textproduct: State College

WHAT HAS CHANGED

* Air Quality Alert upgraded to code Purple for all of PA on Friday

KEY MESSAGES

1) Wildfire smoke from Ontario and northern Minnesota will result in unhealthy air quality and reduced visibility tonight and Friday

2) Strong to severe thunderstorms with heavy rainfall are possible Saturday afternoon and evening

DISCUSSION

KEY MESSAGE 1: Wildfire smoke from Ontario and northern Minnesota will result in unhealthy air quality and reduced visibility tonight and Friday

Widespread smoke continues to reduce visibility (generally 1-3 miles in FU and HZ) and degrade air quality across much of the region. As of 8 PM, PM2.5 AQI of 300-450+ have been observed across parts of central PA, with lower values of 150-250 (less bad) in the Lower Susq Valley. The PA DEP has upgraded the Friday Air Quality Alert to Code Purple (very unhealthy) for potential 24-hour averages in the 200-300 range.

Incremental adjustments to daytime temperatures: 1) lower through Friday due to the extensive smoke layer 2) lower on Saturday due to projected early arrival of clouds and rain showers. Lower adjustments were made to dewpoints across the board due to systematic NBM model bias.

KEY MESSAGE 2: Strong to severe thunderstorms with heavy rainfall are possible Saturday afternoon and evening

A potent cold front trailing a deepening sfc low tracking up the St. Lawrence River Valley is expected to trigger one or more rounds thunderstorms Saturday afternoon and evening. Southwesterly low-level flow ahead of the front will transport a high PW airmass into CPA. Strong synoptic forcing, driven by an amplifying trough over the Northeast and favorable upper jet dynamics, will support the potential for heavy rain rates to produce isolated flash flooding concerns. WPC introduced a D3 SLGT ERO Saturday across southeastern PA, clipping Lebanon, Lancaster, and York Counties, with the remainder of PA in a MRGL ERO.

SPC level 2/5 severe thunderstorm risk continues cover most of central PA for day 3/Saturday. As thunderstorms initiate along the front, outflows may eventually congeal into one or more linear clusters posing a threat for damaging winds. Reduced instability may be a limiting factor if clouds and showers persist from early in the day.

A drying trend is expected Sunday with noticeably lower humidity allowing for a pleasant day and relatively cool night. Additional bouts of rain/t-storms are possible into the workweek. Temperatures will trend near to below historical averages through late week with some noticeable cooler nights possible.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

The primary concern during the 00Z TAF period will continue to be visibility restrictions due to wildfire smoke from Canada and northeastern Minnesota. Tonight may bring the most widespread restrictions as winds lighten behind the front with high pressure moving in, with the HRRR/RRFS models showing IFR to LIFR visibilities across much of Central PA. Improvements from overnight IFR-LIFR on Friday is favored across the northeastern edge of the smoke plume (currently forecast to be KBFD/KIPT), where the wind's northerly component may linger enough to bring smoke south of those sites, with improvements looking less likely over southwestern Pennsylvania (KJST/KAOO) where the winds may lighten as the core of the smoke plume moves overhead.

Outlook...

Sat-Sun...Chance of showers and t-storms, mainly during the afternoon hours on Saturday into early Sunday aft. Smoke moving out.

Mon...Mainly VFR.

Tue...Restrictions possible in showers/t-storms.

CTP WATCHES/WARNINGS/ADVISORIES

Air Quality Alert until midnight EDT Friday night for PAZ004>006-010>012-017>019-024>028-033>037-041-042-045-046- 049>053-056>059-063>066.