textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated aviation discussion for 12z TAFs.

KEY MESSAGES

1. Multiple rounds of heavy thunderstorms will affect the area through today into tonight bringing the threat of both severe weather and flash flooding. Additional thunderstorms are possible on Friday.

2. Hot and humid conditions are forecast across the region on Thursday.

3. The Labor Day Weekend will feature mostly dry weather with at or below normal temperatures and lower humidity.

DISCUSSION

KEY MESSAGE 1...Multiple rounds of heavy thunderstorms will affect the area through today into tonight bringing the threat of both severe weather and flash flooding. Additional thunderstorms are possible on Friday.

There are still some showers and storms ongoing at this hour but these have diminished in intensity compared to earlier this morning. By later in the morning through a good portion of the afternoon there looks to be a lull in storms as some sunshine develops with it getting quite warm and humid as the area breaks into the warm sector with a WSE flow. This will lead to further destabilization to occur. In fact ML CAPES looks to max out in the 2000-3000 j/kg range. This will set the stage for additional storms to occur along a cold front working south mainly as we get into Thursday evening. There's some uncertainty on the timing and coverage of these storms and at this point the Storm Prediction Center has most of the region in a MARGINAL risk of severe thunderstorms meaning isolated severe storms are possible. Only portions of our eastern MD shore counties are grazed by the SLIGHT risk area that's focused mainly to our south and west. Similar to this morning, PWATs will be quite high so flash flooding will be a threat as well.

Friday's SPC outlook only has a small portion of the Maryland Eastern Shore and southern Delaware in a Marginal risk for severe weather. Th rest of the area includes general thunderstorms (no severe risk), though there is still some uncertainty with this. Our latest forecast has about a 40-70% chance of showers and thunderstorms from about I-78 on southwards, including the entire Philadelphia metro area.

KEY MESSAGE 2...Following the brief cool down that occurred Wednesday, hot and humid conditions are once again expected across the region on Thursday as the frontal boundary lifts north again. With the front to the north, high temperatures are forecast to climb into the upper 80s to mid 90s for many areas (low to mid 80s far north). These temperatures combined with dewpoints in the low to mid 70s will again result in heat indices well into the 90s to low 100s south of I-78, perhaps even as high as 105 degrees for Delmarva.

A Heat Advisory remains in effect for portions of southeastern PA, NJ, and Delmarva. North of the current advisory area, maximum heat indices are forecast to generally top out in the low to mid 90s.

KEY MESSAGE 3...The Labor Day Weekend will feature mostly dry weather with at or below normal temperatures and lower humidity.

By Saturday, current model consensus indicates the frontal boundary will be pushing well south of our region, with Canadian high pressure building across the Northeast and Mid-Atlantic. This will largely end any shower and thunderstorm potential. Thus, Labor Day Weekend is still shaping up to feature nice weather, with temperatures trending at or slightly below normal and lower humidity.

AVIATION /12Z THURSDAY THROUGH MONDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Thursday...Mainly IFR cigs lifting to MVFR by around late morning. Showers continue into this morning and still can't rule out some thunder though coverage and intensity of storms has been on the wane the past couple hours. There should then be a break of a few hours this afternoon before yet another round of storms could affect the TAF sites very late in the afternoon into the evening. SW winds 5 to 10 kt. Moderate confidence.

Thursday night...SHRA/TSRA may continue through the evening before tapering off overnight. MVFR/IFR restrictions possible in any storms. Light west winds. Moderate confidence.

Outlook...

Friday...Sub-VFR conditions are possible in showers and thunderstorms (40-60% chance), mainly during the afternoon to evening hours.

Saturday through Monday...Primarily VFR conditions expected.

MARINE

Winds and seas are expected to stay below Small Craft Advisory conditions. However, gusty and erratic winds are possible in the vicinity of any storms, particularly early this morning and again Thursday afternoon/evening.

Outlook...

Thursday night through Monday...Sub-SCA conditions are expected through the period. There is a chance (20 to 50%) for thunderstorms to linger Thursday night. For Friday, the chance of thunderstorms is around 40 to 60%, mainly in the afternoon and early evening hours.

Rip Currents...

For Thursday, winds will mainly be out of the south/southwest around 10 MPH or less. Breaking waves will generally be 2 feet or less with a dominant wave period of 9 seconds. Given light winds, and not much in terms of wave heights, elected for a LOW risk for rip currents at all beaches.

Similar conditions expected Friday though winds turn more offshore, lowering the rip current risk even further. Will continue the LOW risk through the end of the week for the Jersey Shore and Delaware Beaches.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for PAZ060-070-071-101>104-106. NJ...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for NJZ013>015-017>020-027. DE...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for DEZ001>003. MD...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for MDZ012-015-019-020. MARINE...None.