Twin Citians itching for a warm weekend after a series of cool, wet ones may be tempted to make up for lost time with outdoor chores and recreation Sunday.

Proceed with extreme caution.

For the second day in a row, a steamy combination of high heat and humidity will make even modest exertion potentially hazardous.

“It’s pretty dangerous to be outside for an extended period of time,” said Caleb Grunzke, a meteorologist with the National Weather Service in Chanhassen. “It’s a good weekend to stay inside,” even though it looks sunny and inviting. “Deceivingly nice.”

An excessive heat warning for the Twin Cities metro area issued by the National Weather Service began at 1 p.m. Saturday and will continue until 9 p.m. Sunday. Much of the rest of southern Minnesota is under a heat advisory.

Overnight low temperatures Saturday night are expected to fall only into the mid-70s.

And stormy weather is possible overnight Saturday, with heavy rain, high winds, large hail and possibly even an isolated tornado. Storms are expected to develop in North Dakota, then move southeast into central Minnesota.

Sunday’s high in the metro area will be 94, with a heat index of 103, the NWS said.

If you do plan to be outside this weekend, “avoid strenuous activity,” advised Grunzke. “Take many breaks and stay hydrated.”

Be alert to signs of heat exhaustion and heat stroke. “If you’re not feeling well — dizzy or tired — take a break, and seek medical attention,” said Grunzke.

Senior citizens and children under 4 are especially vulnerable, so check on vulnerable relatives and neighbors.

And don’t leave a child or pet in a closed car, even for a very brief period. “It’s extremely dangerous,” said Grunzke. “Pets are very vulnerable.”

Twin Citians who find themselves without power or air-conditioning this weekend may want to seek places to cool off. In addition to libraries, recreation centers, Salvation Army buildings, movie theaters and shipping malls, there are a number of cooling centers in Minneapolis and St. Paul. The National Weather Service offers an interactive map of places to cool off. Some may have limited hours or require a fee, so calling ahead is recommended.