textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Heat Advisory for Carbon and Monroe Counties will be allowed to expire at 8 PM. Also, updated Aviation Section for 00Z TAFs.

Timing of storms Friday night has trended slightly later.

KEY MESSAGES

1. Hot and very humid conditions are expected to persist through Tuesday of next week. A Heat Advisory is in effect for portions of the area.

2. There are daily chances for showers and thunderstorms through Saturday, as well as much of next week. There is at least a marginal risk each day for localized heavy rainfall leading to flash flooding and locally damaging wind gusts with any stronger thunderstorms that develop.

DISCUSSION

KEY MESSAGE 1...Hot and very humid conditions are expected to persist through Tuesday of next week. A Heat Advisory is in effect for portions of the area.

A relatively stagnant synoptic pattern is developing across our region. Quasi zonal mid and upper level flow will be in place through the middle of next week as Bermuda high pressure persists offshore. This will keep a steady stream of very warm and moist air in place across the region through the period.

A prolonged period of hot and, most notably, very humid conditions is expected to remain in place for the next several days. We look to have a weak cold frontal passage early Sunday, ushering in slightly drier air. This will give some slight reprieve from the excessive humidity, though highs will still be in the upper 80s to low 90s. Moisture advection will return again by Monday though, and hot and very humid conditions are anticipated to return Monday through Tuesday. A somewhat stronger cold front looks to come through late Tuesday or Wednesday, which will help break the excessive heat and humidity some.

A Heat Advisory remains in effect for portions of eastern Pennsylvania, much of New Jersey except the far southern counties, as well as New Castle County, Delware.

KEY MESSAGE 2...There are daily chances for showers and thunderstorms through Saturday, as well as much of next week. There is at least a marginal risk each day for localized heavy rainfall leading to flash flooding and locally damaging wind gusts with any stronger thunderstorms that develop.

Diurnally driven instability will mostly be in the 1000-2000 J/kg each afternoon through the next several days along with PWats in the 1.5-2" range. Mean flow will be predominantly southwesterly and relatively weak through Saturday. This will support a continued localized heavy rainfall threat with potential for flash flooding, although water loaded downdrafts could cause localized wind damage. Areas with very saturated ground from recent rains will be most prone to experiencing trees falling.

For this evening, the main limiting factor is the lack of mid level forcing and relatively weak low level convergence. As such, while we will continue to see isolated or even scattered shower and storm development, not expecting anything widespread or particularly long lived. The main threats will be brief heavy downpours and gusty winds.

On Friday, a mid level shortwave trough is expected to approach late in the day, providing more forcing (as compared to today). As such, storm coverage is expected to be more widespread, and storms should be more organized although bulk shear values are still relatively modest (up to 25 kt). All that being said, continued concern for heavy rain/flash flooding and damaging winds. To a lesser extent, there is also some risk for hail, given the deep unstable layer.

Details for Saturday's convection remain unclear, but we could certainly end up with some potential for flash flooding and severe storms again. Current indications in the guidance support a later timing of storms during the evening or even overnight depending on the timing of a weak cold front and shortwave trough.

We look to have a weak cold frontal passage by early Sunday, ushering in slightly drier air. This combined with subtle subsidence aloft will help briefly limit convection potential on Sunday and also give some slight reprieve from the excessive humidity, though highs will still be in the upper 80s to low 90s. Moisture and forcing aloft will return by Monday as mean flow shifts more westerly and mid level flow increases. This should result in a greater severe thunderstorm threat compared to this week, and perhaps slightly lower flash flooding threat, but the details into the period early next week remain unclear. Long story short, Monday and Tuesday look to continue the hot and humid conditions and daily thunderstorm chances.

A somewhat stronger cold front looks to come through late Tuesday or Wednesday, which will help break the excessive heat and humidity some, but the pattern aloft may still support diurnally driven convection.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

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

Tonight...Prevailing VFR expected. Any lingering shower or thunderstorm should cease by 03Z. After 06Z, some fog development is possible, especially at KRDG, KABE, and KTTN, but cannot rule out some patchy fog at some other terminals. Winds will generally settle out of the southwest around 5 kt, but may become light and variable at times. Moderate confidence as a whole.

Friday...VFR expected for most of the day. There will be a chance for showers and thunderstorms late in the day, primarily after 20-23Z from west to east. Opted to carry VCTS in some TAF sites where applicable at this time. Southwest winds generally between 5-10 kt, locally up to 15 kt. Moderate-high confidence.

Outlook...

Friday night...Primarily VFR expected, however sub-VFR conditions are likely at times with any showers or thunderstorms that linger into the evening/overnight.

Saturday-Saturday night...Mostly VFR except lower conditions possible with afternoon/evening showers and thunderstorms.

Sunday-Sunday night...VFR conditions expected.

Monday-Tuesday...Mostly VFR except lower conditions possible with afternoon/evening showers and thunderstorms.

MARINE

Winds and seas should remain below Small Craft Advisory criteria through Friday.

Outlook...

Friday night...gusty and erratic winds are possible in the vicinity of any thunderstorms Friday evening.

Saturday-Tuesday...Conditions expected to remain below advisory levels through Tuesday of next week.

Rip Currents...

For Friday...Southwest to south winds around 10 mph with breaking waves in the surf zone of 1 to 3 feet. A medium period east to southeast swell continues (dominant wave period of 5-7 seconds). Opted to go with a LOW risk for the development of dangerous rip currents at all beaches.

For Saturday...Southwesterly winds continue and should increase some to 10-20 mph. Breaking waves in the surf zone are forecast to increase some, especially for the Jersey Shore, at 2 to 4 feet. Given the increasing surf and southerly winds, went with a MODERATE risk for the development of dangerous rip currents for the Jersey Shore. Breaking waves should be a little lower still for the Delaware Beaches, therefore went with a LOW risk for the development of dangerous rip currents.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory until 8 PM EDT Saturday for PAZ070-071-102-104- 106. Heat Advisory until 8 PM EDT Friday for PAZ060>062-101-103-105. NJ...Heat Advisory until 8 PM EDT Saturday for NJZ010-012>015- 017>020-027. Heat Advisory until 8 PM EDT Friday for NJZ001-007>009. DE...Heat Advisory until 8 PM EDT Saturday for DEZ001. MD...None. MARINE...None.