textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

All of the Heat Advisory now in effect through 8 PM Wednesday.

An isolated strong to severe thunderstorm possible through early this evening mainly north/west of I-95.

Small Craft Advisory has been cancelled.

KEY MESSAGES

1. Hot and humid conditions continue, with record setting early season heat probable, through Wednesday.

2. A strong cold front brings some showers and possibly isolated severe thunderstorms to the region Wednesday afternoon and evening.

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

DISCUSSION

KEY MESSAGE 1...Hot and humid conditions continue, with record setting early season heat probable, through Wednesday.

A strong upper-level ridge centered off the Southeast U.S. coast will remain in place through tonight before it starts to flatten on its northwest side during Wednesday. This along with surface high pressure anchored in the western Atlantic will continue to drive a southwesterly flow of early season (anomalous) heat across our area through Wednesday.

Temperatures peak in the 90s for many areas again this afternoon and again Wednesday afternoon, except cooler along the coasts and the higher terrain. While the surface dew points will mix out some during peak heating this afternoon due to the drier ground and boundary layer, this stretch of early season heat, with additional record breaking high temperatures probable, will have potentially greater impacts. The Heat Advisory remains in place across much of the region, where the heat index criteria starts at 100F (the urban I-95 corridor criteria starts at 96F through June). While some areas will probably fall a little short, the potential for greater impacts (lack of acclimation) continues to drive our decision. Where the wind turns more from the south and especially southeast will result in quite a bit of cooling due to the chilly ocean and therefore continued to not include the immediate coastal zones in the Heat Advisory. Peak heat indices will be near 100 degrees, with the core of the heat shifting a bit more south and east during Wednesday as a strong cold front starts to arrive. Despite it being a little cooler from Berks County, the Lehigh Valley into northern New Jersey Wednesday, it will be another day of hot and humid conditions. Given the consecutive stretch of this early season heat, opted to continue the current Heat Advisory through 8 PM Wednesday but extended the far north and west segment (also through 8 PM Wednesday). High temperatures on Wednesday will depend on how quickly cloud cover arrives along with showers and some thunderstorms, especially north and west of I-95. Either way, we will have a high launching pad once again for temperatures to start Wednesday and it will still be rather hot.

This afternoon and into this evening will mainly precipitation- free, however an isolated shower or thunderstorm cannot be ruled out mainly north and west of I-95 given the amount of heat and instability combined with a thermal trough nearby. This would most likely be terrain induced or isolated activity arriving from the west. Given steep low-level lapse rates, dry boundary layer (high DCAPE) but rather weak forcing, any convection should be short-lived and pulse variety with locally gusty winds (downbursts) possible.

KEY MESSAGE 2...A strong cold front brings some showers and possibly isolated severe thunderstorms to the region Wednesday afternoon and evening.

A strong cold front will settle across our area Wednesday afternoon and Wednesday night before stalling near Delmarva into Thursday. Showers and some thunderstorms will develop ahead of the front, and there looks to be a zone of higher instability across our central and southern zones Wednesday afternoon. It is in these areas where convection may try to organize a bit more as it develops eastward and encounters plenty of instability. The deep-layer shear is on the marginal side (strongest shear to our northwest) as are the mid level lapse rates, however a hot boundary layer will result in steep low-level lapse rates. The combination of the steep low-level lapse rates and any stronger convective cores, locally strong to damaging winds (downbursts) will be possible.

Due to the lower shear convection may struggle to organize, however given the hot air mass in place some linear or small clustering could develop from about the Philadelphia metro on south and eastward late in the afternoon and evening. The coverage of stronger convection is of lower confidence, however given the lower shear values the severe thunderstorm risk looks isolated. General rainfall totals are 0.10- 0.50 inches, however given precipitable water values of 1.5-2.0 inches, localized higher amounts are quite possible with any stronger showers or thunderstorms. Any thunderstorms should end by late Wednesday evening, with some showers lingering in the wake of the cold front.

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

A cold front is forecast to settle and then stall to our south Thursday and 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. This 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, our region needs rain as a long-term drought continues.

AVIATION /20Z TUESDAY THROUGH SUNDAY/

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

This afternoon...VFR. An isolated shower or thunderstorm cannot be ruled out toward evening mainly north and west of KPHL. Southwesterly winds 8-12 knots with some gusts to 20 knots. Low confidence regarding shower/thunderstorm occurrence.

Tonight...VFR. Southwest winds 5-10 knots, becoming locally light and variable. Moderate confidence.

Wednesday...VFR overall with increasing clouds during the day. Some showers and thunderstorms arrive later in the afternoon and evening with areas of sub-VFR conditions. Locally gusty winds possible with a stronger thunderstorm. Southwest winds increasing to 10-15 knots with some gusts to 20 knots. Low confidence on timing details.

Outlook...

Thursday...A period of sub-VFR conditions with some showers.

Friday through Sunday...Periods of sub-VFR conditions with showers at times, especially late Friday and Saturday.

MARINE

Southerly winds will be gusty, to 20 knots, through Wednesday, however a hot air mass over the chilly ocean water is resulting in a very strong inversion just about the surface. This is limiting the vertical mixing. While seas in the southerly flow could build to 5 feet through Wednesday, it is very marginal and therefore the Small Craft Advisory was cancelled. There is the potential for isolated gusty thunderstorms late Wednesday afternoon and especially Wednesday evening.

Outlook...

Thursday...The conditions are anticipated to be below Small Craft Advisory criteria.

Friday and Saturday...Small Craft Advisory conditions should occur.

Sunday...Seas may linger around 5 feet, otherwise the conditions should be below Small Craft Advisory criteria.

Rip Currents...

On Wednesday, south to southwest winds will range from 10 to 15 mph with gusts up to 20 mph. There will be a southeast swell with a period around 6 seconds. As a result, there is a MODERATE risk for the development of dangerous rip currents for the Jersey Shore and a LOW risk for Delaware Beaches.

On Thursday, winds become northeast 10 to 15 mph at the Jersey Shore in Monmouth County, and 15 to 20 mph at the rest of the Jersey Shore and at Delaware Beaches. There will be a southerly swell with a period around 6 seconds. Breaking waves will be 1 to 3 feet. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore in Monmouth County and a MODERATE risk for the rest of the Jersey Shore and Delaware Beaches.

Surface temperatures inland will be in the 90s again on Wednesday, temperatures along the coasts will mainly be in the 70s to near 80 due to ocean temperatures 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

Some record breaking temperatures are forecast through Wednesday. High temperatures today and low temperatures Wednesday morning could challenge monthly all time records for May.

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) 97/May 30 & 31, 1991 Reading (RDG) 97/May 20, 1996 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 Trenton (TTN) 75/May 31, 1895 Wilmington (ILG) 75/May 30, 1895

Record High Temperatures May 19 Site Record/Year Allentown (ABE) 97/1962 AC Airport (ACY) 96/1962 AC Marina (55N) 92/1998 Georgetown (GED) 96/1962 Mount Pocono (MPO) 89/1962 Philadelphia (PHL) 96/1962 Reading (RDG) 96/1962 Trenton (TTN) 96/1962 Wilmington (ILG) 95/1962

Record Warmest Low Temperatures May 19 Site Record/Year Allentown (ABE) 65/2017 AC Airport (ACY) 73/2017 AC Marina (55N) 68/1998 Georgetown (GED) 73/2017 Mount Pocono (MPO) 60/1943 Philadelphia (PHL) 74/1877 Reading (RDG) 68/1962 Trenton (TTN) 70/2017 Wilmington (ILG) 70/2017

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 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 Wednesday for PAZ060>062-070-071- 101>106. NJ...Heat Advisory until 8 PM EDT Wednesday for NJZ007>010-012-013- 015-017>020-027. Air Quality Alert until 11 PM EDT this evening for NJZ012>021- 026-027. DE...Heat Advisory until 8 PM EDT Wednesday for DEZ001. MD...None. MARINE...None.