textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Flood Watch is now in effect for portions of southeast Pennsylvania, northern Delaware, and New Jersey for late tonight through Sunday morning.

The Aviation section was updated for the 12Z TAFs.

KEY MESSAGES

1. Low pressure lifting through the region tonight through Sunday morning will produce showers and thunderstorms, as well as locally heavy rain resulting in a flash flood risk for portions of the forecast area.

2. During the latter half of next week, a slow moving cold front looks to make its way southward across the area bringing the potential for showers and storms.

DISCUSSION

KEY MESSAGE 1...Low pressure lifting through the region tonight through Sunday morning will produce showers and thunderstorms, as well as locally heavy rain resulting in a flash flood risk for portions of the forecast area.

High pressure over the area moves out to sea later today. Return flow sets up behind the high, and low level moisture will increase over the area. Meanwhile, low pressure will organize and develop over the Mid-Atlantic. A warm front associated with that low will lift north through the region this evening, and then southerly flow will allow dew points to rise well into the 60s and low 70s. PWATs will rise to 2-2.25 inches.

Low pressure then follows north through the region late tonight through Sunday morning. This system will be fairly progressive, lifting from Delmarva around midnight tonight to Long Island by late Sunday morning. However, due to the high moisture content in the atmosphere, there will be a 6-8 hour window during the early morning hours on Sunday where as much as 1-2 inches of convective rain will fall. The primary area of concern is the I-95 corridor and surrounding urban areas and small streams where flash flooding is more possible given any rapid runoff.

Precipitation tapers off Sunday morning as low pressure departs. A cold front swings through Sunday afternoon behind the low, and then a secondary front may pass through the region Sunday night.

KEY MESSAGE 2...During the latter half of next week, a slow moving cold front looks to make its way southward across the area bringing the potential for showers and storms.

Once we get through this weekend, attention will turn to Thursday and Friday of next week. It should be first noted there is a lot of spread in the forecast guidance at this time given this is about a week away. But overall, the signal in guidance is for a slow moving cold front to move southward through the area late next Thursday into Friday bring the potential for showers and some storms. PWAT values along this front look to potentially be quite high (around 2 inches) but there remains some questions about how much instability there will be as well as the speed and timing of the front as it moves through. The upshot of all this is that while forecast confidence is low, this period will need to be monitored for heavy rain potential.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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

Today...VFR. Any SHRA/TSRA will hold off until the evening hours. S-SE winds 5 to 10 kt. High confidence.

Tonight...IFR CIGs/VSBYs in SHRA and scattered TSRA. Light E-SE winds. Moderate confidence.

Sunday...Sub-VFR in SHRA in the morning, then VFR. W-NW winds 5 to 10 kt in the afternoon. Moderate confidence.

Outlook...

Sunday night through Wednesday...VFR.

MARINE

Sub-Small Craft Advisory conditions today, tonight, and Sunday. E-SE winds 10 to 15 kt with gusts up to 20 kt today and tonight, then W-SW on Sunday. Seas 2 to 4 ft.

VSBY restrictions in showers, scattered thunderstorms, and fog late tonight through early Sunday afternoon.

Outlook...

N/NW winds will increase later Sunday night into Monday and could briefly reach Small Craft Advisory levels. Winds will diminish by next Tuesday into Wednesday but seas look to increase to around 4-5 feet by this time.

Rip Currents...

Today...Southeast winds will increase to around 10 to 20 mph with breaking wave heights ranging between 1 to 3 feet. There will also be a 1 to 2 foot southeast swell around 8 to 10 seconds. Given strengthening onshore flow, have opted to maintain a MODERATE risk for the development of dangerous rip currents at both the Jersey Shore and Delaware Beaches.

For Sunday...Southwest winds around 10 to 20 mph in the morning will diminish to around 5 to 10 mph in the afternoon. Breaking wave heights will range between 1 to 3 feet. An underlying southeast swell around 8 to 10 seconds will continue. Given these conditions, have opted to go with a MODERATE risk for rip currents for Ocean, Atlantic and Cape May Beaches with a LOW risk for Monmouth and Delaware Beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch from late tonight through Sunday morning for PAZ070-071-103>106. NJ...Flood Watch from late tonight through Sunday morning for NJZ008>010-012>020-026-027. DE...Flood Watch from late tonight through Sunday morning for DEZ001. MD...None. MARINE...None.