High temps, humidity will make for misery in Minnesota

Minnesota is in for a few days of misery as heavy rains swamp some areas during Wednesday morning's commute, followed by three days of downright dangerous tropical heat.

The heat will be so oppressive that it could endanger even the fittest, hardiest people, weather and health officials warned.

"Heat waves are the Number One weather killer, so we need to take it seriously," said Michelle Margraf, a meteorologist at the National Weather Service in Chanhassen.

With temperatures in the 90s and dew points at or near 80, Minnesota will feel more like Bora Bora, Manila or the jungles of Thailand, said meteorologist Paul Douglas.

"It's really the humidity that makes it dangerous," Douglas said. "Without the humidity, it would be Scottsdale. … It would still be uncomfortable, but it wouldn't be as ghastly. What we have is the combination of Arizona heat and Florida humidity."

Wednesday's predicted high of 92 degrees combined with a dew point of 77 will make it feel like 105 degrees, Douglas said. On Thursday, a high of 96 combined with a dew point near 80 will create a heat index of 110 to 112 degrees, he said. Even with a slight dip in temperature and dew point numbers — 92 and 75, respectively — on Friday, it will still feel like it's above 100 degrees.

Nighttime won't bring much relief. Overnight temperatures aren't likely to fall much below 80, according to the Weather Service.

"Studies show that we need to cool the body at night to be effective during the day," Margraf said.

Said Douglas: "A lot of grandparents might remember the '30s, when it was really hot — 108. But they didn't have the high humidity. It was a dry heat."

Nowadays, summer temperatures aren't that different from those years ago, but dew points seem to be rising, with more days featuring dew points in the 70s or more, Douglas said.

"So the heat feels worse because the humidity levels are trending higher," he said. "With this much water in the air, it throws a monkey wrench into our bodies' natural cooling system, which is perspiration."

The dew point is the temperature at which water vapor condenses into liquid water at the same rate at which it evaporates. A dew point of about 60 degrees or lower is considered comfortable.

"We hope we don't get there," Margraf said, noting that this week's dew points will likely top off in the upper 70s. "It will still be oppressive."

With such conditions looming, heat exhaustion can set in quickly, said Dr. Jim Miner, chief of emergency medicine at Hennepin County Medical Center. Symptoms include dizziness, lightheadedness, confusion, nausea or a racing heart rate. A lack of sweating also is a sign, he said.

"Water is the easiest way to keep cool," Miner said. "Drink more than you usually do, whether you feel thirsty or not."

Officials also warned people to limit their outdoor activity, wear light-colored and light-fitting clothing and find places that are air-conditioned.

Hennepin County posted an interactive map (www.hennepin.us/cool) showing public cooling spaces, including Salvation Army buildings, libraries, recreation centers, movie theaters and shopping malls.

The heat can do a number on cars, too, said Sabrina Caprioli of AAA Minneapolis.

"Summer is harder on batteries than winter, to the unhappy surprise of many drivers," she said.

Tires are an issue, too, as "extreme heat increases the pressure in the tires," making them susceptible to blow outs, she said.

Motorists can head off trouble by checking fluid levels, tire pressure and battery cables.

This week's heat wave will follow thunderstorms that could drop 2 to 4 inches of rain overnight Tuesday, prompting the Weather Service to issue a flash flood watch through Wednesday morning for the Twin Cities and southeastern Minnesota.

Relief will arrive Saturday with a slight cooling trend, Douglas said. Temperatures will be in the low 80s and dew points will fall into the low 60s by next week.

marylynn.smith@startribune.com 612-673-4788