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(CNN) -- Like many things in life, it's going to get worse before it gets better.
That lardy layer of humid, sticky heat oozing over the Northeast and Midwest is getting hotter and spreading out Thursday. But cooling rain will wash it away in some places in the evening.
The worst of it started out on the National Weather Service map as bright orange spots over parts of New England and Michigan this week. The spots have grown nearly together like a bad rash, forming a seething blotch from Massachusetts to South Dakota.
Temperatures above 90 degrees will combine with roughly 100% humidity to put heat indexes -- how hot it feels -- in the 100s.
New York City is expected to have a heat index of 103 Thursday. Hartford, Connecticut, along with Detroit and Baltimore, are expected to hit 105, CNN meteorologist Indra Petersons said.
"What makes heat so dangerous is not just the maximum temperature for one day, but how long the heat wave lasts," she said. "New York City is in its fifth day, and we're not over with it yet."
The temperatures are above normal for what is already one of the hottest times of the year, she said.
A cold front from Canada will work its way south, bringing some temperature relief to the Midwest on Friday and to the Northeast on Saturday, but also bringing severe storms, CNN meteorologist Dave Hennen said.
The heat has already caused problems in major cities this week.
Thousands of people were stuck in a commuter train in rush hour Wednesday as they headed out of Manhattan to Long island, New York.
In Detroit, some schools are closed for the rest of the week.
"Daily Show" reporter John Oliver summed up the misery with a joke: "On my way to work this morning, I saw a squirrel stab a pigeon over a piece of ice."
Extreme heat causes more deaths than all other extreme weather conditions, the Centers for Disease Control and Prevention says.
It killed over 8,000 people between 1979 and 2003, more than "hurricanes, lightning, tornadoes, floods, and earthquakes combined."
From 1999 to 2009, there was an annual average of 658 heat-related deaths in the country, the CDC says.
At least six people have died this summer in the Northeast, health officials in Maryland and New York state reported. A elderly man in Kentucky found dead this week after he wandered away from his home passed away from heat exhaustion, a local coroner's office said Thursday.
The hot weather is of particular concern for children, the elderly and people with heart and lung conditions, as air quality plummets while ozone levels soar. Officials advise everyone to stay cool and drink plenty of water.
Letting off some steam
Some people could find no relief from the heat and took their complaints to social media.
"Melting. Way too hot in this place to sleep," tweeted Morgan Sable from Ontario.
Twitter user @annielaa2 in Pennsylvania didn't have air conditioning Wednesday and turned a fan on herself and her cat.
"Getting hot air blown in your face really doesn't help," she tweeted.
From Toronto to Indianapolis, many people chose not to hide indoors in front of the air conditioner and opted for water instead, flocking to beaches, pools, fountains and splash parks.
CNN's Ben Brumfield, Leslie Bentz, Sunlen Miller and Laura Ly contributed to this report.