textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

For today into tonight, confidence in storm coverage and intensity remains low.

A Heat Advisory was issued for portions of the region, valid 11 AM to 8 PM Thursday.

The chance for showers and thunderstorms shifted a bit north on Friday. Friday afternoon high temperatures also inched up a few degrees.

KEY MESSAGES

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

A cold front currently situated across central portions of the region continues to sink southward, and should continue to make southwestward progress through this morning. This will leave our region in the relatively cooler marine layer and more stable air mass north of the front. The net result is that our region should be in northeast to easterly flow, and cloudy conditions for much of the day. As a result, surface based instability is expected to be very limited, with mostly elevated instability present.

Consequently, the risk for severe storms is still quite conditional and uncertain. Given how sharp of a mid level short wave trough is approaching, there will be ample forcing, but it is a question of if the elevated instability will be enough to sustain storms in our region.

With that said, some CAMs do feature scattered storms moving into parts of the area by late this afternoon and into tonight. Confidence in details is 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. 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 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 for today into tonight.

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

A strong marine layer influence today will keep highs mainly in the 70s to low 80s in most areas. 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 new Heat Advisory was issued for portions of southeastern PA and NJ. This advisory may need to be expanded into Delmarva if the forecast for heat indices near 105 degrees holds. 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 a few 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 includes general thunderstorms (no severe risk), though there is still some uncertainty with this. Our latest forecast has about a 30-40% chance of showers and thunderstorms near and south of Philadelphia.

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 very late on Monday.

AVIATION /08Z WEDNESDAY THROUGH SUNDAY/

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

Through 12Z...MVFR and IFR ceilings are expected to rapidly spread across the terminals through 09Z. Winds may briefly be light and variable for a time before settling out of the northeast to east around 5 kts. Areas of fog/mist possible at times resulting in visibility restrictions as well. Low confidence on the timing details, but high confidence in widespread restrictions developing.

Today...MVFR ceilings may remain through much of the day. There is a 20 to 40% chance for showers and storms through the day time that could also result in restrictions, however, there is considerable uncertainty to the extent of storm development tomorrow. Winds through much of the morning are expected to be out of the northeast at 5-10 kts, before shifting more east- southeasterly by 18-21Z. Low confidence.

Tonight...MVFR or IFR ceilings. Scattered showers and storms are possible, but there is low confidence in coverage and intensity. Visibility restrictions are likely in any stronger storms. East- southeast winds around 5 kts. Low confidence.

Outlook...

Thursday and Friday...Sub-VFR conditions are possible in showers and thunderstorms each day (20-40% chance), mainly during the afternoon to evening hours.

Saturday and Sunday...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.

Outlook...

Thursday through Sunday...Sub-SCA conditions are expected through the period. There is a chance (40 to 50%) for thunderstorms Thursday, mainly in the afternoon and evening hours. For Friday, the chance of thunderstorms (30 to 40%) shifts south a bit, basically extending from Manasquan Inlet, NJ down to Fenwick Island, DE.

Rip Currents...

For today, northeast flow will range from 10 to 15 mph with gusts up to 20 mph along the New Jersey coast. There will be southeast swells with a period around 10 seconds and breaking waves of 2 to 3 feet. As a result, there is a MODERATE risk for the development of dangerous and life threatening rip currents for the Jersey Shore along Monmouth, Ocean, and Atlantic counties, and a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore in Cape May county and Delaware Beaches.

For Thursday, winds become southwest at 10 to 15 mph with gusts up to 20 mph along the Jersey Shore. Breaking waves will range from 1 to 3 feet with light easterly swells with a period around 6 seconds. As a result, there is a MODERATE risk for the development of dangerous and life threatening rip currents for the Jersey Shore along Monmouth, Ocean, and Atlantic counties, and a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore in Cape May County 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. MD...None. MARINE...None.