textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

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

2. A cold front will bring some showers and thunderstorms to the region Wednesday through Thursday, then much cooler temperatures by the end of this week.

DISCUSSION

KEY MESSAGE 1...Hot and more 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 late this evening before it starts to flatten on its northwest side during Wednesday. This along with surface high pressure anchored in the western Atlantic will drive a south to southwest flow of early season (anomalous) heat across our area into Wednesday.

Temperatures are forecast to peak into the 90s for many areas again today and Wednesday, except cooler along the coasts and the higher terrain. While the surface dew points will likely mix out some during peak heating due to the drier ground and boundary layer (low evapotranspiration), this stretch of early season heat, with record breaking high temperatures probable, will have potentially greater impacts. The Heat Advisory remains in place across much of the region, including most of the area where the heat index criteria starts at 100F (the urban I-95 corridor criteria starts at 96F through June). While it will probably fall a little short for some areas, the potential greater impacts (lack of acclimation) is driving 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 did not include the immediate coastal zones in the Heat Advisory. Peak heat indices will be 95 to nearing 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. Based on this, portions of the Heat Advisory for areas farther north and west of I-95 only goes through Tuesday. High temperatures on Wednesday will depend on how quickly cloud cover arrives along with showers and some thunderstorms. Either way, we will have a high launching pad for temperatures to start Wednesday and it will still be rather hot.

Temperatures at night will be quite warm by mid May standards. Lows will only be in the mid 60s to low 70s, which will challenge daily warmest low temperature records for May 19th.

Today will largely be dry as well thanks to the strong ridging overhead. However, a stray shower or storm cannot be completely ruled out this afternoon and evening with the amount of heat and instability in the low-levels. The most likely location for any storms would be north of I-78 where terrain would aid in forcing for any convection. Shear will be relatively low though (only around 25-30 kts) so little organization is expected with anything that does form.

KEY MESSAGE 2...A cold front will bring some showers and thunderstorms to the region Wednesday through Thursday, then much cooler temperatures by the end of this week.

As mentioned above, a cold front will approach the area Wednesday and bring an end to the stretch of above normal temperatures by Thursday. Showers and thunderstorms will accompany this front as it passes through the region. Recent guidance trends continue to show the front moving through during the second half of Wednesday into Wednesday night. Given this timing, there is the threat for strong to severe thunderstorms. There remains uncertainty in the more specific details and some timing variations remain. Regarding the threat for strong to severe thunderstorms, the cold front will be moving into an air mass that will feature plenty of instability. The better shear and forcing look to be to our north, but bulk shear values will be supportive of some storm organization along with some supportive lapse rates. All of this said, the potential is there for isolated strong to severe thunderstorms with the SPC maintaining a Marginal Risk in Wednesday's severe weather outlook. The primary concerns with any strong to severe thunderstorm would be damaging wind gusts and large hail. General rainfall accumulation from this frontal passage looks around 0.25" or less. However, given this convection and PWAT values reaching to 1.5-2.0", localized higher amounts are certainly possible with any heavier pockets of rain.

As the front sinks south into Thursday morning, it then looks to linger near or just south of the area through the remainder of the day Thursday. This will lead to the potential for more rain showers, particularly for our southeastern zones. An unsettled pattern looks to continue to take shape for Friday and into the weekend.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...Mainly VFR. Southwesterly winds 5-8 knots increasing to 10-15 knots, with some afternoon gusts up to 20 knots. Can't rule out a stray shower/storm, most likely at KABE, but overall chances are low (15% or less). Moderate confidence.

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

Outlook...

Wednesday through Thursday...Sub-VFR with rain showers and thunderstorms Wednesday afternoon and evening. Rain showers may continue into Thursday.

Friday through Saturday...Sub-VFR with rain showers moving through.

MARINE

Small Craft Advisory continues for the New Jersey Atlantic coastal waters this afternoon and evening. Though southerly flow will be increasing, a hot air mass will be moving over the chilly water and the vertical mixing will be reduced. However, given the strength of the flow, there should be a period of wind gusts to around 25 knots and seas building to about 5 feet. It looks to be more marginal for the Delaware Atlantic coastal waters and therefore held off on issuing an advisory. For Delaware Bay, the conditions should continue to remain below Small Craft Advisory criteria through tonight.

Outlook...

Wednesday...Small Craft Advisory conditions possible. Seas lessen by Wednesday morning but elevated winds out of the southwest remain through the day Wednesday with gusts near 20-25 kt. Rain showers move in Wednesday afternoon into Wednesday night. There is also the potential for isolated thunderstorms Wednesday afternoon into early Wednesday night.

Thursday...Conditions expected to be below Small Craft Advisory criteria but showers may linger.

Friday through Saturday...Small Craft Advisory conditions expected as seas reach 4-6 feet. There is the chance for some rain showers.

Rip Currents...

On Tuesday, south to southwest winds will increase to 15 to 20 mph with gusts up to 30 mph. There will be a southeast swell with a period around 8 seconds. As a result there is a MODERATE risk for the development of dangerous and life threatening rip currents for the Jersey Shore and for Delaware Beaches. Depending on the exact direction of the winds, there is the potential for a HIGH risk for rip currents at the Jersey Shore.

On Wednesday, south to southwest winds will be a bit lighter, and winds look to be more parallel to the shores. 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 and lift threatening rip currents for the Jersey Shore and a LOW risk for Delaware Beaches.

Will continue to note that while surface temperatures inland will be in the mid to upper 90s Tuesday and in the low to mid 90s 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

Near record breaking temperatures are forecast from through Wednesday. High temperatures Tuesday and low temperatures Wednesday morning could challenge monthly all time records for May. Below is a summary of the records broken on Monday, May 18th:

Record High Temperatures broken on May 18 Site Record/Year Georgetown (GED) 91/1974 (new record 96) Mount Pocono (MPO) 87/1962 (new record 88) Philadelphia (PHL) 94/1962 (new record 96) Reading (RDG) 96/1962 (tied record 96)

Record Warmest Low Temperature broken on May 18 Site Record/Year Mount Pocono (MPO) 61/2017 (new record 63)

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 PAZ070-071-102-104- 106. Heat Advisory until 8 PM EDT this evening for PAZ060>062-101- 103-105. NJ...Heat Advisory until 8 PM EDT Wednesday for NJZ012-013-015- 017>020-027. Heat Advisory until 8 PM EDT this evening for NJZ007>010. Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for NJZ012>021-026-027. DE...Heat Advisory until 8 PM EDT Wednesday for DEZ001. MD...None. MARINE...Small Craft Advisory from noon today to midnight EDT tonight for ANZ450>453.