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Severe Weather Awareness Week 2020: Severe Thunderstorms

Today is the first day of Iowa Severe Weather Awareness Week.  Day #1 is focused on severe thunderstorms.

First, all thunderstorms have the threat of lightning. During any thunderstorm, you should be indoors and away from windows.

Learn more about the life cycle of a thunderstorm by watching the video below.

A severe thunderstorm is a little different than your typical, garden variety thunderstorm.  A severe thunderstorm is classified by having winds at least 58 mph and/or hail that is at least an inch in diameter (about the size of a quarter).

Heavy rain does not classify a storm as severe, however if a severe thunderstorm isn’t moving, the heavy rain can lead to flash flooding.

Hail forms in a thunderstorm when the updraft (rising air) lifts water particles high up in the cloud, where temperatures are below freezing.  The hailstones grow as the updraft keeps lifting the particles higher up in the cloud, and they grow.  Eventually, the hailstone becomes too heavy and begins falling through the cloud, and eventually to the ground.  Hail can occur during any season, but larger hailstorms are more common during the spring months where the freezing level is closer to the surface of the atmosphere, as opposed to the higher freezing level in the summer months.

Hail can be as small as peas, or as large as softballs or larger. In fact, the largest in the United States fell on July 23, 2019 in Vivian, South Dakota.  The hail stone was 8″ in diameter and nearly 2 pounds.

The most common type of damage from a severe thunderstorm is straight line winds.  These can sometimes be just as damaging, if not more, than tornadoes.  This is why we need to take every severe thunderstorm warning seriously.

Below is an interesting map from Iowa State University’s Iowa Environmental Mesonet.  It shows the number of Severe Thunderstorm Warnings issued National Weather Service offices across the state during 2019.

Another interesting map shows the number of Severe Thunderstorm Warnings issued by each National Weather Service office in 2019.  Des Moines, Davenport and La Crosse, WI cover the KWWL viewing area.

Of course, severe thunderstorms can and sometimes do produce tornadoes.  That will be our topic Wednesday.

The Storm Track 7 weather team will be here all severe weather season to bring you updates on impending severe weather. Click here for more information on severe thunderstorms.

Here are the topics we will be looking at this week:

Tuesday: Receiving Warning information

Wednesday: Tornadoes

Thursday: Family Preparedness

Friday: Flash Floods

Eileen Loan


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