textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated Key Message 1. The severe threat has diminished though a few storms could produce gusts up to 40 MPH.

The aviation and marine section has been updated.

Climate section added for potential record highs Thursday and Friday.

KEY MESSAGES

1. The severe threat was trended down for this evening, but will pick up again Thursday afternoon and Friday afternoon.

2. Dangerous heat (potentially record heat) and humidity for much of the region Thursday and Friday, then much less humid over the weekend.

DISCUSSION

KEY MESSAGE 1...The severe threat was trended down for this evening, but will pick up again Thursday afternoon and Friday afternoon.

A broken line of showers and thunderstorms are moving through Delmarva and South Jersey at this hour. Have not seen any gusts more than 40 MPH, so these storms are and should remain sub- severe as they move offshore by 8-9 PM. We turn our attention to tomorrow for a better chance for severe thunderstorms.

Warm and muggy tonight with lows in the mid 60s to low 70s. Some fog or low clouds may develop late, especially near any areas that get a decent amount of rain through early this evening.

The pattern in place will continue to support some additional showers and thunderstorms across the region Thursday and Friday, mainly during each afternoon and evening. Better synoptic forcing will arrive on Friday however with a cold front.

Large scale forcing and shear are expected to remain relatively weak on Thursday, however we will have a convectively enhanced shortwave from remnant convection upstream in the Midwest to contend with. This will enhance the forcing at the mesoscale, however the timing and location of this feature is less certain. Hotter temperatures and high dew points will yield steeper low-level lapse rates. This is a pretty typical severe thunderstorm setup for our region, and will likely trigger at least some diurnally driven convection mainly from mid afternoon through early evening. The details remain less certain as it will also depend on how things evolve upstream through tonight. A SLIGHT risk (level 2 out of 5) for a severe thunderstorm potential remains in place for Thursday. Damaging winds will be the main threat as storms may cluster or evolve into one or more short lines with some bowing, however large hail cannot be ruled out given the magnitude of the instability that is forecast.

On Friday, the region is expected to begin feeling the influence of the upper-level trough to the west of the region, in addition to a cold front that will be arriving from the west. Some showers and thunderstorms should focus near and ahead of this front, and with strong destabilization and slightly better wind fields aloft, a severe threat will once again be present. The convective coverage could be once again enhanced by a convectively induced shortwave from the Great Lakes or upper Ohio Valley. This evolution is less certain, however our entire area is highlighted in a SLIGHT risk (level 2 out of 5) for severe thunderstorms. Damaging winds will once again be the main threat as storms may tend to cluster or evolve into one or more short lines with some bowing.

KEY MESSAGE 2...Dangerous heat (potentially record heat) and humidity for much of the region Thursday and Friday, then much less humid over the weekend.

A mid-level ridge slides across the Mid-Atlantic and Northeast Thursday and Friday, however a shortwave trough crosses New England for a time. At the surface, high pressure remains centered off the Southeast U.S. coast which will continue to direct a hot air mass across our area.

Dangerous heat is expected across much of our region on Thursday and Friday, with even some record high temperatures potentially getting challenged. High temperatures both Thursday and Friday are forecast to peak into the low to mid 90s across most of the area, though highs may be a little warmer on Friday compared to Thursday. Dew points will start out in the low to mid 70s, but with afternoon heating and mixing, these should mix down to the upper 60s and low 70s. This still results in peak heat index values of 95 to 105 degrees across the majority of the area, outside of the southern Poconos. This level of heat and humidity can become dangerous quickly for sensitive individuals and those outdoors for extended periods of time. A Heat Advisory remains in effect Thursday through Friday for areas where the heat index criteria starts at 96F and 100F. For coastal Monmouth County, the advisory is only in effect for Thursday as some onshore flow Friday may result in some more immediate cooling off the ocean.

A cold front is forecast to move through the region late Friday, which will knock the temperatures and especially the dew points down several degrees. Though it will still be hot with high temperatures in the upper 80s to low 90s over the weekend, dew points will return to much more comfortable levels and this minimizes the risk for additional heat related advisories.

AVIATION /00Z THURSDAY THROUGH MONDAY/

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

Tonight...Thunderstorms have moved through the I-95 and Lehigh Valley terminals and should move through KMIV/KACY by 01z. A brief period of VFR conditions are expected for tonight but conditions should drop to at least MVFR between 04z-06z. There is a low chance of conditions dropping to IFR with patchy fog and low stratus, mainly between 06z-11z, but would put the probability of IFR around 20-30% or so. Any restrictions should lift by 12z. Winds generally light around 5 kt, favoring a southwesterly direction. Low confidence.

Thursday...Primarily VFR but another round of showers and thunderstorms are expected late in the afternoon, mainly after 22z. Have not included thunder in the TAFs at any site this time given confidence in spatial extent is too low and the probability of thunderstorms at any given terminal is around 30-40%. Winds out of the west around 10 kt. Moderate confidence.

Thursday Night...Scattered showers and thunderstorms could linger for the first half of the night, likely resulting in some restrictions. Cannot rule out some patchy fog late. Westerly winds 5 kt or less. Low confidence.

Outlook...

Friday...Prevailing VFR expected through the period. Brief periods of sub-VFR conditions are possible, with isolated to scattered showers and thunderstorms during the afternoon and evening hours.

Saturday through Sunday...VFR. No significant weather expected.

Monday...Sub-VFR possible with the chance for rain showers.

MARINE

A Small Craft Advisory remains in effect through midnight tonight as winds out of the south/southwest are around 25 kt and seas around 5 feet. Some isolated thunderstorms may impact the nearshore waters and Delaware Bay this evening. Winds and seas decrease overnight.

For Thursday, conditions are expected to remain below Small Craft Advisory criteria. A southwest wind at 10 kt with gusts 15-20 kt and seas of 3-4 feet. Some isolated to scattered showers and thunderstorms may impact the nearshore waters and Delaware Bay Thursday evening.

Outlook...

Friday through Saturday...No marine headlines anticipated. Winds will generally remain between 5-15 kt through Saturday. Seas generally around 2-3 feet. Isolated to scattered showers and thunderstorms possible on Friday. Fair weather expected for Saturday.

Sunday...A brief period of marginal Small Craft Advisory conditions are possible Sunday afternoon and evening with wind gusts near 25 kt.

Monday...No marine headlines anticipated with winds 5-10 kt and seas 2-3 feet.

Rip Currents...

On Thursday, south to southwest winds will be around 10 mph with breaking waves of 1 to 2 feet and a southerly swell with a period around 6 seconds. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents for the Jersey Shore and Delaware Beaches.

Conditions will be fairly similar on Friday, so the LOW risk for the development of dangerous and life threatening rip currents continues for the Jersey Shore and Delaware Beaches.

Ocean water temperatures are mostly in the 60s.

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

CLIMATE

A very hot and humid air mass will overspread our area through Friday. Some record high temperatures could be challenged Thursday and/or Friday.

Location 6/11 Record Highs/Year

Mount Pocono 91/1911 Allentown 94/1984 Reading 98/1911 Philadelphia 95/1986 Trenton 96/1911 Atlantic City Airport 97/1984 Atlantic City Marina 91/1959 Wilmington 95/1973 Georgetown 95/1959

Location 6/12 Record Highs/Year

Mount Pocono 87/1967 Allentown 92/2017 Reading 95/1984 Philadelphia 95/2015 Trenton 94/1933 Atlantic City Airport 94/2017 Atlantic City Marina 93/1914 Wilmington 96/1933 Georgetown 95/2017

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for PAZ060>062-070-071-101>106. NJ...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for NJZ001-007>010-012-013-015-017>020-027. Heat Advisory from 11 AM to 8 PM EDT Thursday for NJZ014. DE...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for DEZ001. MD...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ450>455.