Tropical Storm Fred forms as extreme heat grips the US: Latest

As Tropical Storm Fred takes aim on the Florida Keys, a heat wave is gripping 34 states.

Heat wave

A heat wave is spreading to the Northeast as well as parts of the Midwest, South and Pacific Northwest.

By Thursday, the heat index -- what the temperature feels like -- could climb to 103 degrees in Boston, 105 in New York City and 109 in Philadelphia.

Excessive heat warnings have been issued in New York City and Philadelphia. Boston has declared a heat emergency Wednesday through Friday.

“When it is this dangerously hot during the day and the temperatures do not drop at night, your body doesn’t have time to recover," Boston Mayor Kim Janey said. "I am urging everyone to drink lots of water and find ways to stay cool. Anyone who needs a place to beat the heat can come inside and rest in the air conditioning at one of our cooling centers."

The dangerously high temperatures are also reaching cities including Chicago, Memphis, Dallas, Portland and Sacramento.

Tropical Storm Fred

Tropical Storm Fred formed overnight near Puerto Rico, where gusty winds and heavy rain are hitting Wednesday morning.

A tropical storm warning is in effect for the Dominican Republic, where Fred is expected to reach Wednesday afternoon.

A tropical storm watch was issued for Cuba, where the storm may near Thursday and Friday.

By Friday night into Saturday, Fred is forecast to approach the Florida Keys with gusty winds.

Fred's expected to move up Florida's west coast, parallel to the coastline, on Saturday and Sunday.

By Monday morning, Fred may move ashore along the Florida panhandle, between Pensacola and Tallahassee, bringing rain and wind.

ABC News' Melissa Griffin contributed to this report.