textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated.

No significant changes to the forecast.

KEY MESSAGES

1. Isolated showers and thunderstorms possible through the day and into tonight, but the risk of hazardous weather is low.

2. Heat and humidity will build into the middle of the week, along with daily chances for showers and thunderstorms. There is an increasing threat of severe thunderstorms and potentially flash flooding during the middle of the week, namely Tuesday through Thursday.

DISCUSSION

KEY MESSAGE 1...Isolated showers and thunderstorms possible through the day and into tonight, but the risk of hazardous weather is low.

Rising heights and jet dynamics aloft early in the day should work to suppress convection until the afternoon or evening, when conditions will become more favorable for some isolated showers and storms to develop. Instability will be low (less than 1,000 J/kg) and lapse rates will be poor, but PWats are fairly high (over 1.5"). So not expecting any significant or hazardous weather out of any convection today into tonight, however cannot totally rule out some brief heavy downpours or locally heavy rainfall totaling around 1-2". The greater coverage of convection into the evening should be focused north of Philadelphia, where PoPs are greatest. Any convection persisting into the overnight period should wane toward dawn.

Highs today will be similar to Sunday, ranging from the upper 70s to mid 80s in most areas with similar humidity levels. Skies will be mostly cloudy, but a lucky few could see some more persistent sun. Humid again tonight with lows in the mid 60s to low 70s.

KEY MESSAGE 2...Heat and humidity will build into the middle of the week, along with daily chances for showers and thunderstorms. There is an increasing threat of severe thunderstorms and potentially flash flooding during the middle of the week, namely Tuesday through Thursday.

Broad upper level ridging returns to the eastern US early this week, bringing in surface high pressure across the eastern US. Afternoon highs will creep back into the mid 80s to low 90s for most areas, about 5 degrees above average for early September. Dewpoints will also remain in the upper 60s to low 70s. The hottest day(s) looks to be Tuesday and/or Thursday, when high temperatures could get into the mid 90s across Delmarva and into the Philly metro. Resulting heat indices will be well into the 90s to around 100 degrees. We will continue to keep an eye on the potential for any Heat Advisories.

One other aspect to watch with this ridging pattern is the potential for thunderstorms to rotate around the periphery of the ridge and the position of a frontal boundary during the middle of the week. Guidance maintains a consensus that the pattern will be supportive of multiple rounds of thunderstorms, especially during the Tuesday through Thursday time frame.

SPC maintains a Slight Risk (2 of 5) for severe thunderstorms Tuesday for portions of the area, and a Slight Risk has been introduced for Wednesday. It should be noted that there have been some indications in the guidance that Wednesday's severe potential will be much more conditional, as a frontal boundary looks to be draped across our area with potential for easterly winds and marine influence at the surface. This doesn't mean there would be zero severe potential, but adds a further degree of uncertainty to Wednesday's setup. Thursday remains highlighted with 15% probabilities for severe thunderstorms. The northwest flow pattern that will be in place will be supportive of all severe threats, including damaging winds, large hail, and tornadoes. The threat for flash flooding is typically lower with these patterns, however given multiple days with the potential for storms and recent heavy rainfalls, at least some risk for flash flooding will exist.

Long story short, the Tuesday through Thursday period looks very active with multiple rounds of thunderstorms across the region, but details still remain unclear. Stay tuned for updates on these threats over the coming days.

By Friday, current model consensus indicates the frontal boundary will be pushing into and south of our region with Canadian high pressure building across the Northeast. This should favor an end to severe weather and flash flooding potential. Labor Day weekend is still shaping up to feature nice weather, with temperatures trending slightly below normal and lower humidity.

AVIATION /18Z MONDAY THROUGH SATURDAY/

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

Today...Prevailing VFR conditions expected. Light rain showers track across the western portions of the forecast area, although no restrictions are expected. Only impact will be OVC or BKN ceilings throughout the day. Northeast winds in the morning around 5 kts, shifting to a more south to southeasterly direction by 18Z around 5 kts or less. Winds may become light and variable at times. There is a low (generally less than 20%) chance for isolated showers or storms which could cause brief restrictions in the afternoon. Moderate confidence.

Tonight...MVFR ceilings likely developing by 06Z. Scattered showers and perhaps a few storms around at times, especially between 00Z-06Z. Reductions in visibilities are unlikely outside of any brief rain showers or storms. Will continue to include PROB30 groups for TAF sites that have a meaningful probability of TSRA early in the night. Winds settling out of the south to southwest around 5 kts or less. Moderate confidence. After precipitation moves out of the TAF sites, have introduced IFR ceilings to PHL, ABE, TTN, and PNE. Ceilings improve to MVFR and then VFR throughout Tuesday morning.

Outlook...

Tuesday through Thursday...Restrictions possible in showers and thunderstorms each day, mainly during the afternoon to evening hours.

Friday...Primarily VFR.

MARINE

No prevailing marine hazards expected. East to southeast winds around 5-10 kts. Seas 1-2 feet. Generally fair weather aside from an isolated shower possible.

Outlook...

Tuesday through Friday...No prevailing marine hazards expected. Chances for thunderstorms each day, mainly in the afternoon and evening hours.

Rip Currents...

For today, north to northeast winds in the morning will shift to east in the afternoon around 5-10 mph. Breaking wave heights around 1 foot. A light southeasterly swell with a period around 8 seconds will remain. This will continue to lead to a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and Delaware Beaches.

For Tuesday, south/southwest winds will increase some to around 10-15 mph but they little change is expected so we'll continue with a LOW risk for the development of dangerous rip currents at the Jersey Shore and Delaware Beaches.

For specific beach forecasts, visit weather.gov/beach/phi

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.