Kevin Myatt

Latest entries from the Weather Journal blog

• Weather Journal taking a long break
• Yes, there's still an Atlantic tropical season going on
• Freezing temperatures likely tonight

About Kevin

Kevin Myatt grew up in Arkansas to the tune of tornado sirens and the rhythm of hailstones, aspiring to be a meteorologist before his studies and career were turned to journalism instead. Though he often chases storms, he prefers living in the cooler, more tranquil weather of the Blue Ridge. He moved to Roanoke in 1999 to take a job on the copy desk of The Roanoke Times; writing headlines and editing copy is his principal work for the newspaper today.

Each May, Kevin assists Pulaski County High School / Virginia Tech meteorology instructor Dave Carroll in leading college and high school students to the Plains to observe severe weather firsthand. The accounts of many of his storm chases can be found here on the storm chasing page of his weather blog on roanoke.com.

Kevin was an editor for "Hurricanes and the Middle Atlantic States," a book written by D.C.-area weather enthusiast Rick Schwartz and published by Blue Diamond Books that documents hurricanes striking the mid-Atlantic states since colonial times.

The Weather Journal column began in 2003 and appears on Friday's Virginia section front in The Roanoke Times. The Weather Journal blog began in 2006 and follows weather day-by-day between the larger columns.

Storms blow in bursts, gusts

We don't really have a monsoon season here in Southwest Virginia, but our summer months are often our "wet microburst" season.

So often in the summer, as afternoon thunderstorms crop up, there are reports of trees blown down here and there in a few localized areas, while areas not far away see no significant damage. Many summer wind damage reports are the result of wet microbursts.

This has been an especially active summer for severe weather locally, with the latest round on Wednesday knocking power out for more than 13,500 customers, mostly in Roanoke, Roanoke County and Bedford County. Both wet microbursts and a gust front may have caused that damage. We'll get back to that.

Wet microbursts occur as moisture, lifted by hot updrafts, condenses in cooler air aloft inside a thunderstorm, and then accelerates downward in a mighty cascade of rain, wind and some hail. In fact, melting hail can help intensify the downdrafts by causing air to become even cooler and therefore denser, sinking to the surface faster.

As the downdraft races toward the ground, it fans out along the ground, so that the wind at the surface is blowing outward from a central point.

Functionally, a microburst is the opposite of a tornado.

A microburst is a non-rotating downdraft, while a tornado is a rotating updraft. The winds in a microburst diverge from a central point, while those in a tornado converge toward the center of the storm's path.

Excluding the most violent tornadoes, microbursts are capable of causing similar damage to that caused by a tornado. Microbursts can be accompanied by an audible roar, and sometimes even a low-hanging cloud structure that might be mistaken for a funnel cloud.

Comparing the July 19, 2006, microburst in downtown Roanoke with the June 3 minor tornado in South Roanoke, the monetary damage is similar. The 2006 microburst caused $200,000 damage to the roof of the Virginia Museum of Transportation alone, and also damaged four other buildings, two heavily. The tornado caused a total of $350,000 damage to 10 homes and up to six vehicles, all from falling trees or limbs.

Wind damage in thunderstorms can also be caused by a gust front or outflow boundary. These winds initiate with thunderstorm downdrafts but are pushed ahead of the storm, typically in an arc formation. Usually, the gust front is at the leading edge of a storm, but sometimes it can outrun the storm by 20 miles or more, and on some occasions can even live on after the storm that created it has died.

Gust front winds always blow away from the parent storm. Occasionally, eddies of wind at the edge of the gust front create "gustnadoes," reminiscent of large dust devils or whirlwinds. Gustnadoes occasionally cause some damage.

Many supercell thunderstorms contain both tornadoes and microbursts. In fact, a powerful rear-flank downdraft similar to a microburst is a common feature of the most violent supercells. A supercell's rotation can allow for many downdrafts to occur without wiping out the storm's updrafts. Most of our summer thunderstorms are "pulse storms," where the downdraft kills the updraft and the storm goes out with a blaze of glory.

Wednesday's storm may have involved both microbursts and a gust front. The storm showed some evidence of very weak rotation as it passed over the Roanoke Valley, but winds aloft were not strong enough to keep the storm spinning into a supercell. It merged with other storms into a large cluster, and may have continued pushing out a gust front as it headed east.

Drier air will give us a little breather in this summer's assault of severe weather the next day or two, but heat and moisture will again start building by Sunday, and the pump could again primed again for localized, tree-toppling tempests.