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FXUS61 KPBZ 141805 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 205 PM EDT Tue May 14 2024

SYNOPSIS
Scattered showers, and a thunderstorm or two, remain possible through Wednesday as an upper trough approaches and crosses. Dry weather is forecast Wednesday evening through Thursday evening, before unsettled weather returns to end the week. Temperatures will remain slightly above normal.

NEAR TERM /THROUGH TONIGHT/
KEY MESSAGES:

- Scattered light rain showers will remain possible through the night - Thunderstorm chances remain low, with no severe threat.
- Above-normal low temperatures are forecast.
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Light rain showers/sprinkles continue to lift northward across the region in a pattern of weak vorticity advection. Although a thunderstorm or two remains possible, near-term mesoanalysis shows CAPE values peaking in the 500-600 J/kg range, with ACARS soundings showing generally unfavorable lapse rates.

A better round of vorticity advection exists upstream in the Middle Ohio Valley, where a more robust area of showers currently exists. This forcing should arrive in the southwestern portion of the CWA after 00Z, with higher PoPs appropriate.
Forcing overall appears to weaken as a Missouri upper low fills somewhat, so more scattered activity is indicated overnight for the rest of the region. QPF overall remains muted, and will remain below a quarter-inch in most cases. The abundant moisture and cloud cover will help to keep temperatures above normal, with most lows in the upper 50s and lower 60s.

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/
KEY MESSAGES:

- Shower/thunderstorm chances continue through Wednesday, with little risk of strong thunderstorms or heavy rainfall.
- Ridging provides drier weather Wednesday night into Thursday night.
- Near to above normal temperature expected.
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Most guidance is in good agreement that the main upper trough, centered over the Middle Ohio Valley at 12Z Wednesday, will track slowly eastward through Wednesday night. This will lead to better coverage of showers and a few thunderstorms during this period as compared to today. This will be especially true to the northwest of Pittsburgh, where a slow-approaching surface boundary associated with Hudson Bay low pressure will exert a bit of influence. This is also where thunderstorms will be a bit more likely, as HRRR progs show better surface-based instability of 500-750 J/kg during the afternoon. However, severe risk appears minimal given this low level of CAPE, as well as unimpressive shear.

Precipitation coverage should wane Wednesday evening as the trough departs and as the surface boundary continues to weaken as it crosses. Ridging will then take hold, with minimal precipitation chances late Wednesday night through most of Thursday night, although some showers may begin to show up in the western CWA prior to 12Z Friday ahead of the next shortwave.

Chances of heavy QPF values remain low, with probabilities of an inch or more remaining generally 25 percent or less according to the NBM. Some localized higher totals may be possible in thunderstorms, but in general, 0.25 to 0.50 inch can be expected from tonight through Wednesday evening.

LONG TERM /FRIDAY THROUGH MONDAY/
KEY MESSAGES:

- Forecast uncertainty increases during the extended period.
- Another trough is likely to provide unsettled weather at least for Friday and Saturday, although rain amounts are iffy given the uncertainty.
- Temperatures near or above normal are expected.
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Uncertainty creeps upward in the extended period. The mid-level ridge will be departing the Appalachians Friday morning, with moisture increasing once again ahead of the next trough, which ensembles have crossing the Mississippi River Friday evening and reaching the Upper Ohio Valley by Saturday.

Thereafter, speed and timing differences arise in the models with the handling of this trough. GEFS/GEPS members are favoring a quicker departure and a weaker trough, while ENS favors the opposite outcomes. Differences then continue to get larger from there, with cluster analysis indicating anything from strong ridging to the presence of a closed upper low is on the table by early next week.

For now, will roll with the NBM suggestions for the extended period and wait for increased model clarity. This would suggest a return of unsettled, but likely not overly impactful weather Friday and Saturday, with lower rain chances during the early part of next week. Near to slightly above normal temperatures are currently forecast, but again, there is a sizable range of outcomes here.

AVIATION /17Z TUESDAY THROUGH SATURDAY/
Moist advection ahead of and eventually around an upper level low that will traverse south of the region is expected to promote widespread low VFR to MVFR conditions through the period. Lack of strong forcing will limit probability of terminal impact from isolated showers through 06z, before a more noteworthy wave promotes greater shower coverage between 06z and 21z Wednesday. Of note, models highlight potential frontogenetic banding that would support a swath of showers and low probability thunderstorms across eastern OH into northwest PA.
If these do develop, be prepared for locally lower cigs/vsbys in heavier rain.

Ensemble model guidance has struggled with cig heights and timing of lowering restrictions, meaning lesser confidence in the timing of TAF restriction impacts.

Outlook
Restrictions are likely to linger or see only brief improvements Wednesday afternoon while shower and thunderstorm chances favor eastern OH to northwest PA.

Exiting of the upper trough and high pressure will promote VFR conditions by Thursday afternoon. The next low pressure system is likely to arrive Friday into the weekend.

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

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