As if the steam-bath heat wasn't enough, the National Weather Service has added a severe-thunderstorm watch until 8 p.m. Monday for the entire region, along with the excessive heat warning in effect until 6 p.m.

Heat-index values could reach 105 Monday afternoon, said Lance Franck, meteorologist at weather service's Mount Holly office. At 1 p.m. the official reading at Philadelphia International Airport was 91, with a heat index of 100.

In addition, the government's Storm Prediction Center sees a "slight risk" that thunderstorms could reach "severe" criteria - that's with winds of 58 m.p.h. or better and inch-diameter hail.

Overnight a mouthful called a "mesoscale convective system" wrung out substantial rains from areas north and west of the city to the shore.

That's the technical term for a complex of thunderstorms that organize and generate persistent rains, sometimes for several hours. The 0.62 inches measured at the airport represented the city's heftiest rain total of a bone-dry July.

On Sunday, the high was 95, but it did not deter thousands who turned out for protests in advance of Monday's start of the Democratic National Convention.

In its heat warning Monday, the NWS spoke to the protesters gathered in Philadelphia without addressing them directly.

"The multiple days of excessive heat will greatly affect those who are attending outdoor activities, especially events with large groups of people that are gathering in the direct sun," the NWS said. "Visitors not acclimated to the heat in our region this summer have a greater risk of being affected."

Those thunderstorms early Monday morning prompted DNC organizers to warn that press tents set up outside the Wells Fargo Arena "are not designed to fully protect inhabitants in the event of a direct lightning strike."

Anyone inside the tents was advised to seek shelter in the arena. That might be the case again Monday night.

The storms also knocked out power to hundreds of customers in Southeast Pennsylvania and South Jersey.

The Philadelphia Corporation for Aging has activated its hotline - 215-765-9040 - until 6 p.m. for anyone needing assistance in dealing with the heat.