textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Morris County, NJ was added to the Flash Flood Watch.

The tornado risk has increased, mainly for SE PA, central and southern NJ, and northern Delmarva.

Increasing confidence in the flash flooding potential for today, mainly for New Jersey, southeastern PA, and northern Delaware.

KEY MESSAGES

1. A strong cold front is expected to cross through the region today, resulting in widespread showers and thunderstorms. Flash flooding, damaging wind gusts and a few tornadoes are possible later today.

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 today, resulting in widespread showers and thunderstorms. Flash flooding, damaging wind gusts and a few tornadoes are possible later today.

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 today. 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 of what should transpire later today. 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 this afternoon and evening.

Bulk shear values up to 30-40 kt support organized clusters where locally damaging wind gusts are possible. Some of the 00z CAMs have been suggesting some isolated discrete supercells developing out ahead of the main batch of convection. Thinking the models may have a good idea of how things will evolve today, with scattered showers and thunderstorms starting out as individual discrete cells in the early afternoon with a more linear, multi-cell clusters taking over and pushing through by the mid-afternoon and into the early evening.

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. SPC has introduced a 5% tornado risk over SE PA, northern Delaware, and the southern half of New Jersey. Locally backed flow at the surface near/south of the warm front will enhance low-level shear. In addition to increased low-level shear, we will have a very moist airmass overhead and low LCLs - factors favorable for tornadogenesis. Some question remains in how far north the warm sector gets, which will have implications for where the largest tornado threat is. For now though, the forecast currently has the area south of the PA Turnpike and I-195 as the main area to watch. This does include the Philly metro area. Cannot rule out isolated large hail as well as temperatures aloft are rather cool for this time of year. Cloud cover should increase at the morning goes on, which could be a limiting factor in terms of instability buildup. The Slight (2/5) Risk remains for most of the area.

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 today 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 4 inches per hour. With this being said, WPC has maintained a SLIGHT risk (Level 2/4) for excessive rainfall today over most of the area. There was talk of adding a Moderate Risk around the Philly metro, but have held off for now. Added Morris County, NJ to the Flood Watch since there was a slight trend north with the highest QPF. Otherwise, no changes. The Flash Flood Watch remains in place for most of New Jersey, southeastern PA, and northern Delaware.

In general, expecting anywhere between 1 to 2 inches of rain to fall, with localized higher amounts up to 3 to 5 inches. Guidance has remained relatively consistent all throughout today. We will have efficient rainfall producers where some areas could see multiple rounds, which is not a good combination. This event will also be during the evening commute, with travel disruptions likely. Have multiple ways to receive warnings.

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. The cold front will move offshore tonight but stall just offshore. Multiple waves of low pressure will ride along the stalled front, resulting in unsettled weather through the weekend. The main periods to watch are early Saturday through Saturday evening, with periods of rain increasingly likely, especially along/east of the I-95 corridor. Another round of rain is expected on Sunday as well. The severe threat looks low but there could be further flooding concerns, although no day has more than a Marginal (1/5) Risk for excessive rainfall. The areas to watch for flooding may depend on where most of the rain falls today.

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. Seasonable temperatures are expected.

AVIATION /12Z THURSDAY THROUGH MONDAY/

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

Today...VFR in the morning with restrictions likely developing in the afternoon (primarily after 18Z), 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 01Z. Moderate confidence overall, but low confidence on exact timing details.

Tonight...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 once precipitation moves out. Conditions should lift back to VFR near or not long after daybreak. North/northeast winds around 5 to 10 kt. Low confidence.

Outlook...

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 tonight. Winds increase to around 10-15 kt out of the south/southwest for today. A cold front will move through tonight, with winds becoming more north/northeasterly around 10-15 kt. Seas of 2 to 3 feet. Showers and thunderstorms expected on the waters this evening into tonight.

Outlook...

Friday through Monday...No marine headlines expected. Showers and thunderstorms possible, mainly during the day Friday through Sunday.

Rip Currents...

For today, 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, winds turn more onshore but will be light - around 10 MPH or less. 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 noon EDT today through late tonight for PAZ070-071-101>106. NJ...Flood Watch from noon EDT today through late tonight for NJZ008>010-012>022-025>027. DE...Flood Watch from noon EDT today through late tonight for DEZ001. MD...None. MARINE...None.