textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

1. There is a slight risk of severe thunderstorms today.

2. Dangerous heat (near record heat) and humidity for much of the region through today, then much less humid over the weekend.

3. Chances for showers and storms will return as a cold front passes through late Sunday into Sunday night, then turning more seasonable through the middle of next week.

4. Widespread minor coastal flooding will develop with the evening high tide cycles Saturday and Sunday along the Atlantic coast, Delaware Bay, and on the tidal Delaware River and its tributaries.

DISCUSSION

KEY MESSAGE 1...There is a slight risk of severe thunderstorms today.

The pattern in place will continue to support showers and thunderstorms developing across the region late this afternoon and evening.

We will 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 isolated to scattered showers and thunderstorms should focus near and ahead of this front. One potential limiting factor coverage wise will be a mid level jet streak producing localized subsidence across the center portion of the forecast area, from SE PA across the Philly metro and into southern NJ. PoPs were lowered in this area. Strong destabilization and slightly better wind fields aloft will still support a severe threat where ever storms do materialize (most likely focused across the southern and northern portions of our area).

The convective evolution is more uncertain than the past 2 days given the subtle subsidence aloft likely limiting development and coverage for a while this afternoon, so mesoanalysis will be key in figuring out these details later today. Our entire area remains highlighted in a SLIGHT risk (level 2 out of 5) for severe thunderstorms. Damaging winds will once again be the main threat due to the high DCAPE and steep low level lapse rates. Weak low level shear, high wetbulb zero heights, and modest instability should result in little to no threats for tornadoes or large hail.

KEY MESSAGE 2...Dangerous heat (near record heat) and humidity for much of the region through today, then much less humid over the weekend.

A mid-level ridge will begin to slide offshore of the Mid- Atlantic and Northeast through today. At the surface, high pressure remains centered off the Southeast U.S. coast which will continue to direct a hot and humid airmass across our area.

Dangerous heat is expected to continue across much of our region through today, with several record highs likely to be challenged again. High temperatures are forecast to peak into the low to mid 90s across most of the area. Temperatures may be a degree or two hotter today compared to Thursday, as better mixing out of the dewpoints in anticipated. Dewpoints 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 through 8 PM today for areas where the heat index criteria starts at 96F and 100F.

A cold front is forecast to move through the region tonight, which will knock the temperatures and especially the dewpoints down several degrees. Though it will still be hot with high temperatures in the upper 80s to low 90s over the weekend, dewpoints will return to much more comfortable levels (into the 50s in some areas Saturday). Therefore, the heat index will not be much of a factor in the apparent temperature, and no heat hazards are anticipated for the weekend.

KEY MESSAGE 3...Chances for showers and storms, including a slight risk for severe thunderstorms, will return as a cold front passes through late Sunday into Sunday night. Turning more seasonable through the middle of next week.

A shortwave trough will push another cold front through the region late Sunday into Sunday night. This will give us another round of scattered showers and thunderstorms, however this one will be different than the activity we're experiencing this week. There looks to be plenty of synoptic forcing with more shear present. The biggest question and potential limiting factor as far as the severe threat goes with this setup will be instability. We'll need time to advect sufficient low level moisture northward into the area before the best forcing and shear arrive. Surface dewpoints will be the thing to watch with this. There will be, at the very least, sufficient instability for showers and storms, but we'll need dewpoints nearing 70 degrees to realize any meaningful severe threat. Timing variations remain among guidance for the cold front, which could also make or break our severe threat. A faster evolution will likely lead to a lower severe threat, and a slower progression should lead to a greater one. We'll continue to monitor this potential. SPC has outlooked areas mainly south of I-78 in a slight risk (level 2 out of 5) for severe storms.

The frontal passage will usher in a much more seasonable airmass that will stick around through at least the middle of next week. Highs will be in the mid 70s to mid 80s Monday through Wednesday, with lows in the mid 50s to low 60s. Dewpoints in the 50s and low 60s will make for much more pleasant conditions. Can't completely rule out a few showers, but overall the chances for rain look low during this period.

KEY MESSAGE 4...Widespread minor coastal flooding will develop with the evening high tide cycles Saturday and Sunday along the Atlantic coast, Delaware Bay, and on the tidal Delaware River and its tributaries.

A New Moon this weekend will result in higher than normal astronomical tides. Astronomical tides will generally be 1/2 foot less than minor coastal flooding thresholds, and tidal departures will be around 1 foot or so. This will result in widespread minor coastal flooding with the evening and nighttime high tide cycles Saturday and Sunday, and potentially Monday as well on the Atlantic coast and Delaware Bay. The tidal Delaware River and its tributaries could experience some spotty minor tidal flooding by Sunday night's high tide, and potentially Monday night too, but generally these astronomical tide driven flooding events produce more noteworthy impacts for the Atlantic coasts and Delaware Bay. The highest of the New Moon tides will be Sunday night and Monday night. Tidal flooding is not currently expected along the eastern shore of Chesapeake Bay.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

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

Today...Prevailing VFR expected through the period. Brief restrictions are possible, with scattered showers and thunderstorms, mainly after 21Z. Westerly winds increasing to near 10 kts, with gusts up to 20 kts possible at times after 15Z. High confidence in prevailing VFR through the day, moderate confidence in thunder timing.

Tonight...Any showers and storms ending by 03Z, then VFR. Winds shifting northwesterly near 5-10 kts. High confidence.

Outlook...

Saturday...VFR. No significant weather expected.

Sunday...Restrictions possible with the chance for showers and storms.

Monday...VFR. No significant weather expected.

Tuesday...Restrictions possible with the chance for showers and storms.

MARINE

No prevailing marine hazards expected through tonight. A southwest wind near 10-15 kts, with occasional gusts up to 20 kts and seas of 3-4 feet. Showers and thunderstorms may impact the nearshore waters and Delaware Bay this evening, with brief strong wind gusts possible. Winds shifting northwest overnight.

Outlook...

Saturday...No marine hazards anticipated. Winds will generally remain between 5-15 kt. Seas generally around 2-3 feet. Fair weather expected.

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

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

Rip Currents...

For today, a medium period 8 second swell, offshore winds, and breaking waves 2 feet or less will result in a LOW risk for development of rip currents at the Jersey Shore and Delaware Beaches.

On Saturday, conditions will be relatively similar, though the dominant period will increase to around 10 to 11 seconds. However, with winds offshore, and breaking waves again 2 feet or less, there will not be much energy in the surf zones. This will result in another day of a LOW risk for the development of rip currents.

Ocean water temperatures are mostly in the mid to upper 60s.

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

CLIMATE

Four record high temperatures were set on Thursday, including the Atlantic City Marina, Philadelphia, Wilmington, and Georgetown. The very hot and humid airmass will remain across our area through today. Some record high and warmest low temperatures could be challenged.

Location 6/12 Record Warmest Low/Year

Mount Pocono 69/2005 Allentown 70/1942 Reading 75/1958 Philadelphia 75/2015 Trenton 72/2015 & 2017 Atlantic City Airport 73/1973 & 2015 Atlantic City Marina 79/2016 Wilmington 73/2015 Georgetown 73/1947, 1973, 1986, & 2017

Location 6/12 Record Highs/Year

Mount Pocono 87/1967 Allentown 92/1949, 1961, 2015, & 2017 Reading 95/1984 Philadelphia 95/1947 & 2015 Trenton 94/1933 Atlantic City Airport 94/2017 Atlantic City Marina 93/1880 & 1914 Wilmington 96/1933 Georgetown 95/2017

PHI WATCHES/WARNINGS/ADVISORIES

PA...Heat Advisory until 8 PM EDT this evening for PAZ060>062-070- 071-101>106. NJ...Heat Advisory until 8 PM EDT this evening for NJZ001-007>010- 012>015-017>020-027. DE...Heat Advisory until 8 PM EDT this evening for DEZ001. MD...None. MARINE...None.