textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Added key message regarding minor tidal flooding that is forecast to occur this weekend. Otherwise, no significant changes to the forecast.

KEY MESSAGES

1. There is a risk of severe thunderstorms both this afternoon and Friday afternoon.

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

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

4. Minor tidal flooding is forecast to occur with the evening/night high tides on Saturday and Sunday along the Atlantic coast and Delaware Bay.

DISCUSSION

KEY MESSAGE 1...There is a risk of severe thunderstorms both this afternoon and Friday afternoon.

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

Large scale forcing and shear are expected to remain relatively weak today, however we will have a convectively enhanced shortwave from remnant convection upstream in the Midwest to contend with. This will enhance the forcing and potentially shear at the mesoscale. Guidance has come into a consensus on timing of convection developing locally associated with this feature, mainly between about 5 PM to 11 PM from west to east. Hotter temperatures and high dewpoints will yield steeper low- level lapse rates compared to Wednesday. This is a pretty typical severe thunderstorm setup for our region, but the details in exactly how the storms will evolved still remains less certain. Current indications from the CAMs suggest scattered convection redeveloping along the mid level wave across western and central PA this afternoon, then gradually congealing into a broken line and moving east toward our area by late afternoon and evening.

A SLIGHT risk (level 2 out of 5) for a severe thunderstorm potential remains in place for today. Damaging winds will be the main threat as storms may cluster or evolve into one or more broken 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 today through Friday, then much less humid over the weekend.

A mid-level ridge slides across the Mid-Atlantic and Northeast today 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 and humid airmass across our area.

Dangerous heat is expected across much of our region today and Friday, with even some record high temperatures potentially getting challenged. High temperatures both today 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. 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 today through Friday for areas where the heat index criteria starts at 96F and 100F. The advisory for coastal Monmouth County was extended through Friday. Latest hi-res guidance indicates the westerly flow should be just strong enough to keep the sea breeze boundary just offshore there, though it could certainly start to push onshore later in the day and 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 dewpoints down several degrees. Though it will still be hot with high temperatures in the upper 80s to mid 90s over the weekend, dewpoints will return to much more comfortable levels (into the 50s in some areas Saturday) and this minimizes the risk for additional heat related advisories.

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

A shortwave trough will push another cold front through the region late 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. The unfavorable timing of the front overnight will significantly limit instability, which will probably translate to a lower severe threat. There looks to be plenty of synoptic forcing though. PWats look to recover enough to give us some much needed rainfall. Exact details on this setup remain unclear, but the setup is promising for some decent rain.

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 upper 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 stray showers, but overall the chances for rain look low during this period.

KEY MESSAGE 4...Minor tidal flooding is forecast to occur with the evening/night high tides on Saturday and Sunday along the Atlantic coast and Delaware Bay.

Higher than normal astronomical tides are expected to occur this weekend, associated with the New Moon on Sunday. While tide departure/surge values are not particularly high, mainly less than 1 foot, astronomical tides will only be about a half a foot below minor flood stage. Thus, we should see widespread minor tidal flooding occurring with the Saturday and Sunday evening/night high tides along the Atlantic coasts of New Jersey and Delaware, and Delaware Bay. Tidal flooding is not currently expected along the tidal Delaware River or eastern shore of Chesapeake Bay.

AVIATION /12Z THURSDAY THROUGH MONDAY/

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

Today...IFR ceilings and MVFR visibility should quickly scatter out after 12Z. Thereafter, prevailing VFR, but another round of showers and thunderstorms is expected late in the afternoon, mainly after 22z. Have included a thunder mention for all terminals, as likelihood of thunder is increasing. Storms could produce brief strong wind gusts. Winds out of the west increasing to around 10 kt. High confidence in prevailing VFR through the day, moderate confidence in thunder timing.

Tonight...Scattered showers and thunderstorms should end by 03Z everywhere, likely resulting in some restrictions, then VFR. Cannot rule out some patchy fog late. West to southwest winds 5 kts or less. Moderate confidence.

Outlook...

Friday...Prevailing VFR expected through the period. Brief restrictions are possible, with scattered showers and thunderstorms during the afternoon and evening hours.

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

Sunday night through Monday...Restrictions possible with the chance for showers and storms.

MARINE

No prevailing marine hazards expected through tonight. A southwest wind at 10 kt with gusts 15-20 kt and seas of 3-4 feet. A line of showers and thunderstorms may impact the nearshore waters and Delaware Bay during the evening, with brief strong wind gusts possible.

Outlook...

Friday through Saturday...No marine hazards 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 hazards anticipated with winds 5-10 kt and seas 2-3 feet.

Rip Currents...

For today, 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 mid to upper 60s.

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

CLIMATE

A very hot and humid airmass will overspread our area through Friday. Some record high and warmest low 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 95/1947 & 1959

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 from 11 AM this morning to 8 PM EDT Friday for PAZ060>062-070-071-101>106. NJ...Heat Advisory from 11 AM this morning to 8 PM EDT Friday for NJZ001-007>010-012>015-017>020-027. DE...Heat Advisory from 11 AM this morning to 8 PM EDT Friday for DEZ001. MD...None. MARINE...None.