Thursday…Severe Thunderstorms

The Iowa Homeland Security and Emergency Management Division and the National Weather Service have declared the week of March 23 through March 27, 2015 Severe Weather Awareness Week. Severe Weather Awareness Week is an annual event to remind Iowans that severe weather is part of living in our state and that understanding the risks and how to respond to them can save lives. Each morning during severe weather awareness week, we’ll be focusing on a different severe weather topic. The topics this year include:

  • Monday – Flash Flooding
  • Tuesday – Warning Reception
  • Wednesday – Tornadoes
  • Thursday – Severe Thunderstorms
  • Friday – Family Preparedness

Thunderstorms are a common occurrence each spring and summer across the Midwest and Iowa is not immune. Each year, Iowa sees hundreds of severe and non-severe thunderstorms. Thunderstorms can be extremely dangerous storms which may bring deadly tornadoes and lightning, damaging high winds and hail, and can lead to flash flooding.

The National Weather Service issues severe thunderstorm warnings for thunderstorms that are producing or are capable of producing:

  • Winds of at least 58 mph
  • Hail at least one inch in diameter

SevereCriteriaOftentimes, severe thunderstorms may be much stronger than the minimum criteria.


Straight-Line Winds

Straight-line wind is a term used to describe non-tornadic winds generated by severe thunderstorms. These winds have their origins aloft in the thunderstorm, where rain cools the air in its immediate vicinity. This cold air accelerates downward because it is denser than the surrounding air mass, and spreads out across the ground upon reaching the surface. These winds are typically in the 50 to 70 mph range, but in rare cases can exceed 100 or even 115 mph (similar to a Category 3 hurricane). Unlike tornadoes, downed trees and other debris trails are oriented in a single direction, hence the term “straight-line” winds.

Even though straight-line winds are not as strong as large tornadoes, they still have the ability to uproot trees, down power lines, damage buildings (especially grain bins, storage sheds, and other similar structures), and flatten crops. High profile vehicles are also vulnerable and can be flipped or forced off the road by these winds. Falling trees and other debris pose a hazard to anyone in their path; many deaths in straight-line wind storms are attributed to trees falling onto people in their cars or homes. Downed live power lines can easily electrocute and possibly kill anyone who comes in contact with them. Straight-line winds are responsible for most thunderstorm wind damage, and can cause damage equivalent to an EF-2 tornado. However, while a tornado damage track is relatively short and narrow, the damage swath from a straight-line wind event can be tens of miles wide and affect thousands of square miles.

Terms to Know:
  • Bow Echo: One of the more common straight-line wind storms. Consists of a long band of severe thunderstorms and appears bow-shaped on radar imagery (see image above). Bow echoes tend to produce a wide band of straight-line winds with the strongest winds located at the apex of the bow.
  • Downburst/Microburst: A small scale wind event typically on the order of a few miles in size and lasts for five to ten minutes. Generally associated with individual storm cells.
  • Derecho: A name given to an especially long-lasting straight-line wind storm that produces a large damage path, which may be hundreds of miles long.

Hail

Hail is frozen precipitation that falls from a thunderstorm and can grow to the size of softballs or larger, but is generally less than two inches in diameter. A strong thunderstorm updraft (rising air in a storm) is key for the production of hail. As rain near the base of the storm gets caught in the updraft, these drops get lofted high into the storm and freeze into small ice pellets. These ice pellets cycle through the updraft, repeatedly collecting more water and growing larger as they are lofted into the storm and freeze. Once the thunderstorm updraft can no longer support the hailstones, they fall to the ground. A number of meteorological factors can influence hail sizes, but the strength of the thunderstorm updraft (therefore, the strength of the storm itself) is the most critical.

Hail poses a serious threat to anyone outside and outdoor property. Large hailstones can fall at speeds of over 100 mph and easily injure or kill anyone caught in their path. Pets and livestock are also susceptible to injury or death by hail. In addition, strong winds during a hailstorm can amplify the effects of small hail and damage the sides of buildings. Close to a billion dollars in property damage is caused by hail each year, mostly to automobiles, house roofs, and crops.


Lightning

While undoubtedly the most commonly experienced severe weather threat, lightning is by far the deadliest of these phenomena. An average of 58 people are killed and over 300 injured in the United States each year by lightning, making it the third deadliest weather phenomenon (behind heat and floods). Lightning develops as the result of an electrostatic charge build-up in a thunderstorm, which is caused by colliding ice crystals high in the storm. This charge continues to build over time, with the bottom of the storm becoming negatively charged and the ground becoming positively charged. When the electric charge becomes too great, a large bolt of electricity, a lightning bolt, travels from one charge source to the other.

DSM Lightning 8

The temperature of lightning can exceed 50,000°F, over five times hotter than the surface of the sun. This super-heats the air around the bolt, producing a shock wave that we hear as thunder. Because sound waves travel much slower than light waves, the distance from a lightning bolt can be calculated using the time that elapses from when a lightning bolt is seen to when thunder is heard. Sound waves travel approximately one mile every five seconds, so one can divide the elapsed time by five to find the distance (in miles) between them and the lightning bolt. For example, if the time between the lightning strike and thunder is 20 seconds, 20 divided by 5 would be 4 miles.

Terms to Know:
  • Cloud-to-Cloud Lightning: A lightning bolt that travels from one cloud to another.
  • Cloud-to-Air Lightning: A lightning bolt that travels from a cloud to the air (relatively uncommon)
  • Cloud-to-Ground Lightning: A lightning bolt that travels from a cloud to the ground.