India, June 30 -- Boston and much of New England are bracing for a prolonged spell of dangerous heat as a powerful heat dome settles over the northeastern United States. Forecasters warn that temperatures could approach or exceed 100degF, while humidity will push the heat index as high as 105degF to 110degF in some areas through the Fourth of July holiday.

The National Weather Service has issued an Extreme Heat Watch from Wednesday morning through Saturday for most of Massachusetts and New England, excluding Cape Cod and the Berkshires.

Meteorologists warn that the combination of extreme heat and tropical humidity could pose serious health risks, especially for older adults, children and people working outdoors.

Forecasters said Tuesda...