That radar, a WSR-88D model, is the most powerful one tasked with scanning the skies in northern Virginia, central Maryland, the eastern panhandle of West Virginia, and the District. It’s part of a network of 159 such Doppler radars nationwide maintained by the National Weather Service. Each radar emits high frequency pulses of energy, a portion of which bounce off precipitation targets and offer valuable information from inside a storm.

While the radar is down, forecasters will rely on airport radars and Weather Service radars at adjacent offices in State College, Pa., Pittsburgh, Mount Holly, N.J., Wakefield, Va., Dover, Blacksburg, Va., and Charleston.

This network of radars can stitch together a reasonable representation of storm surveys.

The region has some of the best radar coverage in the country thanks to four smaller, less powerful “terminal” radars at the three major airports, Dulles, Reagan National and BWI Marshall, as well as Andrews Air Force Base. Those radars are very useful for aircraft in detecting changing winds during takeoff and landing.

Until the broken radar is fixed, only the airport terminal radars will be available for radar coverage below 8,000 feet. The nearest National Weather Service radar, situated in Dover, Del., nearly 90 miles east of Washington, D.C., can collect some data above that level as can surrounding radars at somewhat higher altitudes.

Aside from a few showers on Friday, significant weather requiring serious radar monitoring is not expected through the weekend. But next week’s weather pattern could see more turbulent storm systems, which could make the absence of the Sterling radar more consequential.

Meteorologists at the National Weather Service in Sterling don’t know how long this outage will last. A team of specially-trained radar technicians from NOAA’s radar Operations Center in Norman, Okla., are set to arrive here by Saturday morning.

“They’re driving out here,” said Austin Mansfield, a meteorologist at the National Weather Service in Sterling. “They have all their materials with them, so they can assess the damage and make a repair.”

The team is set inspect the gear box inside the radome, the large golf ball-like bubble that houses the actual rotating antenna. A series of gears are responsible for driving that spinning apparatus. When they break, the delays can increase.

“The bull gear would be worst case scenario,” Mansfield explained. “This repair usually takes about ten days.”

The bull gear is about a foot and a half to two feet wide, and replacing it means hoisting the radar antenna off its pedestal inside the dome. That’s no easy task.

He said that the office’s staff was initially alerted to an issue “when red alerts [popped] up on screen” and data scans stopped coming in. Electronics technicians in Sterling weren’t able to fix the issue, so the folks in Oklahoma were called in.

Two years ago, the National Weather Service in Wakefield, Va., suffered a similar failure with the bull gear. Getting that radar back in shape took weeks.

“You start to get a lot of noise with a vibration,” recalled Jeff Orrock, the meteorologist in charge at the National Weather Service in Wakefield. “You could hear it. You could feel the whole tower have a vibration within it. We knew we had a substantial problem.”

Diagnosing the problem and shipping out parts were time-consuming, said Orrock. The team from Norman also had to build scaffolding inside the radar.

“They had to open up a couple of the fiberglass panels [on the radome] to load up the scaffolding,” Orrock explained. “They had a small lift crane loading stuff, but they knocked it out fast. They worked long days.”

The National Weather Service Radar Operations Center is in the midst of the Service Life Extension Program, which seeks to lengthen the lifetime of the U.S. Doppler radar network. Each radar was originally built to last 20 years; the goal of the Service Life Extension Program is to double that.

The Sterling radar was taken offline for several days in July 2017 for the first in a series of upgrades to extend its lifetime through the 2030s.

Jason Samenow contributed to this article.

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