textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

There is an increasing threat of severe thunderstorms during the middle of the upcoming week, especially on Wednesday.

KEY MESSAGES

1. Scattered showers and thunderstorms are anticipated to develop across portions of eastern Pennsylvania and New Jersey later today 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 during the middle of the week, especially on Wednesday.

DISCUSSION

KEY MESSAGE 1...Scattered showers and thunderstorms are anticipated to develop across portions of eastern Pennsylvania and New Jersey later today into tonight. Temperatures will be warmer with increasing humidity as well.

As high pressure shifts offshore through the day, southwesterly flow will ramp up into the afternoon. Temperatures will warm into the mid 80s for most areas (even some upper 80s in spots), on the order of 5 degrees warmer than Saturday. Dewpoints will also surge back well into the 60s to around 70 degrees by the afternoon hours, making for a noticeable return in summer-like humidity. Skies will turn mostly cloudy by the afternoon, especially along and NW of I-95, which should help negate the degree of unpleasant humidity.

The main forecast challenge for today will be the potential for scattered showers and thunderstorms developing later this afternoon and into tonight. The latest trend in the CAMs has been for an even later convective initiation time than before, which will significantly limit any potential for severe and flash flood threats. The greatest limiting factor appears to be a lack of substantial instability until close to sunset, with even the highest modeled CAPE largely less than 1,000 J/kg. This combined with poor lapse rates and weak forcing during the peak diurnal time frame should limit coverage for much of the daytime hours. However, better forcing does look to arrive during the evening to even overnight hours. So at least some scattered showers and potential for a few thunderstorms are still anticipated to develop at some point. We still cannot totally rule out a couple stronger storms, with gusty winds, small hail, and brief heavy downpours possible.

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 during the middle of the week, especially on Wednesday.

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 low 100s. We will have 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. Guidance is coming into a consensus that the pattern will be supportive of multiple rounds of thunderstorms, especially during the Tuesday through Thursday time frame. We could see some scattered showers and storms on Monday, though the severe and flash flood risks appear to be low for Monday.

SPC maintains probabilities for severe thunderstorms for our region on Wednesday (Day 4), and has introduced a Day 3 SLIGHT Risk (level 2 out of 5) for Tuesday. Thursday could also pose a severe thunderstorm risks depending on how all the synoptic and mesoscale factors shake out. The northwest flow pattern that will be in place will be supportive of all severe threats, though the threat of flash flooding is typically lower in these patterns. 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 the threat for severe thunderstorms in 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, which would bring an end to the risk of thunderstorms and severe weather. This extended range pattern also supports dry and very pleasant weather for Labor Day weekend.

AVIATION /07Z SUNDAY THROUGH THURSDAY/

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

Through 12Z...Prevailing VFR, though there is a low chance (less than 20%) of some patchy fog, particularly for MIV and ACY. Winds favoring a south-southwesterly direction at less than 5 kts, with calm periods likely. Moderate confidence overall.

Today...Mainly VFR except some low stratus is likely in the morning for RDG and ABE where we've introduced a TEMPO group for MVFR cigs. Isolated to scattered showers and storms will be around in the afternoon and evening, which could lead to restrictions after 20Z. RDG and ABE have the highest chance for restrictions, but they cannot be ruled out as far southeast as the I-95 terminals. Have maintained VCTS and VCSH for most areas, though TEMPO groups may be needed eventually. South to southwest winds 5-10 kts. Moderate confidence.

Tonight...MVFR ceilings anticipated after 06Z or so in the wake of the afternoon and evening showers and thunderstorms. South to southwest winds around 5 kts or less, shifting northeast after 06Z. Low confidence.

Outlook...

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

MARINE

No prevailing marine hazards expected. Winds will shift south- southwesterly at 10-15 kts. Seas 2-3 feet. Showers and storms are possible beginning this evening, particularly for the northern NJ coastal waters.

Outlook...

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

Rip Currents...

For today, south to south-southwest winds of 5-10 mph increase to 10-15 mph in the afternoon. Breaking waves of 1 to 3 feet. A light southeasterly swell with a period around 8 seconds will remain. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and Delaware Beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

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