Straight-line winds vs a tornado: What caused Saturday’s storm damage?

Nearly 20,000 people were thrown into the dark Saturday after storms toppled dozens of trees and power poles throughout Cullman, Morgan, Madison, Limestone and Jackson counties.

The National Weather Service in Huntsville determined that wind gusts as high as 90 mph occurred Saturday evening.

With such extensive storm damage that occurred Saturday, many of us are wondering what exactly happened. Was it a straight-line wind event? Was it a tornado? What could have downed so many trees and powerlines in the Valley Saturday?

'Severe thunderstorm warning in effect through 6pm'

At 5:27pm, the National Weather Service issued a severe thunderstorm warning for Madison, Morgan and Limestone Counties, specifically for areas that included Lacey's Spring, Madison and Huntsville. The warning was issued for a storm that quickly moved through those counties between 5:30pm and 7pm (an additional severe thunderstorm warning was issued from 6:30pm to 7pm). The storms' main threats included damaging 60 mph winds.

Severe thunderstorm warning issued at 5:27pm CT Saturday, May 20, 2017.

A strong color gradient between the reds and oranges versus the blues and greens on the reflectivity imagery indicates very heavy rain is falling near "drier" air. This set up can lead to very damaging thunderstorm winds.

As the storms approached Triana, Madison and downtown Huntsville, doppler radar continued to detect wind speeds in excess of 60 mph. This was indicated not only by the precipitation/rain gradient present in the reflectivity imagery, but in the velocity (wind speed) imagery as well.

However, the orientation of the winds' direction was in such away that strong, tornadic rotation was not suspected, and the 60+ mph wind speeds spanned as much as 20 miles wide.

Doppler radar velocity imagery indicates 60+ mph wind speeds moving towards the northeast, in the direction of downtown Huntsville.

However, there were a few instances where quick, extremely small rotations did indeed develop. These are called mesovortices, which means they are extremely small and circular in nature (a few hundred feet to half a mile wide).

Meteorological studies indicate that mesovortices can produce quick "spin up" tornadoes, especially in a straight-line wind event.

It seems that a mesovortex did develop in south Huntsville Saturday evening, as caught on radar by the severe research teams at UAH:

Many shelf clouds were reported through the Valley as the storms moved through, which indicates the presence of strong, straight-line winds. However, a few wall clouds (lowering protrusions) were spotted hanging from the shelf clouds, and a few viewers did report seeing a few funnel clouds develop as well.

A shelf cloud moves over Lake Guntersville. (Photo: Jordan Anken)

Funnel clouds are clouds that exhibit rapid rotation, but remain suspended in the air and do not reach the ground. Given the intense wind speeds moving through the rapidly changing terrain of the Tennessee Valley (including Monte Sano and Chapman Mountain), it is possible that a quick "spin" up would develop amongst the straight-line winds. However, the majority of the storm damage from Saturday occurred due to strong, straight-line winds.

In conjunction with WHNT News 19, the National Weather Service in Huntsville is also analyzing radar data, damage reports, and viewer photos/videos to understand what happened. The National Weather Service issued a statement Monday saying that the widespread damage was caused by a a straight-line wind event that produced wind gusts as high as 90 mph.

A possible wall cloud moves through Gurley. (Photo: Garrett Cole)

"Most likely, the totality of the damage was due to the straight line winds with at least one or two gustnadoes [mesovortices] along the leading edge of the shelf cloud," said Chris Darden, the Meteorologist in Charge at the National Weather Service in Huntsville.

"The location of the reports and characteristics of the observations compared to radar data would indicate they were gustnadoes. If you map out the damage it extended all the way East Limestone across Triana and the Zeirdt/Martin Road area to Owens Crossroads as the storm was moving almost due north."

"But we had a couple of folks that witnessed/saw funnels in the Hampton Cover area and also perhaps down near Lake Forest Estates. The one video I saw from Hampton Cove showed a funnel probably 50 yards wide," Darden concluded.

Straight-line winds can be just as damaging as a weak tornado

It is important to note that severe thunderstorms capable of producing straight-line wind damage can be just as powerful as an EF0 to EF1 tornado.

Wind damage caused by a downburst (left) and a tornado (right). Photo: NWS La Crosse

Severe thunderstorm warnings that indicate straight-line winds are a possibility should be taken with the same seriousness as a tornado, since falling trees and powerlines can significantly impact businesses and communities.

The image at right shows a comparison between a microburst (straight-line wind) damage versus tornado wind damage. A microburst is a straight-line wind event that spans less than 2.5 miles in diameter. You may hear something different, like a macroburst, which is a straight-line wind event that spans greater than 2.5 miles in diameter.

How a microburst works - NOAA

Both macrobursts and microbursts can be considered downbursts, which are extremely fast-moving winds that occur when extremely dense, rain-cooled air surges to the ground and fans out in all directions. Downbursts are one particular type of straight-line wind events.

As the imagery indicates, both straight-line wind events and tornadoes can create extensive damage to trees, crops, powerlines, and other property. For this reason, severe thunderstorms should be taken very seriously, since the wind damage involved can be quite intense, even without the presence of a tornado.