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FXUS61 KPBZ 181819 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 219 PM EDT Sat May 18 2024

SYNOPSIS
Scattered showers and thunderstorms will dissipate this evening, with areas of fog overnight. High pressure will provide mainly dry weather from Sunday into early next week, with a warming trend in temperature.

NEAR TERM /THROUGH TONIGHT/
KEY MESSAGES:

- Mainly isolated/scattered showers and storms will end this evening.
- Areas of fog, possibly dense locally, will develop overnight.
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The axis of an upper trough currently lies through western PA southwestward to the Middle Ohio Valley, with a slowly filling upper low swirl clearly visible in northern Ohio on water vapor imagery.

Showers to this point remain quite isolated in most cases, with the bulk of the activity along the ridges. Latest PIT ACARS sounding shows 1100 J/kg of CAPE available with no cap, although moisture drops off significantly above 650mb or so.
With very little flow and forcing in the trough, it appears that shower and thunderstorm activity west of the ridges will remain widely scattered/isolated at best, and CAMs continue to advertise this thinking as well. Also, with precipitable water slowly dropping off (now 0.93 on that ACARS sounding), any threat of very localized heavy rain will be driven mainly by the slow steering flow as opposed to efficient rain processes.
Mesoanalysis plots show the best downdraft CAPE, in the 700-800 J/kg range, across the western CWA With some dry air aloft, gusty downburst wind cannot be totally ruled out in a strong storm.

The trough will continue to slowly sag south and east through tonight, eventually to be replaced by upper ridging nosing in from the Ohio Valley. Any showers/storms will collapse during the evening with the loss of heating. Convective cloud will also dissipate, leaving most of the region with a mostly clear overnight period. That, plus the light wind and the lingering low-level moisture/wet ground, will likely lead to areas of fog overnight into Sunday morning, and it would not be a surprise if it was locally dense in spots. Overnight low temperatures, mainly in the 50s, will remain near to slightly above normal.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
KEY MESSAGES:

- Outside of isolated afternoon showers in the ridges on Sunday, dry weather can be expected through Monday night.
- Temperatures will climb some 10 degrees above normal by Monday.
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Upper ridging builds overhead on Sunday, with its axis oriented from the middle Mississippi Valley northeastward into the Ohio Valley and lower Great Lakes region. Dry weather is still forecast for the majority of the region. However, hi-res models continue to depict isolated showers along the PA/WV ridges, where subsidence may be a bit weaker and where low-level easterly flow may aid development. Any such activity should end with the loss of daytime heating Sunday evening. Higher heights and increased sunshine should add roughly five degrees to afternoon highs as compared to today.

The ridge then dominates our weather Sunday night through Monday night, with the axis passing through during the daylight hours. Dry weather and temperatures some 10 degrees above normal can be expected, with most non-ridge locations reaching the lower to mid 80s on Monday.

LONG TERM /TUESDAY THROUGH FRIDAY/
KEY MESSAGES:

- Dry and quite warm Tuesday.
- Rain chances return for Wednesday and Thursday, with some potential for strong thunderstorms.
- Seasonable temperatures and lower rain chances are expected Friday and Saturday.
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Dry weather continues into Tuesday night as the upper ridge slowly works off of the Atlantic Seaboard. Plenty of sunshine and another day of drying will help add a degree or two to Monday's highs, pushing values further into the 80s.

The models continue to show some differences with the strength and timing of the next trough crossing the Plains into the Great Lakes, which will likely have the most impact on the Wednesday and Thursday forecast. GFS/GEFS seem to prefer a faster/stronger wave, which could bring precipitation back sooner than the overall grand ensemble solution, while the ENS leans towards a bit more shallow trough. In any case, good precipitation chances continue to favor the Wednesday through Thursday period. We will continue to watch for the potential for severe weather as the midweek period approaches, as CIPS/CSU severe guidance showing some potential, particularly on Wednesday. Well above-normal temperatures Wednesday may be knocked back closer to climatology by Thursday due to a cold frontal passage.

Model divergence increases further for Friday and Saturday, but additional, smaller shortwaves may provide lower-end chances for rain, with temperatures not straying far from climatology.

AVIATION /18Z SATURDAY THROUGH THURSDAY/
A scattered to broken VFR convective type cloud deck has taken over this afternoon as convective temperatures are achieved.
Very weak boundary layer flow will generally keep wind variable at 5 knots or less through this evening, perhaps favoring a northerly direction. Widely scattered showers and thunderstorms are expected to develop this afternoon after 19z (20-40% chance... highest to the east), but with no real defined surface forcing mechanism aside from orographic influence, confidence is low in impacts to any one terminal at any given time. As stated, hi res ensemble favors development primarily east of PIT and potentially affecting at LBE/DUJ with brief restrictions in TSRA.

Showers and storms will dissipate after 00z and give way to more widespread restrictions again possible overnight tonight with lingering low level moisture in place and wind going mostly calm. Probability for MVFR to IFR VIS returns to 40-70% and with less cloud coverage and no rain expected, confidence is a bit higher tonight than Friday night for fog development. FKL/DUJ have a low to moderate probability of LIFR VIS.

Ridging builds on Sunday and VFR returns with high confidence after mixing dissipates the fog and erodes any lingering scattered cloud deck in the morning. Wind will again be light during the day with a weak pressure gradient in place.

Outlook
VFR and dry weather remain high probabilities into Monday and Tuesday under the influence of high pressure. Precipitation chances and associated restrictions return Wednesday.

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

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