UPDATED: March 8, 2013, at 6:31 p.m.

Heavy snowfall in Cambridge slowed traffic across the city on Friday. But on JFK Street near Harvard, traffic stopped completely. On Friday morning, underground electrical cables ignited, causing a manhole fire and prompting Cambridge Police to temporarily close JFK Street between Mount Auburn Street and Memorial Drive.

The incapacitated cables caused power outages for 700 customers of NSTAR Electric and Gas, the city’s electricity provider, according to NSTAR spokesperson Michael Durand. He said the incident was probably not related to the snowstorm.

NSTAR received reports of a smoking manhole at the intersection of JFK Street and Winthrop Street at around 11 a.m., Durand said. The smoldering, he said, was due to energy released when the cables failed.

The cables burned for about an hour, according to Cambridge Fire Department Deputy Chief Francis E. Murphy III, before firefighters dumped water onto the cables and extinguished the fire. NSTAR restored power for all but 100 customers within the hour, and expects to have all customers back online by 7 p.m., Durand said.

Traffic signals at the corner of JFK Street and Memorial Drive were without power due to the incapacitated cables, so state police directed traffic for some of the afternoon, according to a statement on the Cambridge Police Department website asking motorists to avoid the area.

University spokesperson Kevin Galvin wrote in an email that 65-67 Winthrop Street, which houses Harvard University Dining Services offices, was the only Harvard building to lose power.

Soon after the fire was put out, Mount Auburn Street reopened entirely, and by around 2 p,m., all of JFK Street was open to vehicles. NSTAR employees remained at work on the scene.

Firefighters, police, and electrical workers attended to the cables and closed streets as heavy snow fell on the greater Boston area. As of 1 p.m., more than a foot of snow had fallen in the area, and the National Weather Service extended its winter storm warning until 7 p.m. for Eastern Massachusetts.

MBTA subway service remained unaffected by the storm Friday afternoon, except for the replacement of Mattapan Line trolleys with buses due to “weather-related issues.”

According to the NWS, snow showers will be mostly finished by 9 p.m., and the area can expect sunny weather with a high of 42 degrees on Saturday.

—Staff writer Samuel Y. Weinstock can be reached at sweinstock@college.harvard.edu. Follow him on Twitter @syweinstock.