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NWS Area Weather Forecast Discussion
County Warning Area [CWA]: PBZ
Regional NWS Weather Office: Pittsburgh, PA

FXUS61 KPBZ 272212

Area Forecast Discussion
National Weather Service Pittsburgh PA
612 PM EDT Mon Sep 27 2021

Shower and thunderstorm chances increase overnight through early
Tuesday with a crossing cold front. Building high pressure Tuesday
should then return dry and seasonable conditions for the remainder
of the week.


Llvl wswly flow will persist as high pressure begins to erode in
response to an approaching cold front, currently situated across the
Great Lakes and slowly advancing south. Flow ahead of this feature should
gradually advect and maintain moisture within the boundary layer
near 850mb as the evening progresses, with sfc dewpoint values
increasing into the low 60s overnight. Meanwhile, a trough extending
outward from closed low pressure centered east of James Bay, Canada,
will dig south across the Great Lakes this evening, helping to
steepen mid-level lapse rates and eventually pull the cold front
across the forecast area through Tuesday. Moistening llvls combined
with cooling mid-levels will support MUCAPE of 1000-1200 J/kg ahead
of the cold front, which will be aided in support for ascent from the
left exit region of a southward plunging jet across the Great Lakes.
Scattered showers and thunderstorms should then develop after
midnight, with 30-40 knots of 0-6km bulk shear supporting isolated
instances of large hail in stronger, more discrete storms. SPC
maintains our region within a marginal risk for severe potential
given the notable, aforementioned ingredients, but most convection
should remain elevated due to diurnal sfc cooling trends overnight
beneath an 850mb inversion. As such, PoPs should remain rather
limited through daybreak Tuesday. Otherwise, llvl warm advection
ahead of the approaching cold front will maintain temperature 7 to 10
degrees above averages.


The surface cold front will be in the vicinity of I-80 at sunrise on
Tuesday.  The boundary and its associated shortwave will continue to
trek across the region, eventually exiting during the afternoon.
Scattered showers and thunderstorms will continue ahead of the
front.  Instability will continue to be mainly elevated in nature.
0-6km shear of 30-35 knots may linger across the southern counties
as well, and the threat of isolated large hail may continue into
Tuesday afternoon.  Clouds and showers will mute high temperatures
as compared to today, but will still be above normal.

Dry weather returns Tuesday night behind the front in northwest flow
aloft Building surface high pressure will keep the weather quiet
through Wednesday night as well, with a return to fairly seasonable


The upper low sinks into the New England/Canadian Maritime region by
Thursday, and persists over that area into Saturday at least. Our
CWA remains in northwest flow aloft between this low and a building
ridge which will extend from the Gulf Coast to the Upper
Midwest/Great Lakes. With surface high pressure remaining in control,
dry weather and seasonable temperatures will continue.

Uncertainty increases thereafter as the ridge flattens starting
Sunday.  This will be in response to shortwave energy from further
west, although the strength and timing of said shortwaves remains a
point of contention in the various models.  Will keep the blended
model idea of mostly very low/slight chance PoPs for Sunday and
Monday for now, pending future model clarity.


VFR conditions will give way to sct shra/iso tstm and MVFR/IFR cigs
early Tuesday morning with the approach/passage of a cold front.
Increasing subsidence and intruding dry air will diminish showers
after fropa and improve cigs to VFR Tuesday afternoon.

Gusty SW wind up to 25kts will subside near sunset, but a llvl jet
ahead of the cold front may create brief bouts of LLWS overnight.
Confidence in 2kft wind reaching and sustaining criteria strength
remains too low for TAF mention.

VFR conditions are likely Tuesday night through the rest of the week
under the influence of high pressure.





Forecast Discussion from: NOAA-NWS Script developed by: El Dorado Weather