( 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 . Sweltering conditions . 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 . '' Health dangers . 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 did n't have air conditioning Wednesday and turned a fan on herself and her cat . `` Getting hot air blown in your face really does n'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 .
NEW : Heat exhaustion kills man in Kentucky . The longer the heat wave lasts , the greater the dangers , meteorologist says . Severe storms are expected Saturday in the Northeast , Friday in the Northeast . Heat indexes are expected to hit 658 in some cities Saturday .