textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The Heat Advisory was extended through Saturday for the I-95 corridor and portions of central and southern New Jersey, and now includes coastal Monmouth County.

The end time of the Heat Advisory for Carbon and Monroe Counties was changed to end at 8 PM this evening.

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, Delware. The Heat Advisory was expanded to include coastal Monmouth County, as southwesterly surface winds should prevent the sea breeze from making much inland progress there. The end time of the Heat Advisory for Carbon and Monroe Counties was changed to end at 8 PM this evening, as the forecast for Friday has trended well below advisory criteria. The Heat Advisory for the I-95 corridor and portions of central and southern New Jersey was extended through 8 PM Saturday, where confidence is highest in heat indices reaching near or over 100 degrees on Saturday. Elsewhere in eastern Pennsylvania and northern New Jersey, the Heat Advisory remains in effect through 8 PM Friday.

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.

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, with varying kinematics and forcing aloft. Mid level flow does appear to increase by early next week, which may be more supportive of better organized and stronger convection compared to this week.

Diurnally driven instability will 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.

Convection today should be confined primarily northwest of the I-95 corridor where forcing aloft is most supportive, though cannot rule our some storms making it as far southeast as the I-95 corridor. SPC and WPC maintain a Marginal Risks in this area. On Friday, convection looks to be greater in coverage, and perhaps more organized given the forcing and shear profiles. So we could see a somewhat greater damaging wind threat if any organized linear structures materialize. However, low level lapse rates will be poor and there's a notable lack of mid level dry air to strengthen downdrafts. Overall, looking at marginal risks of flash flooding and damaging wind gusts with greater coverage and organization of storms anticipated on Friday.

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 /12Z THURSDAY THROUGH MONDAY/

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

Today...Any restrictions around 12Z will quickly improve to VFR by 13-14Z. Afternoon SHRA/TSRA may impact KRDG/KABE, then KTTN/KPNE/KPHL/KILG. Have opted to include PROB30 groups for TSRA at the I-95 terminals, where the probability of direct impact form this activity is lower. Southwest winds 5-10 kts, becoming gusty at ACY/MIV by 18Z. High confidence in prevailing VFR conditions much of the day, but low confidence on SHRA/TSRA.

Tonight...Any SHRA/TSRA taper off in the evening. Generally VFR, though patchy fog possible. South winds 5 kts or less. Moderate confidence overall, low confidence on any fog developing.

Outlook...

Friday through Monday...Restrictions possible in afternoon and evening SHRA/TSRA each day, though Sunday looks dry, and patchy fog possible at night.

MARINE

No marine hazards expected through tonight. Southerly winds increasing to 15-20 kts by this afternoon, diminishing some overnight. Seas 2-4 feet.

Outlook...

Friday through Monday...No prevailing marine hazards are forecast. However, there could be some afternoon and evening showers and thunderstorms Friday, Saturday, and Monday, which could locally increase winds and seas. South to southwest winds 10-20 kts and seas 2-4 feet, greatest in the afternoon and evening periods.

Rip Currents...

For today, south to southwest winds will increase to 15 to 20 mph in the afternoon with breaking waves in the surf zone of 1 to 3 feet. There will be a light easterly swell with a period around 10 seconds. Opted to go with a MODERATE risk for the development of dangerous and life threatening rip currents for Cape May and Atlantic Counties, where the wind will have a greater onshore component, and a LOW risk elsewhere for the Jersey Shore and for Delaware Beaches.

Conditions will be fairly similar on Friday, thus maintained the same risk levels as today.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory from 10 AM this morning to 8 PM EDT Saturday for PAZ070-071-102-104-106. Heat Advisory from 10 AM this morning to 8 PM EDT Friday for PAZ060>062-101-103-105. Heat Advisory from 10 AM this morning to 8 PM EDT this evening for PAZ054-055. NJ...Heat Advisory from 10 AM this morning to 8 PM EDT Saturday for NJZ010-012>015-017>020-027. Heat Advisory from 10 AM this morning to 8 PM EDT Friday for NJZ001-007>009. DE...Heat Advisory from 10 AM this morning to 8 PM EDT Saturday for DEZ001. MD...None. MARINE...None.