textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Aviation section was updated for the 00Z TAFs.

KEY MESSAGES

1. Through tonight, a shortwave trough could spark another round of showers and storms, but it will be working against a relatively stable boundary layer.

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

3. Unsettled conditions will continue on Thursday with the potential for severe thunderstorms. Scattered thunderstorms are again possible on Friday, mainly south of Philadelphia.

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

DISCUSSION

KEY MESSAGE 1...Through tonight, a shortwave trough could spark another round of showers and storms, but it will be working against a relatively stable boundary layer.

An additional round of severe thunderstorms is possible tonight. Model guidance has struggled with the convection today with uncertainty continuing into tonight. Currently, it looks as though the backdoor cold front moves back through the area as a warm front late this evening into tonight. Confidence in details regarding this remains relatively low, but should the region experience more destabilization and erosion of the marine layer than is currently anticipated, storms will be capable of damaging winds, some hail, and perhaps an isolated tornado. The greater risk of any severe storms will be toward our southern and western zones. As of right now, model soundings continue to show a step inversion into tonight with instability remaining elevated. However, forcing and shear values remain favorable. Locally heavy rainfall will also be a threat with any training showers and storms, even if a severe thunderstorm threat does not materialize. Portions of eastern PA have been highlighted in a Slight (level 2 out of 4) Risk for excessive rainfall, which could lead to localized flash flooding.

SPC maintains a Slight Risk (level 2 out of 5) for severe thunderstorms for much of our area, but again, this risk is highly conditional and low confidence into tonight.

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

Following the brief cool down in the marine layer today for most areas, 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 and NJ. This Advisory has been expanded into Delmarva given the continued trend of forecast heat indices near 105 degrees. North of the current advisory area, maximum heat indices are forecast to top out in the low to mid 90s, thus the advisory is unlikely to need to be expanded farther north.

KEY MESSAGE 3...Unsettled conditions will continue on Thursday with the potential for severe thunderstorms. Scattered thunderstorms are again possible on Friday, mainly south of Philadelphia.

Another round of severe thunderstorms are possible on Thursday. Friday could also experience a few severe storms across Delmarva, depending on how quickly the frontal boundary pushes south.

SPC has a Slight Risk (level 2 out of 5) for severe thunderstorms on Thursday for Delmarva and a Marginal Risk (level 1) to as far north as the I-78 corridor.

The northwest flow pattern will remain in place on Thursday and will continue to be supportive of all severe threats, including damaging winds, large hail, and isolated tornadoes, depending on how all the mesoscale details pan out. The threat for flash flooding is typically lower with these patterns, but not zero. Training can still occur and a single locale could have multiple days with heavy rainfall.

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 40-60% chance of showers and thunderstorms from about I-78 on southwards, including the entire Philadelphia metro area.

KEY MESSAGE 4...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. The only road bump at the moment, could be a cold front that approaches the forecast area on Tuesday.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight...IFR CIGs, possibly as low as LIFR. Several rounds of SHRA/TSRA will impact the terminals tonight, the first being from 02Z to 06Z at KRDG/KABE, 03Z to 07Z at the I-95 corridor terminals, and 04Z to 08Z at KMIV/KACY. The second round will impact the terminals around 12Z. VSBY restrictions in the stronger TSRA, and VSBY restrictions in BR. LGT/VRB winds. Low confidence.

Thursday...IFR CIGs in the morning, lifting to VFR after 15Z. SHRA/TSRA may impact the terminals in the 12Z to 16Z timeframe, and then the next round of convection may impact KRDG/KABE after 22Z. SW winds 5 to 10 kt. Moderate confidence.

Thursday night...SHRA/TSRA taper off by 06Z. VFR. Light W to NW 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 tonight and again tomorrow afternoon/evening.

Outlook...

Thursday night through Monday...Sub-SCA conditions are expected through the period. There is a chance (20 to 40%) for thunderstorms to linger Thursday night. For Friday, the chance of thunderstorms is around 30 to 50%, 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 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 to 8 PM EDT Thursday for PAZ060-070- 071-101>104-106. NJ...Heat Advisory from 11 AM to 8 PM EDT Thursday for NJZ013>015- 017>020-027. DE...Heat Advisory from 11 AM to 8 PM EDT Thursday for DEZ001>003. MD...Heat Advisory from 11 AM to 8 PM EDT Thursday for MDZ012-015- 019-020. MARINE...None.