I get that question often from viewers who have recently experienced a severe weather outbreak. “How could this tree damage not be from a tornado?” they’ll often ask. As much as they might want it to be a tornado (which is a topic for another discussion), most severe thunderstorm damage in Maine is caused by downdrafts or micorbursts. Folks are typically surprised when I tell them microbursts can and often produce stronger winds and worse damage than a weak tornado. So what’s the difference?
Under certain atmospheric conditions, Thunderstorms can begin to develop a circulation within the cloud. These storms are called meso-cyclones. Updrafts and downdrafts in these storms can persist for hours as the storm moves along its path. Severe winds and hail are also more likely with meso-cyclones, and if the rotation within the storm becomes intense, there is an increasing possibility that the storm might produce a tornado. Doppler radar allows meteorologists to monitor air movement within these storms and to see the development and strength of any circulation within the storm. It was doppler radar that indicated a TVS or (Tornado Vorticity Signature) in the severe thunderstorms in Oxford, Franklin, and Somerset Counties last night that prompted the National Weather Service to issue three Tornado Warnings.
Tornado development within a supercell thunderstorm
In some thunderstorms, intense downdrafts develop. When these downdrafts reach the ground, they spread out quickly causing strong and often damaging winds at ground level. These intense downdrafts are called downbursts and can cause significant wind damage over large areas. In the case of downbursts, the damage is generally referred to as straight-line wind damage since fallen trees generally line up in the same direction.
A special type of downburst is a micorburst. Microbursts get their name because they generally affect a much smaller geographical area, but the winds in a microburst can be very intense. Like the general downburst, most of the damage with microbursts lines up in one direction. Microbursts are usually accompanied by heavy rainfall and/or hail and can have winds as strong and sometimes stronger than a weak tornado.
Downdrafts in a thunderstorm producing straight line winds
National Weather Service survey teams will often head out to survey damage left behind from severe thunderstorms in the following days of the severe event. They rely on archived radar, and also eye witnesses to conduct a full survey. That survey usually includes estimated wind, path length, width, and duration. Damage extent and tree damage patterns are also important in determining the whether the damage was caused by a micoburst or tornado and its strength. These two images provided by NWS show the difference in damage patterns between a micorburst and tornado.
In the case of the York microburst yesterday, there was a pretty strong signature on radar indicating it was caused by straight line winds. NWS survey team surveyed the damage today, and confirmed it was caused by a microburst. Here are the details
DATE/TIME: 7/15/14 5:16 PM
ESTIMATED PEAK WIND: 70 to 80 MPH
WIDTH: 2.5 Miles
DURATION: Less than 5 minutes
We’re told NWS Survey crew will be headed out to survey the damage in a line roughly running from Farmington, Mercer, to St. Albans tomorrow. The cell that produced that damage was rotating and a clear Tornado Vorticity Signature. We’ll see what they find.
Thanks to viewers who provided great damage photos. Special thanks to NWS for info and damage description graphic.