textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Flash Flood Watch has been issued for portions of southeast PA, northern DE and New Jersey from 12 PM Thursday through 2 AM Friday.

The Slight Risk (2/4) for Excessive Rainfall has been expanded further north into portions of northern NJ.

A Slight Risk (2/5) for Severe Weather has been introduced across portions of southeast PA, southern NJ, and the Delmarva.

Aviation section updated for 00Z TAFs.

KEY MESSAGES

1. A strong cold front is expected to cross through the region on Thursday, resulting in widespread showers and thunderstorms. Flash flooding and severe weather, including all hazards are possible.

2. Conditions look to remain unsettled through the weekend, with daily chances of showers and storms each day.

3. Fair weather is expected to return for early next week.

DISCUSSION

KEY MESSAGE 1...A strong cold front is expected to cross through the region on Thursday, resulting in widespread showers and thunderstorms. Flash flooding and severe weather, including all hazards are possible.

To the south of a closed upper low over eastern Canada, broad troughing will be in place across much of the Northeast and Mid- Atlantic. An embedded mid-level shortwave will pass through the base of the trough on Thursday. At the surface, a warm front will move into portions of the area tonight as a cold front will begin to approach the area on Thursday.

With guidance coming into better agreement with the timing and placement of frontal boundaries in our area, there has been an uptick in confidence in what the evolution could be on Thursday. For now, multiple rounds of widespread showers and storms are expected. With the cold front approaching from the west, and a warm front expected to set-up across the southern half of the area across the Delmarva and into southern New Jersey, we will have to watch these areas for severe weather Thursday afternoon and evening. In fact, SPC has upgraded portions of the area to a SLIGHT risk for severe weather (Level 2/5) with a MARGINAL risk elsewhere. Bulk shear values up to 30-40 kt support organized clusters where locally damaging wind gusts are possible. With the warm front draped over portions of the area, there will be a localized corridor of a tornado threat, especially where any storm is able to latch onto the boundary. The best guess of where this may occur is somewhere in the Delmarva to southern New Jersey corridor where SPC has introduced a 2% tornado risk with locally backed flow at the surface. Cannot rule out isolated large hail as well as temperatures aloft are rather cool for this time of year. There should be an abundance of clouds in place on Thursday, so this could hinder the magnitude of the severe threat to some degree.

The other, and possibly more concerning threat may be the risk and potential for flash flooding across portions of the area. Most guidance continues to depict a very moisture rich airmass surging northward on Thursday where PWAT values are expected to top out around 2.2-2.6 inches. Model forecast soundings also show tall, skinny CAPE profiles and a relatively deep warm cloud layer. The one saving grace is that the overall shear vectors indicate that storms should be 'relatively' quick moving. However, any given storm will be capable of producing highly efficient rain rates, at times around 2 to 3+ inches per hour. With this being said, WPC has maintained a SLIGHT risk (Level 2/4) for excessive rainfall on Thursday, but has expanded the risk further north to incorporate northern New Jersey. In general, expecting anywhere between 1 to 2 inches of rain to fall, with localized higher amounts up to 3 to 5 inches. This is supported by most of the 12Z guidance suite today, especially where the HREF LPMM show localized bullseyes of higher rainfall. Given the increased confidence in flooding potential, have opted to issue a Flash Flood Watch for portions of southeast PA, northern Delaware, and New Jersey from 12 PM Thursday through 2 AM Friday.

The severe and flooding rain threat should diminish overnight into Friday morning.

KEY MESSAGE 2...Conditions look to remain unsettled through the weekend, with daily chances of showers and storms each day.

Mid to long 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 are currently in the 40-80% range on Saturday and 30-60% range on Sunday. Given the present uncertainties in timing and placement of the frontal features and progression in the long wave pattern, have opted to keep very close to the NBM.

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

KEY MESSAGE 3...Fair weather is expected to return for early next week.

High pressure looks to bring fair and tranquil weather to start next week. The pattern potentially becomes more active again towards the middle to end of next week.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight...VFR with increasing high clouds. West-southwest winds around 5 kt or less early, becoming light and variable overnight. Moderate-high confidence.

Thursday...VFR in the morning with restrictions likely developing in the afternoon (primarily after 16Z), as widespread showers and thunderstorms are likely at all terminals. Any storms will be capable of producing very heavy rainfall and LIFR conditions. Winds will vary depending on location, with northern terminals experiencing east- northeast winds around 5 kt where southern terminals will encounter southeasterly winds around 5-10 kt. Winds may locally become gusty and erratic near any storm. Showers and storms ending by 00Z. Moderate confidence overall, but low confidence on exact timing details.

Outlook...

Thursday night...Sub-VFR conditions probable at times with showers and thunderstorms continuing into the evening hours, before diminishing overnight. Fog and/or low stratus possible at night.

Friday through Friday night...Primarily VFR, though cannot rule out isolated showers/thunderstorms for southern terminals which could result in some restrictions.

Saturday through Sunday...Periods of VFR and sub-VFR at times. Restrictions expected with daily chances for showers and thunderstorms.

Monday...VFR. No significant weather expected.

MARINE

No marine headlines expected through Thursday. South-southwest winds around 10-15 kt with seas around 1 to 2 feet.

Fair weather expected through Thursday morning, before widespread showers and thunderstorms develop in the afternoon and evening hours. Locally gusty and erratic winds will be possible near/in any thunderstorm. Waterspouts are also possible.

Outlook...

Friday through Monday...No marine headlines anticipated. Daily showers and storms possible on all waters Friday through Sunday. Fair weather returns on Monday.

Rip Currents...

For Thursday, south to southeast winds increasing to near 10 mph. 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 Friday, north to northeast winds will be near 10 to 15 mph. Breaking waves will average around 1 to 2 feet. There will continue to be a southeasterly swell with a 6 to 9 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...Flood Watch from Thursday afternoon through late Thursday night for PAZ070-071-101>106. NJ...Flood Watch from Thursday afternoon through late Thursday night for NJZ009-010-012>022-025>027. DE...Flood Watch from Thursday afternoon through late Thursday night for DEZ001. MD...None. MARINE...None.