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FXUS61 KPBZ 170540 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 140 AM EDT Fri May 17 2024

SYNOPSIS
Other than isolated showers in the higher elevations, dry weather will continue this evening. Unsettled weather returns for Friday through the weekend, with showers and thunderstorms possibly providing isolated heavy rainfall totals. Near to above normal temperatures will continue.

NEAR TERM /THROUGH TODAY/
KEY MESSAGES:

- Isolated higher-elevation showers will end this evening.
- After several quiet hours, more showers arrive from the west towards sunrise.
- Above-normal low temperatures are expected.
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Updated PoPs over the far east for this evening. Seeing popup showers over portions of the eastern border, mainly north of Indiana PA. This activity should dissipate after sunset. Rest of the forecast is in good shape.

Previous discussion...

CAMs still depict a few isolated showers/storms popping up over the terrain this afternoon, and this seems reasonable as weak convergence and lingering moisture should aid the process.
Satellite shows some towering cumulus in the ridges east of Pittsburgh, and expect some radar returns to start showing up shortly. Elsewhere, scattered to broken cumulus will remain.
Clouds, and showers, will collapse somewhat by sunset.

The evening hours will be generally quiet as an upper ridge departs. Towards morning, increasing vorticity advection and moisture on WSW flow aloft will begin to bring some light rain into the region, although the bulk will wait until after 12Z as a shortwave approaches. Low temperatures will remain several degrees above normal.

SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/
KEY MESSAGES:

- A pair of upper disturbances will bring shower and thunderstorm chances to the region.
- Severe weather risk is low, but isolated heavy rainfall is possible.
- Near-normal high temperatures and above-normal low temperatures can be expected.
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A pair of shortwave troughs will impact the Upper Ohio Valley during this period. The first crosses the Great Lakes on Friday, with a very weak to nearly nonexistent surface low reflection. Upper jet support will be fading as the wave rotates across the eastern lakes. HRRR mean instability progs generally keep CAPE at 750 J/kg or less. While 0-6km bulk shear of 35-40 knots is possible, suggesting storm organization, the severe threat still appears low, with CSU machine- learning guidance in agreement.
Localized heavy rainfall does appear to be possible with near- saturated model soundings and precipitable water values of around 1.4 inches progged, which is essentially at the top end of the climatology. That said, NBM QPF expectations are not excessive, with 24-hour probabilities of 0.50 inch or greater through 8 AM Saturday in the 30-50 percent range, and chances of an inch under 15 percent.

The second is a larger trough that will impact more of the eastern CONUS through Saturday night. With an upper jet rotating through the trough across the Tennessee Valley, favored left entrance region jet dynamics lie closer to the Middle Ohio Valley and areas south of the Mason-Dixon Line. Thus, higher PoPs appear appropriate across the southern CWA Again, NBM 24-hour half-inch probabilities remain below 50 percent with the same north-to-south gradient. Severe threat remains very low as well.

Diurnal temperature spreads will be fairly low given the expected clouds and rain, with near-normal maximums but above-normal minimums.

LONG TERM /SUNDAY THROUGH THURSDAY/
KEY MESSAGES:

- Shower chances linger into Sunday as the second disturbance departs.
- Ridging should result in dry and warmer weather Monday and Tuesday.
- Rain chances return for Wednesday and Thursday.
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The earlier trend towards a more southerly track for the main shortwave trough on Sunday continues in the recent guidance. This does lead to generally low PoPs on that day, focused on areas south of Pittsburgh where moist easterly flow lingers during the morning.

Ensembles then seem to be in decent agreement in keeping this trough/upper low near or just off the southeast CONUS coast, while slowly rising 500mb heights take control over the Upper Ohio Valley.
This points to mainly dry weather from Sunday night through Tuesday, with increasingly above-normal temperatures through early next week.

The next shortwave trough then drives across the Upper Mississippi Valley towards the Great Lakes during the midweek period. Model cluster guidance shows strength and timing differences with this, which point to various scenarios for precipitation amounts and severe weather chances. In any case, PoPs topping in in the likely range and muted, but still warm high temperatures looks to be the forecast of least regret at this time.

AVIATION /06Z FRIDAY THROUGH TUESDAY/
The 06Z update suggests a slowing trend of cig falls early this morning, with the most pronounced drops in cigs most likely to arrive with the precipitation onset. Morning guidance suggests that most regions drop to low VFR or high MVFR in morning showers, though temporary drops in heavier showers are possible and denoted in tempo groups. Some cig recovery is expected into the day save FKL and DUJ.

Rain coverage should taper off and become more isolated/scattered Friday afternoon, though the chance for lightning will increase. For now, will continue to run with PROB30 TSRA for the afternoon hours given lower spatial confidence of impacts.

MVFR cig restrictions will favor FKL/DUJ after 18z, with probabilities increasing areawide after 06z Saturday.

Outlook
Upper trough passage to the south Saturday will maintain shra/tstm chances focused south of KPIT, with varying degrees of restriction potential.

VFR and dry weather become high probabilities Sunday into Monday under the influence of high pressure.

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

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