textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Slight Risk (2/4) for Excessive Rainfall was added from the I-95 corridor and points east for Thursday.

Marginal Risk (1/5) for Severe Weather expanded northward, now extending from the I-78 corridor on south for Thursday.

QPF Amounts increased for Thursday.

Updated 12z aviation discussion.

KEY MESSAGES

1. High pressure nudges in for today with mainly dry and comfortable conditions. Some patchy fog possible this morning.

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...High pressure nudges in for today with mainly dry and comfortable conditions.

High pressure building in today will yield mostly sunny skies with seasonable temperatures topping out in the mid to upper 80s and lower, more comfortable humidity levels. The only other thing of note for today is a very low chance (15-20%) of showers forming along the sea-breeze this afternoon. No significant impacts are expected from this though.

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 tonight. 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 tonight.

As noted by the previous shift, there remains considerable differences in timing with the frontal passage though the area. Most of the 00Z suite of guidance has continued to slow down the forward progression of the front. With the continued slowing of the front, this will yield more of a convective/hydro threat as some of the mid-range CAM guidance shows potential for multiple rounds of storms on Thursday. Some CAMs, such as the 00z HRRR and NAMNest both had a convective line pushing through our area on Thursday evening. There is some uncertainty but confidence is increasing in that there will be some severe weather and flash flooding concerns. WPC has actually added a Slight (2/4) Risk for excessive rainfall from the I-95 corridor on east. This makes sense since guidance has PWATs around 2 to 2.3". Soundings show a long/skinny CAPE profile as well, which usually results in more of a hydro than severe weather threat. However, with moderate instability and decent shear, severe weather is definitely on the table, mainly in the form of damaging wind gusts. This is especially true if linear clusters of storms form upstream and move towards our area, like some of the CAMs suggest. The Marginal (1/5) Risk was expanded northward, now going to I-78. If we see another guidance cycle similar to this morning however, it would not be a surprise for the introduction of a Slight (2/5) Risk. The main timing window is for Thursday afternoon and evening, though some showers may develop in the morning ahead of the main window.

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.

High pressure looks to nudge in next week, which should shake the unsettled pattern.

AVIATION /10Z WEDNESDAY THROUGH SUNDAY/

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

Today...VFR. Westerly winds around 5 to 10 kt. High confidence.

Tonight...VFR. Winds generally out of the west around 5 kt or less. Moderate confidence.

Outlook...

Thursday through Thursday Night...VFR to start but restrictions expected in the afternoon with widespread showers and thunderstorms expected to move through. Restrictions likely continue into Thursday Night.

Friday through Friday Night...Primarily VFR, though cannot rule out some showers/thunderstorms from KILG on south which could result in some restrictions.

Saturday through Saturday Night...Restrictions expected with showers and thunderstorms anticipated.

Sunday...Primarily VFR though cannot rule out a few showers and thunderstorms.

MARINE

Sub-SCA conditions expected through tonight. Winds out of the west/southwest around 10 kt or less. Seas 1 to 2 feet.

Outlook...

Thursday through Sunday...No marine headlines anticipated. Showers and thunderstorms expected at times on the waters, especially Thursday and Saturday.

Rip Currents...

For today and Thursday, westerly winds 5 to 10 mph early will shift southerly by the afternoon (more southwest today, 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.