Photo Courtesy of NSSL
A radar image of a bow echo associated with a severe straight-line wind storm over central Iowa in July 2011. The strongest winds were located at the tip of the bow, near the towns of Garwin and Toledo at the time of this image.
Heavily damaged grain elevators after a straight-line wind storm.
Photo Courtesy of Kip Ladage
Hailstones compared to a golf ball and baseball (on the left side of the image).
Photo Courtesy of NSSL
Photo Courtesy of Vortex 2

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Severe Weather: Introduction

Before it Strikes . . .  
Introduction
Planning Ahead
Understanding the Forecast
Receiving the Warning

Staying Safe . . .  
Tornadoes
Lightning
Hail and Straight Line Winds

 


Iowans are quite familiar with the severe weather that can rake across the state in the spring and summer months. Located at the northern end of Tornado Alley, Iowa typically sees about 50 tornadoes a year, along with frequent severe storms that can unleash dangerous hail and straight-line winds, and hundreds of additional thunderstorms that can produce deadly lightning. However, the chances that a tornado will strike your exact location are very low, and most people will only experience a few severe thunderstorms in a given year. Therefore, it is crucial that you are ready each year for severe weather and have a safety plan in place. Hopefully, you will never need to implement your safety plan, but having one can mean the difference between life and death in a dangerous weather event.

The remainder of this page contains a quick introduction to the various types of severe weather one can expect in Iowa, while the links at the top will help you develop a safety plan, become familiar with weather forecasts and warnings, and provide tips on staying safe when severe weather threatens.


An Introduction to Iowa Severe Weather

Tornadoes

TornadoA tornado is defined as a violently rotating column of air generated from a thunderstorm in contact with the ground. Winds in a tornado can range anywhere from 50 to 200+ mph. Tornadoes take on a variety of shapes, sometimes resembling a thin rope that is only a few feet wide, other times appearing as a monstrous mile-wide wedge that fills the horizon. Rain sometimes surrounds a tornado and makes it impossible to see until it is too late. In addition, tornadoes may last only a minute or so, while others may stay on the ground for over an hour.

Iowa has experienced tornadoes during every month of the year, but they are most frequent from March to July, with the climatological peak in activity occurring in June. This does not mean that violent and deadly tornadoes are confined to this five month window. Deadly tornado outbreaks have occurred in Iowa as early as late January and as late as mid November. Tornadoes tend to touch down during the late afternoon and early evening hours, but overnight and morning tornadoes are also possible.

Terms to Know:

  • Wall Cloud: A lowering in the storm clouds that often precedes a tornado. Many times, this cloud is rotating.
  • Funnel Cloud: A funnel-shaped cloud that sometimes descends from a wall cloud. If this cloud comes in contact with the ground, it becomes a tornado.
  • Enhanced Fujita (EF) Scale: The official scale used to rank tornado strength. This scale goes from EF-0 to EF-5 and is based on the amount and type of damage caused by a tornado. From this damage, an estimated wind speed is determined.

Additional Information on Tornadoes:

NWS Storm Prediction Center
National Severe Storms Laboratory
Tornadoes (pdf)


Straight-Line Winds

Straight-Line WindsStraight-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.

Bow EchoEven 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.

Additional Information on Straight-Line Winds:

National Severe Storms Laboratory
NWS Storm Prediction Center (Focuses on Derechos)
Severe Thunderstorms (pdf)


Hail

HailHail 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.

Additional Information on Hail:

National Severe Storms Laboratory
Severe Thunderstorms (pdf)


Lightning

LightningWhile 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.

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.
  • Heat Lightning: Lightning that can be seen, but is too far away to be heard.

Additional Information on Lightning:

National Severe Storms Laboratory
Severe Thunderstorms (pdf)


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