Weather Service forecaster attributes Grand County wind damage to microbursts
July 1, 2011
GRAND COUNTY – According to meteorologist Frank Cooper of the National Weather Service in Boulder, yesterday’s storm was caused by atypical conditions.
Unusual to this time of year, the jet stream is still lingering over Northern Colorado, he said, which brought a strong low pressure system into the area. Normally the jet stream has moved farther north by now, he added.
At the same time, a system of localized afternoon thunderstorms developed ahead of the system due to the normal heating at the surface that occurs in the mountains during summer.
When the strong winds aloft collided with the thunderstorms, they were pulled down to surface in a downdraft, Cooper said, causing what is known as microbursts, or straight-line winds.
These winds are essentially the powerful jet stream, moving at 70 to 80 knots, being dragged down to the surface.
Cooper said he wouldn’t be surprised if there were some weak cold core funnels associated with the storm, but that most of the winds should have been “straight line” winds.
He does not believe a tornado actually hit the area, but he suggested that looking at the direction of the trees that have fallen would be a good indication of what really happened in Grand County on Thursday afternoon, June 30.
If the trees fell in the same direction, it’s likely straight line winds, he added.
Cooper added that the National Weather Service does not have its own weather station in Granby and has to rely on the reading from other weather stations and eye witness reports to determine what happened up here.
The station at Middle Park High School in Granby took a reading showing winds of 83 mph yesterday.
Damage is widespread from Granby to Grand Lake, primarily affecting roofs, trees and lawn furniture.
Karla Booth wrote in an email, “I was in this storm in my F650 (truck) at Carquest (in Granby) Thursday. It blew out six car windows including my back window from pressure like a tornado.”