textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Heat Advisory issued for the PA and NJ urban corridor, in effect today through Tuesday.

Added a Climate Section, which includes record high and warmest low temperatures for the area through Wednesday.

KEY MESSAGES

1. Unusual and potentially record setting early season heat is expected through Wednesday. High temperatures will generally be upward of 10 to 20 degrees above normal, with low to mid 90s expected for many areas today through Wednesday.

2. A cold front will bring some showers and thunderstorms to the region Wednesday through Thursday, along with moderating temperatures closer to normal by the end of this week.

DISCUSSION

KEY MESSAGE 1...Unusual and potentially record setting early season heat is expected through Wednesday. High temperatures will generally be upward of 10 to 20 degrees above normal, with low to mid 90s expected for many areas today through Wednesday.

An upper level ridge will remain in place across the East Coast through Tuesday then shift offshore and weaken into Wednesday. This will bring a period of well above normal, summer-like temperatures which could challenge monthly all time records for May. At the surface, Bermuda high pressure will remain in place offshore as a low pressure system develops across the Plain, passing across the Great Lakes Tuesday and passing well to our north by Wednesday. This will result in a very warm south to southwesterly flow for several days.

The warming trend that began over the weekend will continue into early this week. High temperature for inland areas today will be generally in the upper 80s to mid 90s for eastern PA, inland NJ and Delmarva. However, the flow be more out of the south to even southeast, so there will be more of cooling influence near the coast by the afternoon as the marine layer pushes inland. This cooling will extend a bit farther inland compared to Sunday given the surface flow direction. This will keep highs in these areas near the coast mainly in the mid 70s to mid 80s, occurring around midday before cooling off into the afternoon and evening.

Dewpoints should mix out a bit this afternoon due to a lack of deep moisture return, cool ocean temperatures, and low evapotranspiration, but will still be mainly in the low to mid 60s. Not terribly humid, but certainly uncomfortable since we aren't acclimated to it yet. The dewpoint values will translate to heat indices being close to the air temperatures. With temperatures getting into the mid 90s in the urban corridor, we've opted to start the Heat Advisory at noon today, though temperatures and heat indices will likely be hotter on Tuesday (more on that below). Given the likely bay breeze developing in Wilmington today, we've excluded New Castle County from the advisory.

Mostly sunny skies today with scattered cirrus around. Can't rule out an isolated shower/storm in our western zones this afternoon, but this potential is low enough that we don't have much in the way of precip mentioned in the forecast.

Temperatures tonight 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.

Confidence continues to increase that Tuesday will be the hottest day across the region. Widespread temperatures in the mid 90s are expected. Even with many of the aforementioned factors keeping heat indicies close to air temperatures will continue into Tuesday, the hotter air temperatures will push heat indicies higher and exceed the early season criteria in the urban areas. Thus, the Heat Advisory will continue through Tuesday evening.

The heat forecast remains a bit more uncertain for Wednesday, as a faster progression of an approaching front may bring more clouds ahead of the showers and storms, and cap temps and heat indices a bit lower. Regardless, it will still be abnormally warm and keep portions of the area close to record high temperatures, particularly near and southeast of I-95 where skies will remain clear the longest.

The heat looks to break by Thursday following the cold frontal passage late Wednesday. Temperatures will moderate closer to normal for the remainder of the week and into next weekend.

KEY MESSAGE 2...A cold front will bring some showers and thunderstorms to the region Wednesday through Thursday, along with moderating temperatures closer to normal 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. At this range there is still uncertainties in timing, but if the front moves through during the day Wednesday, then the potential for stronger storms increases as there will be plenty of instability present. Currently, the better forcing looks to be displaced to the north of the front. All of this said, it remains still a bit early to get very specific on the details and extent of the convection, impacts, and hazards. The front then looks to linger nearby or just south of the area on Thursday leading to more potential for showers. By the end of the week and into next weekend, the trend continues to show a more unsettled pattern.

AVIATION /06Z MONDAY THROUGH FRIDAY/

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

Through 12Z...VFR. Winds mostly calm, if not SSW less than 5 kts. High confidence.

Today...VFR. S to SW winds near 5 kts in the morning, increasing to around 10 kts by 18Z. Winds a little more east of south for ILG, MIV, and ACY, and perhaps as high as 15 kts at times, but not anticipating any prevailing gusts. High confidence.

Tonight...VFR. SSW winds 5-10 kts. High confidence.

Outlook...

Tuesday...VFR prevailing with no significant weather expected.

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

Friday...Sub-VFR possible with the chance for rain showers.

MARINE

Southerly winds increase to around 10-15 kts, gusting up to 20 kts by this afternoon, but the conditions are expected to stay below Small Craft Advisory levels. Seas around 3-4 feet. Winds and seas lessening some overnight.

Outlook...

Tuesday through Wednesday...Southwest winds increase through the day Tuesday with Small Craft Advisory conditions possible by Tuesday afternoon as wind gusts reach near 25 kt. Seas look to start to reach near 5 feet as well Tuesday evening. Seas lessen by Wednesday morning but elevated winds out of the southwest remain through the day Wednesday with gusts near 25 kt. Showers and potentially some isolated thunderstorms move through Wednesday afternoon into early Wednesday night.

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

Friday...Conditions are expected to be below Small Craft Advisory criteria. There is the chance for some rain showers.

Rip Currents...

Monday, south-southeast winds will be 10 to 15 mph with breaking waves of 1 to 2 feet. There will be a southeasterly swell with a period of around 8 seconds. As a result, there is a MODERATE risk for the development of dangerous and life threatening rip currents at the Jersey Shore and a LOW risk at Delaware Beaches where winds will be more shore parallel.

Tuesday, south-southwesterly winds of 10-20 mph with breaking waves of 2 to 3 feet. A southeasterly swell will remain, though the period may be slightly shorter. For now, have opted for a MODERATE risk for the development of dangerous and life threatening rip currents for all NJ and DE beaches.

Important to note that while surface temperatures inland will be in the upper 80s to low 90s on Monday, and low to mid 90s Tuesday, 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 today through Wednesday. High temperatures Tuesday 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 18 Site Record/Year Allentown (ABE) 96/1962 AC Airport (ACY) 93/1987 AC Marina (55N) 90/1987 Georgetown (GED) 91/1974 Mount Pocono (MPO) 87/1962 Philadelphia (PHL) 94/1962 Reading (RDG) 96/1962 Trenton (TTN) 96/1986 Wilmington (ILG) 94/1962

Record Warmest Low Temperatures May 18 Site Record/Year Allentown (ABE) 65/2017 AC Airport (ACY) 70/2017 AC Marina (55N) 66/2017 Georgetown (GED) 72/2017 Mount Pocono (MPO) 61/2017 Philadelphia (PHL) 72/2017 Reading (RDG) 67/1900, 1943, & 2017 Trenton (TTN) 71/2017 Wilmington (ILG) 72/2017

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 from noon today to 8 PM EDT Tuesday for PAZ070- 071-104-106. NJ...Heat Advisory from noon today to 8 PM EDT Tuesday for NJZ015- 017>019. DE...None. MD...None. MARINE...None.