textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

There is an increasing threat of severe thunderstorms and potential flash flooding during the middle of the upcoming week.

KEY MESSAGES

1. Scattered showers and thunderstorms are possible across portions of eastern Pennsylvania and New Jersey into tonight. Temperatures will be warmer with increasing humidity as well.

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...Scattered showers and thunderstorms are possible across portions of eastern Pennsylvania and New Jersey into tonight. Temperatures will be warmer with increasing humidity as well.

This afternoon, the high pressure that has been dominating our weather for the past day has shifted off the Mid-Atlantic coast, and will continue to slide eastward with time. Warm and moist southwesterly flow has commenced across the region, with temperatures rising into the mid 70s to upper 80s from north to south. Widespread cloud cover over the northern half of the area will continue to limit heating this afternoon, but temperatures may rise a couple more degrees into late this afternoon across southern NJ and Delmarva.

The atmosphere across portions of southeastern PA and southern NJ is modestly destabilizing, with Objective Analysis indicating around 500 J/kg of MLCAPE. Some additional destabilization is likely through the afternoon, with at least pockets of 1000 J/kg expected. With modest forcing overspreading the region, isolated to scattered showers are developing across portions of southeastern PA. Thus far, this activity has mostly struggled. However, some increase in storm coverage and intensity is expected as we head toward the evening hours. Locally heavy rainfall is possible with the strongest storms. Some localized flooding can't be ruled out especially in areas that have received substantial rainfall recently. However, overall this potential is expected to remain fairly limited. WPC has placed eastern PA and NJ under a Marginal Risk (1 of 4) for excessive rainfall today. The threat for severe thunderstorms is low, though a few instances of gusty winds or small hail will be possible with the strongest storms.

Scattered showers may persist well into the overnight hours as forcing remains aloft.

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 next 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 looks to be Tuesday, 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. We will continue to keep an eye on the potential for 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. Latest guidance suggests that Monday may be mostly dry, though isolated storms can't be ruled out. Guidance continues to come into consensus that the pattern will be supportive of multiple rounds of thunderstorms, especially during the Tuesday through Thursday time frame.

SPC has placed portions of the area in a Slight Risk (2 of 5) for severe thunderstorms Tuesday. Wednesday and Thursday are also 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 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 south of our region with Canadian high pressure building across the Northeast. This should favor an end to severe weather and flash flooding potential.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

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

Rest of today...Primarily VFR. Isolated to scattered showers and storms will be possible for RDG, ABE, and the I-95 terminals. Trends this afternoon suggest that the greatest chance for storms will be across the I-95 terminals, but coverage should be limited. South-southwest winds generally 5-10 kts, though a few gusts 15-20 kts will be possible. Low confidence regarding potential shower and storm coverage this afternoon.

Tonight...Ceilings will once again lower after 04-06Z in the wake of the afternoon and evening showers and thunderstorms. Currently forecasting MVFR ceilings for at least a few hours at all terminals. South to southwest winds around 5 kts, becoming light and variable after 06Z with calm periods possible. Low confidence in the extent of MVFR restrictions.

Monday...MVFR ceilings in the morning should lift/scatter to VFR by around 15Z. Northeast winds in the morning around 5 kt, shifting to a more southerly direction during the afternoon. There is a low (generally less than 20%) chance for isolated showers or storms which could cause brief restrictions in the afternoon. Moderate confidence.

Outlook...

Monday night 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. South-southwesterly winds of 10-15 kts into the first part of tonight, with winds turning more westerly and diminishing late. East to east- southeast winds Monday around 5-10 kts. Seas 2-3 feet. Showers and storms are possible beginning this evening. Generally fair weather Monday.

Outlook...

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

Rip Currents...

For Monday, north to northwest winds in the morning will shift to east in the afternoon around 5 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 to 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.