textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Severe Thunderstorm Watch 317 was canceled for Berks, Lehigh, Northampton, Carbon, and Monroe Counties, and remains in effect elsewhere until midnight tonight.

The severe weather threat has trended a bit later for today. There is also the potential for severe weather on Sunday.

Updated the key message for tidal flooding and updated the rip current forecast for Friday and Saturday.

KEY MESSAGES

1. There is a risk of severe thunderstorms today and Friday.

2. Dangerous heat (potentially record heat) and humidity for much of the region through Friday, 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 risk of severe thunderstorms today and Friday.

Severe Thunderstorm Watch 317 was canceled for Berks, Lehigh, Northampton, Carbon, and Monroe Counties, and remains in effect elsewhere until midnight tonight.

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 Friday with a cold front.

For today's severe weather threat, the larger scale forcing and shear are expected to remain relatively weak. A shortwave moving out of the lower Great Lakes region will help trigger shower and thunderstorm development. One change through the day with recent guidance has been a later arrival of the convection. The general timing for severe weather for our area is mainly 6 PM to midnight tonight from west to east. In terms of the atmospheric environment, shear values are weak, but there is plenty of instability with the hot and humid conditions that are present. These conditions will also allow for supportive lapse rates. This afternoon, convection is already blossoming across western portions of PA with this scattered activity expected to congeal into a broken line that arrives through the evening hours.

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.

Tomorrow, 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 near 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 through Friday, then much less humid over the weekend.

A mid-level ridge slides across the Mid-Atlantic and Northeast through Friday. 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 through 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 slightly warmer Friday compared to today. 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 Friday for areas where the heat index criteria starts at 96F and 100F.

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 low 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 late Sunday into 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 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. Also, there will be instability and supportive lapse rates. Timing variations remain among guidance for the cold front, but the atmospheric conditions are supportive of convection during the afternoon and evening hours. Given these atmospheric conditions, the Storm Prediction Center has outlooked us for severe weather Sunday, particularly during the afternoon and evening hours. PWAT values will also recover for Sunday allowing for some heavy rain with the convective activity.

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 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 departure s 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 on the Atlantic coast, Delaware Bay, and the tidal Delaware River and its tributaries. The higher of the two will be Sunday night. Tidal flooding is not currently expected along the eastern shore of Chesapeake Bay.

AVIATION /01Z FRIDAY THROUGH TUESDAY/

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

Tonight...Scattered showers and thunderstorms should end by 04-06Z 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.

Friday...Prevailing VFR expected through the period. Brief restrictions are possible, with scattered showers and thunderstorms, most likely after 18-21Z. High confidence in prevailing VFR through the day, moderate confidence in thunder timing.

Outlook...

Friday night...Sub-VFR possible with a chance for lingering showers and storms.

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 Friday. A southwest wind at 10 kt with gusts 15-20 kt and seas of 3-4 feet. Showers and thunderstorms may impact the nearshore waters and Delaware Bay during this evening and again Friday evening, with brief strong wind gusts possible.

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 Friday, 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

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