The temperature soared to 91 degrees Sunday -- and that's likely to be the coolest day all week.

Philadelphians should prepare for the longest, hottest heat wave so far this summer. The National Weather Service says temperatures are expected to top 95 degrees from today through Friday and potentially Saturday.

Highs in the mid-90s are expected, with heat index values of 100 to 105 degrees. The highest heat indices should occur between 1 p.m. and 6 p.m.

The weather service is calling for high temperatures of 96 degrees today and Tuesday, 97 degrees Wednesday, 96 degrees Thursday and Friday and 93 degrees Saturday.

Those temperatures are well above the normal high for mid-July of 87 degrees.

But the forecasts are also a bit shy of the record highs for this week. The records for today through Friday in Philadelphia are all above 100 degrees.

Even overnight, there will be little relief from the heat in the region. A weather service excessive heat warning says overnight lows should only drop to around 80 degrees.

And don't expect relief from summer thunderstorms, either. Forecasters aren't expecting rain until Saturday, when there's a 50 percent chance of showers and thunderstorms.

The week should be the fourth heat wave in Philadelphia this summer.

There were 17 heat-related deaths in Pennsylvania last year and one in New Jersey, according to the weather service. Elderly people living in non-air-conditioned homes are at the greatest risk of suffering heat illnesses.

The Philadelphia Corporation for Aging will activate its heatline from noon to midnight today, from 8:30 a.m. to midnight Tuesday and from 8:30 a.m. to 8 p.m. Wednesday. Callers can get information about hot-weather precautions, signs of heat stress and heat-related medical issues. The phone number is 215-765-9040.