Four construction workers jackhammer a bridge sidewalk in the heat (copy)

Construction workers cope with 90-degree-plus heat Wednesday, as they use jackhammers on the Anthony J. Sacco Sr. Memorial Bridge in North Adams, which is undergoing renovations. The heat is expected to last through Friday.

As stifling heat and humidity blanket the Berkshires and the rest of the Northeast for a second day, the region’s power utility group is asking residents and businesses to lower their energy usage to protect the electrical grid.

Based on peak consumption predictions for Thursday, ISO New England, which serves all six states in the region, is recommending that energy users plan to conserve power starting at midafternoon, since demand peaks between 4 and 6 p.m. during the summer. That is later than in the past because of greater solar power usage earlier in the day.

The summer’s peak regional demand was 25 gigawatts June 29, in the midst of the season’s only previous heat wave.

On Wednesday, actual demand on the grid just after 5 p.m. was 23.5 gigawatts.

The oppressive heat and humidity — North Adams reported a high of 91 — fueled the development of strong storms to the west of the region, according to meteorologist Brian Montgomery at the National Weather Service in Albany, N.Y.

After temperatures topped out at 89 degrees in Pittsfield on Thursday, a line of severe thunderstorms dumped heavy rain and caused numerous lightning strikes across parts of the Berkshires.

About 2,400 people were left without power in Great Barrington after the storm took down power lines. The National Weather Service reported trees down on Christian Hill Road in Great Barrington, Washington Mountain Road in Hinsdale and Yokum Pond Road in Becket.

In Cheshire, firefighters respond to a tree that was struck by lightning and caught on fire, and Pittsfield reported some minor street flooding.

The hot spell will continue Friday,

The National Weather Service issued another heat advisory for 11 a.m. to 8 p.m. for a third and final day of high humidity and temperatures near 90, but the leading edge of cooler air will approach the Berkshires after nightfall and into Saturday afternoon, he stated. Thundershowers are possible until the slow-moving cold front clears the area before dusk, with a noticeable drop in temperatures and humidity accompanied by breezy northwest winds.

With temperatures dropping into the 50s by dawn Sunday, one of the best days of the season is expected, with daytime highs in the 70s, under clear skies. A repeat performance follows Monday, before a warming trend returns later in the week.

State House News Service contributed to this report.

Clarence Fanto can be reached at cfanto@yahoo.com, on Twitter @BE_cfanto or at 413-637-2551.