2:57
In Caribou, Maine, the heat index -- what the temperature feels like -- hit an all-time high of 103 degrees.
![\"PHOTO:](\"http://a.abcnews.com/images/US/ny-heat-wave_1718878514504_hpEmbed_3x2_384.jpg\")
![\"PHOTO:](\"http://a.abcnews.com/images/US/dc-main_1718879533268_hpMain_4x3t_384.jpg\")
The heat won't relent on Thursday.
Temperatures are forecast to hit a scorching 99 degrees in Concord, New Hampshire; 97 degrees in Hartford, Connecticut; 95 in Boston and Pittsburgh; 93 in Philadelphia and Indianapolis; and 91 in New York City.
![\"PHOTO:](\"http://a.abcnews.com/images/US/heat-map-2-abc-bb-240620_1718888691684_hpEmbed_16x9_384.jpg\")
In the Northeast, the heat might force Amtrak trains to slow down, causing up to one-hour delays, according to Amtrak.
High temperatures may require trains to operate at lower speeds, resulting in delays of up to 60 minutes between the hours 12 noon and 7:30 p.m. for the remainder of the week. Check your train status for the most up-to-date information.
— Amtrak Northeast (@AmtrakNECAlerts) June 20, 2024
In Washington, D.C., the heat is forcing Metro trains to slow to below 35 mph.
This weekend, the heat dome is heading south and west, with the heat index forecast to climb as high as 110 degrees in some places.
By Sunday, the temperature is forecast to skyrocket to 101 degrees in Oklahoma City and Washington, D.C., and 99 degrees in Nashville, Tennessee.
![\"PHOTO:](\"http://a.abcnews.com/images/US/heat-map-1-abc-bb-240620_1718888604628_hpEmbed_16x9_384.jpg\")
There are hundreds of deaths each year in the U.S. due to excessive heat, according to CDC WONDER, an online database, and scientists caution that the actual number of heat-related deaths is likely higher.
Click here for tips on how to stay safe in the heat.
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