textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

A Severe thunderstorm Watch has been issued for parts of South New Jersey, metro Philadelphia and Delmarva until 1AM Monday.

Updated for the 00Z TAFs.

Heat Advisory has been discontinued.

KEY MESSAGES

1) Multiple rounds of heavy showers and thunderstorms are expected to impact the area late today and continuing through Monday bringing the threat for flash flooding.

2) Severe weather remains a threat with showers and storms later this afternoon through this evening with the greatest threat existing for areas near the I-78 corridor southward.

DISCUSSION

KEY MESSAGE 1... Multiple rounds of heavy showers and thunderstorms are expected to impact the area tonight and continuing into Monday bringing the threat for flash flooding.

A Flood Watch remains in effect for the I-95 corridor and points northwest through 8 PM Monday.

A somewhat diffuse frontal boundary remains situated across the area. As of the evening hours, showers and thunderstorms have developed over many areas in response to diurnal heating plus weak impulses moving through aloft. The atmospheric column is more saturated compared to yesterday with PWATs progged to be over 2 inches. At the surface, the front is stationary over portions of eastern PA into central portions of NJ. Aloft, the area will be under the right entrance region of a jet streak over New England extending into the Canadian Maritimes. This pattern looks to lock in place right through Monday. Heavy showers and some storms will become widespread across the area this evening into tonight. Rain rates of over 2 inches per hour are likely at times and model guidance indicates there will be areas likely to see 3-4+ inch rainfall amounts in just a matter of a few hours within the heaviest storms. Concern remains for flash flooding impacts, some potentially significant, especially for the I-95 corridor and points northwest.

Overnight, showers and storms should temper down at least somewhat due to the loss of daytime heating, however, there will still be plenty of frontal forcing at the surface and jet support aloft to at least keep some showers and possible storms around. This is not a setup where convection will be completely dying after sunset. Lingering elevated instability will mean even overnight lightning will be possible.

Unfortunately, very little change is likely into Monday. Expect showers/storms to linger over the area and potentially become quite widespread and heavy by the afternoon period once again leading to more instances of flash flooding.

Impacts from flash flooding could be significant over the next couple days. Areas facing the greatest threat from flash flooding will be urban and lower lying areas and areas around streams and creeks as these are likely to exceed their banks in spots.

The heaviest of the rain should diminish overnight Monday night but even into Tuesday there are likely to be some more showers and storms around even though they are not likely to be as widespread and intense.

KEY MESSAGE 2... Severe weather remains a threat with showers and storms later this afternoon through this evening with the greatest threat existing for areas near the I-78 corridor southward.

Severe thunderstorm Watch #465 is in effect for South NJ, metro Philadelphia and Delmarva until 1AM Monday.

The storms discussed above will also have the potential to produce severe weather into the early overnight period. MLCAPE values will be less than the previous couple of days given some cloud cover and weak lapse rates, however, it will still be in the 1000-1500 J/kg range, especially from Philly southward. Given modest shear, multi-cell clusters are expected to be the dominant mode with damaging winds being the primary hazard. The Storm Prediction Center has the majority of our area in a Slight Risk (2 of 5) for severe storms. The highest threat for severe weather looks to be areas near and south of the I-78 corridor where instability will be the greatest.

AVIATION /00Z MONDAY THROUGH FRIDAY/

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

Tonight...Sub-VFR conditions expected. Widespread showers and storms are expected to spread across all terminals between 00Z and 05Z. Strong winds and visibility restrictions are likely with this activity. MVFR ceilings to start, with IFR expected after 08-10Z. Winds generally East to Southeast at 5-10 kt. Low confidence overall.

Monday...Sub-VFR conditions expected. While there may be a relative lull in shower and storm activity in the morning, widespread showers and storms are again expected during the afternoon and evening hours. IFR ceilings may improve to MVFR by 14-16Z. East-northeast winds at 5-10 kt. Low confidence.

Outlook...

Monday night...Scattered to widespread showers and thunderstorms with sub-VFR conditions expected at times.

Tuesday...Still some showers around that could bring sub VFR conditions.

Wednesday...Mainly VFR.

MARINE

No marine headlines are anticipated through Monday. East to southeast winds around 10 kt this afternoon, becoming east to east to northeast tonight into Monday over northern and central waters. Seas generally 2-3 feet. Scattered to widespread showers and storms are expected beginning this evening and continuing through Monday. Gusty winds, rough seas, and visibility restrictions are likely in the strongest storms.

Outlook...

No marine headlines are currently anticipated through midweek, with winds below 25 kt and seas remaining below 5 ft. However, a stalled cold front will again be the focus for showers and storms tomorrow afternoon and evening, with additional isolated showers and storms also possible on Tuesday. Winds back to the E-NE 5-10 kt north of the front, remaining E-SE south of the front over far southern waters through Tuesday.

High pressure builds across New England through midweek, and will eventually push the stalled boundary south of the area Tuesday night through Thursday. Winds back to the N-NE behind the front during this period, with seas also building gradually later Tuesday through Wednesday, especially north of Barnegat Light, though again likely remaining below 5 ft.

Rip Currents...

Through Tuesday...Winds remain east to southeast around 10 kt, with gusts to 20 kt in the afternoon. Resulting increasing NE wind wave will return an elevated MODERATE Rip Current Risk along much of the Jersey Shore Monday and Tuesday, with LOW risk persisting over Cape May/Atlantic Counties and the Delaware Beaches, where winds remain lighter and more SSE through Tuesday afternoon.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Flood Watch through Monday evening for PAZ054-055-060>062-070- 071-101>106. NJ...Flood Watch through Monday evening for NJZ001-007>010-012>027. DE...Flood Watch through Monday evening for DEZ001. MD...None. MARINE...None.