A "dangerous" heat wave this weekend in the Philadelphia area is expected to bring scorching temperatures and soaring heat indexes as a heat dome approaches just as the first day of summer is set to arrive.
Delaware and Philadelphia counties are under heat advisories with heat index values, or feels-like temperatures, nearing 100 degrees through Thursday, June 19. The brutal shift from a cooler mid-June will feel significant as uncomfortable, muggy, excessive heat arrives by the weekend.
"The magnitude and longevity of this heat wave will be a shock to the system," AccuWeather Senior Meteorologist Chad Merrill said in a briefing.
Here's the latest forecast in Pennsylvania and the Philly area, from how hot it'll get and feel and when the year's first heat wave will start.
Heat advisory Philly
The NWS Philadelphia/Mount Holly office issued a heat advisory that will extend through 8 p.m. on Thursday, June 19. The advisory covers Delaware and Philadelphia counties in Pennsylvania, as well as Camden, Gloucester and Burlington counties in New Jersey. Heat indexes are expected to reach 99 degrees, NWS said, which can lead to heat illnesses. NWS urges residents to take extra precautions when outdoors by wearing lighter clothing and staying hydrated, and if possible, to stay in an air-conditioned room and check on relatives and neighbors.
NOAA's National Weather Prediction Center forecasts heat indexes are "most likely" to reach temperatures that may surpass 110 degrees on Monday, June 23.USA TODAY NETWORKWhat is a heat advisory?
In the Philadelphia region, a heat advisory is issued by the NWS Philadelphia office usually within 12 to 24 hours before the heat index value is expected to reach between 96 degrees and 104 degrees for two consecutive hours through June 30. After July 1, an advisory is issued when the heat index reaches between 100 and 104 degrees for two consecutive hours.
Heat index values are expected to reach 100 degrees in Philly on Juneteenth, which is Thursday, June 19.
Heat wave weather forecast
In Philadelphia, NWS and AccuWeather are forecasting temperatures in the 90s and potentially at or above 100 degrees for several days:
Saturday, June 21: 89 degrees
Sunday, June 22: 92-94 degrees
Monday, June 23: 97 degrees
Tuesday, June 24: 95-98 degrees
Wednesday, June 25: 94 degrees
Thursday, June 26: 100 degrees
Friday, June 27: 96 degrees
Saturday, June 28: 95 degrees
Sunday, June 29: 92 degrees
How hot will it feel?
The heat wave will be triggered by a change in the jet stream, called a heat dome or bubble of high pressure that traps heat in. This will make it feel much warmer outside to the human body. Here's the "real feel" temperatures expected in the Philly area:
Saturday, June 21: 97 degrees
Sunday, June 22: 97 degrees
Monday, June 23: 104 degrees
Tuesday, June 24: 103 degrees
Wednesday, June 25: 100 degrees
Thursday, June 26: 108 degrees
Friday, June 27: 108 degrees
Saturday, June 28: 96 degrees
Sunday, June 29: 99 degrees
What is considered a heat wave?
A heat wave is a period of abnormally hot weather when temperatures are well above average than usual for that area and time of year, according to the National Weather Service. They can occur with or without humidity and last more than two days, NWS said.
In Philly, a heat wave is considered a three-day stretch with temperatures above 90 degrees.
Heat waves can vary in length and intensity, and cause harm to the human body but also to transportation, water, the electric lines, agriculture and more.
What is heat index?
The heat index is also known as the apparent temperature or the "feels-like" temperature, meaning what the temperature feels like to the human body when relative humidity and air temperature combine, according to the NWS.
What is a heat dome?
A heat dome occurs when a persistent region of high-pressure traps heat over an area, causing it to sink toward the surface, according to NOAA. The sinking air acts as a dome capping the atmosphere, trapping the heat and leaving very little chance for rain.
A heat dome results from strong changes in water temperature in the Pacific Ocean during the previous winter. The warmth from the ocean heats the atmosphere above it, driving heat and humidity east, according to NOAA. In the summer, the jet stream moves north and the hot air from the Pacific becomes trapped underneath, where it sinks to ground level.
They can last anywhere from a few days to weeks. On rare occasions, they last longer than two weeks, causing serious heat-related illnesses to Americans, and even death. One such instance occurred in the summer of 1995 in Chicago, where a heat dome caused more than 500 deaths and hundreds across the Midwest.
This article originally appeared on NorthJersey.com / USA TODAY Network via Reuters Connect
