The Interstate 95 corridor, from Richmond to Boston, faces the prospect of the summer's most widespread outburst of heat and humidity between Thursday and Sunday.

While actual air temperatures may not rival their July peaks in some areas (especially in the Mid-Atlantic), triple-digit heat index values are likely to cover more territory over a longer period than in any prior heat wave. (The heat index is a measure of how hot the air feels, factoring in humidity.)

The brutal combination of heat and humidity arrived Wednesday in the Mid-Atlantic, when the heat index eclipsed 100 in Washington. Such levels are forecast to ooze as far north as southern New England on Thursday. Predicted heat index levels near 100 degrees in eastern Massachusetts have prompted the National Weather Service to issue heat advisories.

Thursday heat index forecast


GFS heat index forecast at 5 p.m. Eastern. (PivotalWeather.com)

Friday and Saturday are likely to see the most widespread coverage of punishing heat index values. That's when it will feel like 100 to 105 degrees from Richmond to Boston.

Friday heat index forecast


GFS heat index forecast at 5 p.m. Eastern. (PivotalWeather.com)

Saturday index forecast


GFS heat index forecast at 5 p.m. Eastern. (PivotalWeather.com)

By Sunday, the core of the heat and humidity is predicted to retreat south of Boston but will continue to roast the corridor from Richmond to Providence, R.I.

Sunday heat index forecast


GFS heat index forecast at 5 p.m. Eastern. (PivotalWeather.com)

The source of the steamy air is a high-pressure system parked over the Atlantic whose circulation is pumping a deep plume of tropical air from the Gulf of Mexico directly northeastward.


GFS forecast of precipitable water, an indicator of atmospheric moisture, on Friday evening. It shows a deep plume of moisture (red and purple shades) streaming northeast from the Gulf of Mexico into New England. (WeatherBell.com)

Dew points, which are direct measures of the humidity, are forecast to be well into the 70s throughout this area. Anything over 70 degrees is very uncomfortable.


Forecast dew points from GFS model late Saturday. (PivotalWeather.com)

The abundant moisture in the air has raised concerns about flash flooding and thunderstorms that could develop in this region over the weekend into early next week.