough clears the region with weak northwesterly
flow slowly bringing in drier air.
Come Sunday, forcing will be lost for most of the area on the back
side of the low as it departs and ensembles build high pressure from
the west. That said, with the trough axis still lingering across the
Laurel Highlands, precipitation chances are favored to continue for
the higher elevations until Sunday night when stronger subsidence
builds. Elsewhere, temperatures Sunday will increase back above
normal with clearing skies and probability for highs >85F at 80+%.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
KEY MESSAGES:
- Slower upper trough will maintain periodic precipitation chances
into the weekend.
- Temperatures dip back closer to normal.
-------------------------------------------------------------------
Some disagreement comes into view with the upper level pattern to
begin next week, but the consensus is that upper troughing develops
across the ECONUS placing us in west-northwest flow aloft with
periodic precipitation chances owing to shortwave movement.
Confidence in any details is low given the difference in ensemble
cluster heights, but Tuesday into Wednesday sees precip chances ramp
back up with the passage of the first wave, albeit with differences
in timing and amplitude. Machine learning paints a low end severe
threat across our area. Temperatures are favored to be above average
to start the week returning to more seasonable levels by mid-week.
&&
.AVIATION /05Z SATURDAY THROUGH WEDNESDAY/...
Highly variable, low certainty forecast through 14z as an upper
level low spins just west of the region. The moist airmass and
boundary layer create the potential for MVFR/IFR cigs at area
terminals (at least 50% probability by 10z) and localized
IFR/LIFR fog. However, broad ascent creating mid-level cloud
decks and isolated showers and thunderstorms (favoring MGW to
LBE) could either enhance those probabilities or disrupt
radiational cooling processes aiding development. Blended
ensemble guidance along with persistence, but users should
remain alert to the potential for rapid fluctuations of cig/vsby
conditions.
The upper low will slide over the region through the daylight
hours, promoting fairly widespread shower and thunderstorm
activity between 18z-00z. VFR conditions are favored in between
storms, but lowered cigs/vsby are likely with storm impact as a
result of higher rainfall rates. Precipitation chances diminish
after 00z as storms shift east with the exiting low and high
pressure encroaches.
.Outlook...
Ensembles suggest little dry advection at the surface in the
wake of the exiting low, creating potential for widespread
MVFR/IFR fog and low stratus. The degree of impacts will likely
be tied by surface moisture/temperature after convection ends
and the degree of cloud clearing behind the wave.
High pressure under drier northwest flow will promote VFR quiet
weather Sunday through most of the day Monday. A series of
shortwaves within the NW flow will create periodic storm chances
late Monday through Wednesday.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...MLB
NEAR TERM...WM/MLB/CL
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...Frazier
Source:
PBZ issues Area Forecast Discussion (AFD) at Aug 3, 1:13 AM EDT---------------
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