textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

There is an increasing threat of severe thunderstorms for late this afternoon and evening along and northwest of I-95.

A portion of the Heat Advisory for northern New Jersey and southeastern Pennsylvania was extended through Saturday.

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. A portion of the Heat Advisory for northern New Jersey and southeastern Pennsylvania was extended through Saturday. This includes Morris and Hunterdon Counties in NJ, and upper Bucks, western Montgomery, and western Chester Counties in PA.

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 (except for Sunday) 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 late this afternoon and evening, a mid level shortwave trough will approach the area, providing more forcing (as compared to Thursday). 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 kts). Latest environmental parameters and CAMs indicate we should see some scattered thunderstorms developing across eastern PA by mid afternoon. Given the weak low level shear, cold pool development should begin to propagate eastward fairly quickly. This should ultimately support an MCS or two of sorts developing and moving toward the I-95 corridor ahead of maximum boundary layer thermodynamics. This will support damaging wind gusts for a couple hours or so as this MCS(s) moves east. Exactly where the greatest severe threat will be will depend on where the early activity begins developing upstream. As the activity moves east, a loss of diurnal heating and accelerating cold pool cutting off inflow should cause storms to weaken toward the coast. Forward motion of the storms should preclude a significant flash flood threat, so we have not issued a Flood Watch, however PWats of 2-2.25" will support very heavy rainfall rates of 2-3" per hour. If these were to impact urbanized areas for long enough, there could certainly be some flash flooding concerns.

All of that said, the threat of severe thunderstorms today has increased, and SPC has add a Slight Risk for damaging winds along and northwest of I-95 (Marginal Risk elsewhere in the forecast area). We remain in a Marginal Risk for excessive rainfall from WPC.

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. PWats look slightly lower than today, and mean flow is a bit more westerly than southwesterly, but overall the thermodynamics appear similar. There will be less forcing, so convection probably won't be as widespread as today. We remain in a Marginal Risk for excessive rainfall from WPC and general thunder from SPC for Saturday.

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 /12Z FRIDAY THROUGH TUESDAY/

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

Today...Any fog or stratus will clear quickly by 13Z. VFR expected for most of the day. Scattered showers and thunderstorms will develop late in the day and organize into a line of sorts, primarily after 20-23Z from west to east. Highlighted this with VCTS at all TAF sites, and included TEMPO groups for IFR visibility where thunderstorms are most likely. Southwest winds generally between 5-10 kts, locally up to 15 kts. Moderate to high confidence.

Tonight...Primarily VFR expected, however restrictions are possible at times prior to 04Z with any showers or thunderstorms. Patchy fog possible late. Moderate confidence overall.

Outlook...

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

No prevailing marine hazards are expected through tonight. Southerly winds increasing to 15-20 kts. Seas 2-4 feet. A line of showers and thunderstorms may move offshore late in the evening, with gusts of 30-40 kts and locally higher seas possible.

Outlook...

Saturday-Tuesday...No prevailing marine hazards expected. Thunderstorms possible Saturday night and Tuesday night.

Rip Currents...

For today...Southwest to south winds around 10-15 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 much of 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 and coastal Monmouth County, 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 PAZ070-071-101>106. Heat Advisory until 8 PM EDT this evening for PAZ060>062. NJ...Heat Advisory until 8 PM EDT Saturday for NJZ008>010-012>015- 017>020-027. Heat Advisory until 8 PM EDT this evening for NJZ001-007. DE...Heat Advisory until 8 PM EDT Saturday for DEZ001. MD...None. MARINE...None.