Kevin Myatt is The Roanoke Times' weather columnist.

kevin.myatt
@roanoke.com
981-3341

Weather with Kevin Myatt

Recent columns

• We got graupel, but not on official record
• Moisture could get caught up in cold blast
• Forecast for Weather Journal: Partly print, with frequent Internet
• Column archive

Read the Weather Journal blog

• Sprinkles or flurries possible Tuesday, but maybe something bigger for the weekend?
• For now, it looks like a quiet, mostly mild week ahead for SW Virginia
• Coldest morning of winter so far likely across much of Southwest Virginia; Tuesday precipitation looking doubtful
• Weather Journal blog

#swvawx on Twitter

@KevinMyattWx

• Follow Kevin on Twitter at @KevinMyattWx

Two significant weather events that occurred late last week will forever be lost in the shadow of powerful Hurricane Charley. One was Tropical Storm Bonnie; the other was a massive cool air mass that rewrote the August temperature record book from the Canadian border to the Gulf of Mexico.

Bonnie will not be inducted into any tropical weather hall of fame: Bonnie was just breezy showers when it came ashore near Appalachicola, Fla., on Thursday. But the tropical storm did conspire with the advance of Canadian air to spawn tornadoes in North Carolina and Southside Virginia.

Danville was hit by a tornado late Thursday afternoon that rated F1 on the Fujita scale, according to a National Weather Service on-site analysis conducted the next day by meteorologists Hendricus Lulofs and William Perry of the Blacksburg office. The weather service's Doppler radar had indicated spin in the thunderstorm before the tornado touched down, and a tornado warning was in effect for Danville before it hit.

An F1 tornado, the second weakest category on the Fujita scale, contains winds of 93 to 112 mph. This tornado landed right in Danville's commercial district, ripping roofs off department stores and restaurants and felling trees and power lines across busy streets. It made a mess, but fortunately, killed no one. However, another tornado that hit on Friday morning killed three people at Rocky Point, N.C.

A common attribute of tropical storms and hurricanes is that they spawn tornadoes during and after landfall. Big spin begets little spin. As the large fluid motion of a hurricane encounters the friction of land, the big spin breaks down into smaller areas of spin. Ultimately, this is the process that kills the storm, but it can spin off twisters before it does.

A zone northeast of the tropical cyclone's center is the most common area for tornado development. This is where the strongest squalls and thunderstorms tend to develop, as moisture is pulled out of the Gulf of Mexico and Atlantic Ocean in front of the storm.

Another important factor is that the counterclockwise winds at the surface are usually blowing at a different direction than winds high in the atmosphere. This is called "shear" and it can cause rotation in a column of air. Imagine taking a baseball bat and moving your hands in opposite directions. The bat spins. When this spin translates to the surface, a tornado is born.

True to form, the tornadoes in Danville and in eastern North Carolina were both northeast of the weakening center of what was then tropical depression Bonnie.

Tornadoes spawned by hurricanes and tropical storms are usually weaker and much shorter lived the supercell thunderstorm-driven tornadoes of the Great Plains. Winds aloft are much stronger in Midwestern outbreaks -- strong winds aloft actually deter tropical storm and hurricane development. Also, there are no great differences in temperature or humidity amid tropical systems to keep tornadoes going, like with the cold fronts and dry lines that signify supercell development.

Last week's tornadoes in Virginia and the Carolinas had one component similar to Midwestern severe weather outbreaks. A strong cold front, the leading edge of the record cool air mass I mentioned, was being forced into the muggy, tropical air mass on the East Coast. So Bonnie's spin was compounded by a clash of air masses more like April than August.

The pattern is still more April than August. Another strong push of atypically cool, dry air is set to push into the temporarily warm, humid air mass that's settled in. That promises a threat of strong thunderstorms and heavy rain this weekend -- but without any assistance from anything that has a name.