Severe Weather Awareness Week: Severe Thunderstorms

What It Takes To Go From A Garden-Variety Storm To A Severe Storm

Severe weather season is upon us and this week is Severe Weather Awareness Week across South Dakota. Each day this week we will tackle a different severe weather topic to help you understand individual weather phenomenon.  Yesterday, Brandon broke down the difference between a watch and a warning. Today’s topic is severe thunderstorms! This topic transitions very well from watches and warnings because both severe thunderstorm watches and warnings can be issued.

Example of a Severe Thunderstorm Watch

Let’s start with the basics of a severe thunderstorm and what is needed to take a thunderstorm from garden-variety to severe. A thunderstorm, severe or not, needs a couple of basic ingredients in order to form. We covered them on this blog post but in case you forgot, we’ll go over them again (they’re very easy to remember… we’ve even come up with an acronym for it).  The basic ingredients that any thunderstorm needs to form are shear, lift, instabilty and moisture or S.L.I.M.

Some of these ingredients are found together, like moisture and instability (energy or food for storms) while ingredients like shear and lift mostly stand alone. Moisture can be brought up from the Gulf of Mexico, which also brings up instability, or it can move eastward over the Rockies and sources of lift include topography to fronts and dry lines (which aren’t fronts by definition but sometimes called a dew point front).

A thunderstorm forming Sioux Falls, photographed by Brent McCown

So let’s assume that we have all of these ingredients and we now have a thunderstorm moving through the Sioux Empire. Usually, thunderstorms will not start off severe, they have to work to get to that point. In order for a storm to be considered “severe” the thunderstorm must be capable of producing at least 1″ in diameter hail (the size of a quarter) and/or wind gusts equal to or greater than 58 miles per hour. The and/or is important because a severe thunderstorm can have strong winds and no hail and still be severe or have a large hail and no winds (most of the time, though, strong winds and hail are found together). In addition to producing strong winds and hail, severe thunderstorms can also produce tornadoes, heavy rains as well as frequent, dangerous, lightning.

Example of Severe Threats Forecasted During Severe Weather

While you may have woken up to thunderstorms a time or two more often than not thunderstorms, severe or not, occur during the late afternoon through the evening. Warning Coordination Meteorologist for the National Weather Service Todd Heitkamp explained that late afternoon isn’t the only time that meteorologists see severe thunderstorms form, he explained that after midnight is another time that meteorologists usually see severe weather. That’s because, especially on sunny days (which are more favorable for thunderstorm development), it takes a long time for energy to build up and is usually the strongest in the late afternoon around the hottest point of the day.

Due To Clear Skies, Thunderstorms Were Expected To Form in Blue Circle

Severe Storms Moved Through Forecasted Area Hours Later

In addition, if severe weather is forecasted to impact an area, the Storm Prediction Center does daily Convective Outlooks leading up to the day of, with their forecast beginning 8 days out.

Example Of Convective Outlook

For the KDLT forecast area, our season for severe thunderstorms ranges from the middle of March through July, sometimes even into August. The reason for that is because, going back to the ingredients needed, the instability needed takes a couple of months to move this far north thanks to the retreating jet stream. Another interesting thing about severe thunderstorms in the Sioux Empire is that they often occur more frequently than tornadoes. According to Todd Heitkamp, Warning Coordination Meteorologist at the National Weather Service, straight-line wind events, associated with squall lines, not only occur more frequently but can sometimes bring wind gusts upwards of 100 miles per hour. One such event occurred in June of 2015 when 100 miles per hour straight-line winds moved through Garretson, South Dakota, knocking down trees, power lines as well as did damage to homes.

We still have more topics to go over like lightning, tornadoes as well as floods. Make sure you’re tuning into KDLT News during Severe Weather Awareness Week for special stories about these topics and make sure you’ve followed/liked KDLT Weather on Facebook and Twitter for the latest weather information. In addition to KDLT Weather, you can also follow the individual meteorologists on Twitter or like their pages on Facebook as well to receive weather updates.

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