CNN — (CNN) -- High temperatures forced Amtrak to keep the brake on rail speed in the Northeast on Friday, while a cold front bearing thunderstorms hinted at some relief from the Great Lakes to northern New England.
Amtrak said the speed restrictions were necessary along rail lines between Philadelphia and Harrisburg, Pennsylvania, because of extreme heat that can distort tracks. Highs in the high 90s were forecast for Philadelphia on Friday.
The National Weather Service said there was a slight risk of severe thunderstorms, damaging winds and hail from northern Illinois and southeastern Wisconsin east to Maine.
The storms should help break the grip of the humid heat wave that has smothered the northeast quarter of the nation, forecasters say.
The relief will arrive in the Upper Midwest late Friday before cooling off the Northeast on Saturday night, the National Weather Service said.
People should avoid celebrating outdoors, as thunderstorms are predicted to electrify the ground with lightning in many areas. Michigan could see large hail, the weather service said. High gusts may blow loose objects around.
Tornadoes are also possible.
But the sticky heat will not retreat without a fight. It was steam cooking the Midwest during the day Friday and will do the same to the mid- to north Atlantic until late Saturday.
Temperatures above 90 degrees will combine with roughly 100% humidity to put heat indexes, which measure how hot it feels, near 100.
The index is likely to hit 105 in the heavy populated corridor from Washington to the northern exurbs of New York City and over a broad swath around Detroit, the weather service warns.
In New York, utility company Con Edison urged customers to conserve energy as electric use hit an all-time high in its service area, a peak of 13,214 megawatts at 2 p.m. Friday.
Extreme heat causes more deaths than all other extreme weather conditions, the Centers for Disease Control and Prevention says.
It killed more than 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. This week, an elderly man who wandered away from his home in Kentucky died of heat exhaustion, a local coroner said.
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.
As the high-pressure area that caused the heat relinquishes in the Northeast, a new one will form in the Southwest.
It will bring "record heat" to western Nevada, the weather service predicts, with temperatures topping 100 degrees in some places every day for nearly a week.
Fire precautions are in place there because of a long-standing drought and dry heat.