textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Severe Thunderstorm Watch issued until 9 PM for our entire area.

KEY MESSAGES

1. Hot and humid with record setting early season heat will end by this evening.

2. Severe Thunderstorm Watch in effect until 9 PM this evening for our entire area.

3. Significantly cooler temperatures and several rounds of showers for the end of this week and the holiday weekend.

DISCUSSION

KEY MESSAGE 1...Hot and humid with record setting early season heat will end by this evening.

A strong cold front will arrive this evening which will end this stretch of hot conditions and record heat. Temperatures in the 90s for many areas again this afternoon, except cooler along the coasts and the higher terrain. While the surface dew points have mixed out some this afternoon with significant heating of the boundary layer, this stretch of early season heat will still have potentially greater impacts. Peak heat indices of 95-100 degrees this afternoon, however as more clouds develop and convection starts to arrive from the west and southwest by early evening, areas of significant cooling will start to take place. The Heat Advisory continues for now through 8 PM this evening although we might be able to cancel it early depending how quick and widespread the cooling is due to showers and thunderstorms.

KEY MESSAGE 2...Severe Thunderstorm Watch in effect until 9 PM this evening for our entire area.

A strong cold front will settle across our area late this afternoon and this evening before stalling near Delmarva into Thursday. Showers and thunderstorms are expected ahead of and with the front. Instability continues to increase with MLCAPE values of 1000-1500 J/kg and SBCAPE of 1500-2500 J/kg. This will help to intensify some of the convection as it arrives from the west. The deep-layer shear is on the marginal side though (strongest shear to our northwest) as are the mid level lapse rates, however a hot well-mixed boundary layer is resulting in rather steep low-level lapse rates (DCAPE values up to 1300 J/kg!). The combination of the steep low-level lapse rates and any stronger/organized convective cores should locally enhance the damaging wind (downburst) threat.

Despite the lower shear, convection should still organize some given ample instability and the hot air mass in place. Some clustering may tend to occur with some guidance also hinting at some bowing segments. The primary severe weather threat from thunderstorms should be damaging downburst winds given the low- level inverted-V profile in the model soundings. These soundings also show that cloud tops could extend upwards of 40,000 feet therefore some hail is possible. Much of our region is within a level 2 out of 5 from SPC (Slight Risk) for severe thunderstorms. The main timeframe looks to be between 4 PM to 9 PM. General rainfall totals are 0.10-0.50 inches, however given precipitable water values of 1.5-2.0 inches, localized higher amounts are likely with any stronger showers or thunderstorms. Any thunderstorms should end by late this evening, with some showers lingering in the wake of the cold front. It will turn noticeably cooler tonight as the winds becoming northerly.

KEY MESSAGE 3...Significantly cooler temperatures and several rounds of showers for the end of this week and the holiday weekend.

A cold front will settle and stall to our south as we get into the holiday weekend. Some ripples of energy ride along and north of it and this looks to bring some rounds of showers to our region. A more organized surface low well to our west may provide an increase in showers or a period of rain later Friday and Saturday. The precipitation may become enhanced some as surface high pressure to our northeast wedges southwestward into our area creating some cool air damming. Furthermore, with PWAT values around 1.50-1.90 inches over the weekend, which is around the climatological maximum for this time of the year, there is at least a marginal chance for excessive rainfall on Saturday. We will be keeping an eye on how strong the baroclinic forcing gets along the stalled boundary wedged between the high to the northeast and the low to the west. Northeast flow along with several rounds of showers will all result in much cooler temperatures Thursday through Saturday, then some warming should take place Sunday and into Memorial Day. The pattern looks active though and therefore some additional showers are quite possible right into Memorial Day. While not great timing for the holiday weekend and unofficial start of summer, our region needs rain as a long-term drought continues.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

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

This afternoon...VFR overall with increasing clouds. Some showers and thunderstorms arrive from the west between 19-22z with areas of sub-VFR conditions. Locally gusty winds, 30-40 knots, with any stronger thunderstorms. Winds varying from west- northwest to west-southwest 8-15 knots with some gusts to 20 knots. Low confidence on the timing details.

Tonight...A period of MVFR/IFR conditions with showers and thunderstorms this evening, then some lingering showers may also contribute to sub-VFR conditions overnight. Northwest winds becoming north-northeast mainly 10 knots or less. Low confidence.

Thursday...A period of sub-VFR conditions with some showers. Northeast winds 8-12 knots, with some local gusts to 20 knots especially at KACY. Low confidence.

Outlook...

Thursday Night through Monday...An unsettled pattern takes over with periods of MVFR/IFR conditions expected with several rounds of rain showers. Most widespread restrictions look to come Saturday and Saturday Night though the entire period likely will feature showers and low clouds with perhaps sporadic stretches of VFR if ceilings lift a bit.

MARINE

Southerly winds to 20 knots this afternoon will diminish this evening and becoming west and northwest, then north to northeast on Thursday. The northeast flow increases during Thursday and with a much cooler air mass moving in the mixing should become more efficient over the chilly ocean water. Seas will also build during Thursday. Opted to issue a Small Craft Advisory for Thursday afternoon for all waters except for the waters off Monmouth County and the upper Delaware Bay. Some thunderstorms could be strong enough this evening to warrant the issuance of Special Marine Warnings.

Outlook...

Thursday Night through Friday...A Small Craft Advisory is in effect for ocean zones south of Manasquan Inlet and the lower Delaware Bay as periods of 25-30 kt winds and seas nearing 5 feet are expected. Sub-SCA conditions expected on the upper Delaware Bay and coastal waters north of Manasquan Inlet.

Friday Night through Monday...SCA conditions expected with seas of 4 to 7 feet and periods of 25-30 kt winds, mainly on Saturday.

Rip Currents...

On Thursday, east to northeast winds will ramp up to 15 to 20 mph with gusts up to 25 mph. There will be a southeast swell with a period around 6 seconds and breaking waves of 1 to 3 feet. As a result, there is a MODERATE risk for the development of dangerous rip currents for the Jersey Shore and for Delaware Beaches.

On Friday, east to northeast winds will range from 15 to 20 mph with 25 to 30 mph gusts. There will be a southeast swell with a period of 10 to 13 seconds and breaking waves of 2 to 4 feet. This should result in a HIGH risk for the development of dangerous and lift threatening rip currents.

Although temperatures inland will mostly be in the 60s or so Thursday through Saturday, and even in the 50s well inland on Saturday, ocean temperatures will remain in the 50s. These cold water temperatures can quickly cause hypothermia and physical incapacitation to anyone suddenly immersed in the water.

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

CLIMATE

More record breaking temperatures are possible today.

Records for our climate sites are listed below:

Monthly Record High Temperatures for May

Site Record/Date Allentown (ABE) 97/May 19, 1962 AC Airport (ACY) 99/May 29, 1969 AC Marina (55N) 95/May 31, 1895 & May 23, 1925 Georgetown (GED) 98/May 28, 1991 Mount Pocono (MPO) 94/May 22, 1911 Philadelphia (PHL) 98/May 19, 2026 (new) Reading (RDG) 97/May 20, 1996 & May 19, 2026 Trenton (TTN) 99/May 23, 1911 & May 31, 1986 Wilmington (ILG) 98/May 10, 1895 & May 30, 1895

Monthly Record Warmest Low Temperatures for May

Site Record/Date Allentown (ABE) 71/May 31, 1975, May 28 & 31, 1991 AC Airport (ACY) 73/May 24, 2004 & May 19, 2017 AC Marina (55N) 73/May 30 & 31, 1987, & May 31, 1991 Georgetown (GED) 74/May 23, 1953 & May 20, 2019 Mount Pocono (MPO) 68/May 21, 1996 Philadelphia (PHL) 78/May 31, 1895 Reading (RDG) 72/May 28 & 31, 1939, May 31, 1991, & May 19, 2026 Trenton (TTN) 75/May 31, 1895 Wilmington (ILG) 75/May 30, 1895

Record High Temperatures May 20 Site Record/Year Allentown (ABE) 92/1962 & 1996 AC Airport (ACY) 96/1996 AC Marina (55N) 93/1996 Georgetown (GED) 95/1962 Mount Pocono (MPO) 89/1996 Philadelphia (PHL) 94/1962 & 1996 Reading (RDG) 97/1996 Trenton (TTN) 94/1996 Wilmington (ILG) 96/1996

Record Warmest Low Temperatures May 20 Site Record/Year Allentown (ABE) 66/2019 AC Airport (ACY) 68/1996(ties) AC Marina (55N) 69/1996 Georgetown (GED) 74/2019 Mount Pocono (MPO) 61/1959 Philadelphia (PHL) 71/2019 Reading (RDG) 69/1959 & 1996 Trenton (TTN) 68/1903, 1959, & 1962 Wilmington (ILG) 70/2019

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 NJZ007>010-012- 013-015-017>020-027. DE...Heat Advisory until 8 PM EDT this evening for DEZ001. MD...None. MARINE...Small Craft Advisory from 2 PM Thursday to 6 PM EDT Friday for ANZ431-451>455.