If you think it’s been hot so far, Tuesday could be an even bigger shock to the system.

The National Weather Service is forecasting a high of 100 degrees. If it does reach that, it would be the first 100-degree day since July 18, 2012, according to an Inquirer analysis of weather data supplied by the National Oceanic and Atmospheric Administration.

But it would fall just shy of the record of 102 for a July 16, set in 1988.

Monday’s high is forecast to reach 97 degrees.

Why is it so hot?

Lee Robertson, a meteorologist with the National Weather Service in Mount Holly, said a Bermuda high pressure system is parked offshore, and should remain there until the middle of the week, bringing excessive heat and humidity through Wednesday.

The jet stream is keeping cooler air “basically trapped up in Canada” Robertson said.

That’s also keeping nightly lows uncomfortably high. The overnight low for Tuesday is forecast to be very sticky 80 degrees.

An approaching cold front should bring temperatures back to normal for this time of year overnight Wednesday into Thursday. The front is expected to stall out and linger near or just south of the area through the end of the week.

But, for now, above-normal high temperatures and humidity continue to bring the heat index to over 100.

The city’s Department of Public Health has declared a heat health emergency that began Monday in response to those heat index values. The declaration is scheduled to end Wednesday but could be extended.

A heat health emergency activates the city’s emergency heat programs, which include the Philadelphia Corporation for Aging’s Heatline, cooling centers, home visits by special field teams, and enhanced daytime outreach for people experiencing homelessness.

It also serves to remind people to check on older relatives, friends, and neighbors. The city operates a hotline to provide information on where to find cooling centers, and ways to keep cool.

The hotline, 215-765-9040, will be open between 8:30 a.m. and midnight on Monday, Tuesday, and Wednesday.

The forecast for the week

Monday is expected to see a heat index of 102, but could see some relief with a chance of showers and thunderstorms after 5 p.m.

Tuesday looks to be sunny and hot with a high near 100 degrees, and heat index values as high as 108.

Wednesday is expected to be hot with a high of 95 degrees and slight chance of showers.

Thursday could bring the first day of relief in a while, as the third heat wave of the summer ends with a high of 85.

Friday and Saturday look to being more of the same more typical summer weather with highs in the mid to upper 80s.

Sunday, however, could bring Philly back into the 90s.