textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Marginal Risk for the Severe Weather Outlook on Thursday has expanded a bit across portions of southern New Jersey and the Delmarva. Otherwise, no significant changes to the forecast have been made.

Updated aviation discussion for 00Z TAFs.

KEY MESSAGES

1. A weak cold front will sink south through the region toward the coast overnight, with dry conditions and lower humidity expected for Wednesday.

2. A stronger cold front is expected to cross through the region on Thursday, resulting in greater chances for showers and thunderstorms. Conditions look to remain unsettled through the weekend, with daily chances of showers and storms each day.

DISCUSSION

KEY MESSAGE 1...A weak cold front will sink south through the region toward the coast overnight, with dry conditions and lower humidity expected for Wednesday.

Diurnally driven convection is moving offshore as of 730 PM. This will continue to diminish and move offshore, and a dry overnight period is forecast. Some patchy fog possible, especially southeast of I-95, where rain has fallen from this afternoon. High pressure building in on Wednesday will yield mostly sunny skies with seasonable temperatures topping out in the mid to upper 80s and lower, more comfortable humidity levels.

KEY MESSAGE 2...A stronger cold front is expected to cross through the region on Thursday, resulting in greater chances for showers and thunderstorms. Conditions look to remain unsettled through the weekend, with daily chances of showers and storms each day.

An upper-low will pivot around portions of Ontario and Quebec through the end of the week, ultimately passing north of our area Wednesday night. To the south of the upper low, broad troughing will be in place across much of the Northeast and Mid- Atlantic, where a few embedded shortwaves, will be pivoting through the base of the trough. At the surface, a cold front will begin to approach the area late Wednesday night.

As noted by the previous shift, there remains considerable differences in timing with the frontal passage though the area. Most of the 12Z suite of guidance has continued to slow down the forward progression of the front. With the continued slowing of the front, this may yield more of a convective/hydro threat as some of the mid-range CAM guidance shows potential for multiple rounds of storms on Thursday. Given the uncertainty, confidence remains low whether there will be any severe weather or flash flooding concerns; so we'll have to continue to monitor these potentials going forward. As of now, WPC has a Marginal risk of excessive rainfall for the southern half of the area, while SPC has a Marginal risk for severe weather across portions of southern New Jersey and the Delmarva. Regardless of convective/hydro threats, PoPs remain in the 60-80% range for Thursday afternoon and evening.

Longer range NWP and AI guidance suggests that the pattern could become unsettled Friday night and into the weekend. Depending on how the frontal boundary evolution pans out on Thursday, will impact the weather over the weekend. Most guidance does suggest that the frontal boundary will stall over or in close proximity of the area where several areas of low pressure may ride along the stalled boundary through the weekend. PoPs currently are mainly in the chance to likely range each day due to the present uncertainties in timing and placement of these features and any progression in the long wave pattern.

In terms of temps, highs and lows will both be quite seasonable for late August, but humidity levels may be uncomfortable at times.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

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

Tonight...Prevailing VFR expected. Low stratus and patchy fog is anticipated to develop after 03Z at KMIV and KACY. Elsewhere, probability of any restrictions in fog is low, but not zero. Northwest winds around 5 kt or less, becoming light and variable at times. High confidence, but lower confidence with respect to any fog restrictions.

Wednesday...VFR with mostly clear skies. West winds around 5-10 kt. High confidence.

Outlook...

Wednesday night...VFR. No significant weather expected.

Thursday through Sunday...Periods of scattered showers and storms are probable, with the greatest chance and coverage on Thursday. Restrictions are possible with any showers or storms.

MARINE

No marine headlines are in effect through Wednesday. Winds will generally remain less than 10 kt through tonight. On Wednesday, southerly winds will increase to around 10-15 kt. Seas around 2 feet through Wednesday.

Outlook...

Wednesday night through Friday night...No marine headlines expected. Showers and thunderstorms are likely to occur on both Thursday and Friday afternoon/evenings.

Saturday through Sunday...SCA conditions possible due to seas around 5 feet. Scattered showers and storm possible each day.

Rip Currents...

For Wednesday and Thursday, westerly winds 5 to 10 mph early will shift southerly by the afternoon (more southwest on Wednesday, more southeast on Thursday). Breaking waves will average around 1 to 2 feet. There will continue to be a southeasterly swell with a 6 to 8 second period. The result of these conditions is a LOW risk for the development of dangerous and life threatening rip currents for 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.