Monsoon Safety Awareness Week: Downbursts

Weather Talk
Wednesday, June 11, 2014 - 1:17pm

Downburst winds are much different from tornadoes, but can be just as powerful

Monsoon Safety Awareness Week rolls on with today’s weather focus on “Downburst Winds”. The goal of Monsoon Safety Awareness Week is to reduce the number of deaths, injuries and property damage caused by weather related dangers that occur during our monsoon season. Our monsoon season is from June 15th through September 30th. Some of these dangerous weather phenomena like flash floods or dust storms can happen any time of year, but they are more likely to happen during this season. Today’s “Weather Talk” is on what are downbursts, the different types, what kind of danger and damage they produce and how to stay safe.

Downburst winds are created as rain-cooled air hits the ground and spreads out forcefully in all directions. Unlike winds with a tornado, downburst winds are directed outward from the point where they hit the ground. NOAA (National Oceanic and Atmospheric Agency) describes downburst as “Cold air begins to descend from the middle and upper levels of a thunderstorm, falling at speeds of less than 20 miles an hour. As the colder air strikes the Earth's surface, it begins to "roll" - much like water as a boat moves through it. As the colder air "rolls" out, it is compressed causing winds to increase dramatically - at times even stronger than tornado winds!”.

Dry downbursts are associated with thunderstorms that contain very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. On our typical monsoon day the thunderstorm activity will end during the late evening hours. This is the most common time for a downburst to occur, resulting in severe wind gusts that can cause extensive damage.

Downbursts create vertical wind shear or microburst which is dangerous to planes, especially during take off and landing where the winds can force the nose or the tail down. Any aircraft is more vulnerable at those times because they are closer to the ground.

Wind shear is the meteorologist's way of describing a rapid change in either wind speed or wind direction over a short period of time or distance. Wind shear can describe the changes either horizontally (along the Earth's surface) or vertically.

Microbursts don’t happen very often, but they do happen fast. Mobile homes, Telephone poles, trees and other objects can be knocked over instantly. Dense blowing dust can be a dangerous and potentially deadly driving hazard. The Borderland saw this recently when six people were killed on Thursday May 22, 2014. An evening “severe dust storm” produced by powerful thunderstorm downburst winds caused visibility to drop to zero. This resulted in a traffic accident involving several vehicles and a semi truck. New Mexico State police said the accident occurred about 5:30 P.M.. in Hidalgo County about six miles from the Arizona state line.

Here is a NOAA comparison of a microburst and the larger macro burst shows that both can cause extreme winds.

Microburst
Damaging winds extending 2 1/2 miles or less
Lasts 5 to 15 minutes
Can cause damaging winds as high as 168 MPH!

Macroburst
Damaging winds extending more than 2 1/2 miles
lasting 5 to 30 minutes
Damaging winds, causing widespread, tornado-like damage, could be as high as 134 MPH!

How frequently do downbursts occur?
NOAA says "Downbursts are much more frequent than tornadoes - in fact, for every 1 tornado there are approximately 10 downburst damage reports!"

Tornadoes-Average of 800 per year in the U.S..

Thunderstorms- Average of 100,000 per year in the U.S..

Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall no or little rainfall. Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation.

“Derecho” Straight-line winds
"Derecho" is a Spanish word meaning "direct" or "straight ahead." All downbursts can occur over large areas. A derecho is a widespread, long-lived wind storm. Derechos are associated with bands of rapidly moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi-linear convective systems. By definition, winds in a derecho must meet the National Weather Service criterion for severe wind gusts, greater than 57 mph, at most points along the derecho path. But in stronger derechos, winds may exceed 100 mph. In the extreme case, a derecho can cover a huge area! NOAA says; “if the swath of wind damage extends for more than 240 miles (about 400 kilometers), includes wind gusts of at least 58 mph (93 km/h) along most of its length, and several, well-separated 75 mph (121 km/h) or greater gusts, then the event may be classified as a derecho.” Some Derechos can last to 12 hours or more, and is associated with some of the most intense straight-line winds.

How do we stay safe?
NWS Motto: Pull Aside – Stay Alive!
Thunderstorms frequently produce strong downward rushes of air, called microbursts
• These winds can gust in excess of 100 mph, and cause extensive property damage
• Downbursts may generate areas of dense blowing dust
• If downbursts approach move inside a sturdy building and stay away from windows
• Pull over well off the side of the road and wait for the visibility to return

There have been many downburst and even straight line wind damaging weather events in the 19 years forecasting here in the Borderland. I have seen Mobile homes overturned in Montana Vista in Far East El Paso. I have seen numerous trees and power poles knocked down and even a few roofs fly off! I have also seen many instant storm caused dust storms dropping visibility dangerously low causing accidents. Knowing about what downburst are, the different types and how to stay safe is safe during our monsoon is very important.

As a forecaster we know when strong weather systems are going to produce our windy season days full of blowing dust as much as a week or so in advance. The scary thing with downburst winds is the forecast window is much smaller within a few hours to a few minutes. Tomorrow Monsoon Awareness Safety Topic is “Dust Storms”,

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX
cdebroder@ktsm.com
www.facebook.com/pages/Charles-DeBroder/
https://twitter.com/wxchuckNC9
 

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