textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

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

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

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 conditions continue, with record setting early season heat probable, through today.

A strong upper-level ridge centered off the Southeast U.S. coast starts to flatten on its northwest side today. Though, with surface high pressure anchored in the western Atlantic a southwesterly flow of early season (anomalous) heat will continue to flow across our area today.

Temperatures peak in the 90s for many areas again this afternoon, except cooler along the coasts and the higher terrain. How quickly cloud cover arrives, along with showers and some thunderstorms from an approaching cold front, will also impact high temperatures today, especially north and west of I-95. Either way though, we will have a high launching pad once again for temperatures, with lows this morning only in the upper 60s to low 70s, so it will still be rather hot this afternoon. 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. Peak heat indices will be near 100 degrees, with the core of the heat shifting a bit more south and east today as the strong cold front starts to arrive.

The Heat Advisory remains in place for across much of the region through 8 PM this evening. While some areas will probably fall a little short of criteria, particularly in Berks County, the Lehigh Valley, and into northern New Jersey where it will be cooler today ahead of the approaching front, it will still be another hot and humid day regardless. The potential for greater impacts, due to lack of acclimation this early in the season and the accumulative effect with this being the third straight day of high heat, continues to drive our decision. Along the immediate coast, 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 these areas in the Heat Advisory.

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

A strong cold front will settle across our area this afternoon and tonight 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 this 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 this evening, with some showers lingering in the wake of the cold front.

KEY MESSAGE 3...Significantly cooler temperatures and several rounds of 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. 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, our region needs rain as a long-term drought continues.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

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

Today...VFR overall with increasing clouds during the day. Some showers and thunderstorms arrive after 19Z 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.

Tonight...MVFR conditions possible with fog/low stratus, particularly at terminals which receive showers/storms this evening. Some lingering showers may also contribute to sub-VFR conditions, particularly at KMIV and KACY. Northerly winds around 5 knots. Low confidence.

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 today, 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 today, it is very marginal and therefore largely sub-SCA conditions are expected. There is the potential for isolated gusty thunderstorms late this afternoon and especially this evening.

Outlook...

Thursday...Small Craft Advisory conditions are possible with northeast winds gusting around 20kts.

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

Sunday...Seas may linger around 4 to 6 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

Record-setting temperatures occurred on Tuesday, May 19th. Below is a summary of records that were tied or broken.

Record High Temperatures broken on May 19th Site Record/Year Philadelphia (PHL) 96/1962 (new record 98*) Reading (RDG) 96/1962 (new record 97*) Trenton (TTN) 96/1962 (record tied) Wilmington (ILG) 95/1962 (new record 96) *The record high of 98 at Philadelphia (PHL) and 97 at Reading (RDG) also breaks or ties the all time record high for the month of May at both sites.

Record Warmest Low Temperatures broken on May 19th Site Record/Year Allentown (ABE) 65/2017 (record tied) Mount Pocono (MPO) 60/1943 (new record 66) Reading (RDG) 68/1962 (new record 72*) *The record warmest low of 72 at Reading (RDG) also tied the all time record warmest low of 72 for the month of May, set back on May 28 & 31, 1939 and May 31, 1991.

More record breaking temperatures are possible 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) 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...None.