Sunrise 5:55 AM   Sunset 8:35 PM Moonrise 6:00 PM   Moonset 3:37 AM

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FXUS61 KPBZ 211136 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 736 AM EDT Tue May 21 2024

SYNOPSIS
Another very warm day with plenty of sunshine and mainly dry conditions. The risk for showers and storms returns Wednesday and Thursday, with severe weather a possibility.

NEAR TERM /UNTIL 6 PM THIS EVENING/
KEY MESSAGES:

- Isolated showers/storms are possible this afternoon, mainly north and over the ridges.
- Well-above normal temperatures today that may approach record levels at a couple of climate sites.
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Upper level ridge will flatten and be pushed eastward this morning as a weak shortwave trough slowly travels across the region today. The best lift within the trough will sink well south of the area. A popup afternoon shower or storm cannot be completely ruled out as the trough drifts across PA, but moisture will be limited. The main impact from the wave will likely be an increase in cirrus clouds and scattered cu this afternoon. Temperatures will again be well above normal.

SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/
KEY MESSAGES:

- The risk for severe storms increases Wednesday afternoon and evening, especially north and west of Pittsburgh.
- Scattered showers/storms on Thursday as a cold front crosses.
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A quiet and warm night, under brief ridging aloft.

A strong shortwave trough will swing through Ohio Wednesday morning and then clear western PA by early evening. This wave will increase the risk for showers and storms as it moves through. A cold front will trail behind the upper level wave, but it likely won't begin to impact the area until sometime after dark.

The degree and placement of the severe risk will depend on timing of the boundary/trough mentioned above. At this time, western portions of the CWA would seem to have higher risk given the likely lean towards evening arrival, with perhaps some weakening as storms moved east across the CWA SPC and NBM CWASP guidance seem to agree with this general thinking, leaning towards areas north and west of Pittsburgh. Deep shear in the 0-6km layer appears fairly marginal for a supercell threat at this distance, but given the ongoing heat and humidity ahead of the boundary, instability should be sufficient for some threat of damaging wind and hail.
Storm strength will likely wane after midnight as noted above, but at least scattered showers and storms will linger as the cold front enters the forecast area. High temperatures on Wednesday will depend greatly on how soon clouds and rain arrive, but they still will be well above normal.

The cold front will very slowly cross the region on Thursday as zonal to southwest flow aloft will work against its southward movement. Showers and storms are likely to continue on Thursday with the crossing boundary. The severe weather risk on Thursday should be tempered by the clouds and rain limiting daytime heating, and thus the buildup of instability. This will also keep temperatures a bit closer to climatology.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/
KEY MESSAGES:

- Relatively dry with seasonable temperature behind the front Thursday night into Friday.
- Near to just above seasonable temperatures chances of scattered showers/storms return for the holiday weekend.
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Meanwhile, an upper low over Ontario will likely weaken and drift into or near New England by Friday evening. While this happens, upper flow over our region will transition through zonal to weak ridging, according to ensembles/cluster analysis.
The cold front should also be able to clear our CWA to the south as well, but there is a bit of uncertainty with this. Thus, while the Thursday night into Friday night period should be relatively dry, some chance-level PoPs will need to be maintained, especially in areas south of Pittsburgh closer to the potential position of the front.

There are indications that a weak shortwave will attempt to undercut the ridging by later Friday night into Saturday, which could push moisture back northward and lead to a bit better precipitation coverage. The forecast for the remainder of the holiday weekend is muddled, as it will depend on the possibility of the old frontal boundary moving back northward, as well as shortwave details in the general WSW flow aloft. As such, a dry day cannot be guaranteed from Saturday through Memorial Day.
Temperatures overall from Friday on will be a bit above normal for the most part, given the overall flow pattern.

AVIATION /11Z TUESDAY THROUGH SATURDAY/
VFR is expected through the TAF period with relatively light south southwesterly wind as high pressure centered to the east remains the dominant feature. Afternoon heating will result in scattered cumulus.

Outlook
Shower and thunderstorm chances increase Wednesday afternoon into Thursday with the passage of a low pressure system; timing on the arrival of the first wave of storms remains varied.
Thunderstorms will be capable of producing gusty and erratic wind.

Periodic thunderstorm chances and associated restrictions are likely through the weekend as an active weather pattern develops.

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

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