Image Courtesy: Alan Cressler

Why is the U.S. a Hotbed for Severe Storms?

By Paul Trambley @yourmetpaul April 12, 2018 8:18 am CDT

With the onset of the spring season, thoughts of warmer days and budding tulips often come to mind. Unfortunately, less peaceful images also come to the forefront, as spring marks the beginning of another severe weather season across the U.S. Residents across the Plains know this more than any others across the globe. The U.S. sees more tornadoes than any other area in the world annually, with many of them occurring in the central and southern Plains states. On average, 1200 tornadoes occur in the U.S. each year. Texas sees the highest number on average, with 155 expected. Kansas takes second place, with an average of 96. In 2016, the SPC (storm prediction center) counted a total of 1059 tornadoes across the U.S.

The peak season for tornadoes arrives during the months of May and June, but many outbreaks can be expected outside of this time frame. The general start of the season is during the month of March, with activity climbing steadily during the month of April.


Why Does the U.S. See So Many Tornadoes Each Year?

The U.S. is the incontestable leader in all severe weather statistics, including tornadoes. The country with the second most tornadoes per year is Canada. But, Canada takes a very distant second place, with an average of only 100 tornadoes per year. This is an astounding 1100 less than what the U.S. can expect.

In order to understand why the U.S. is the epicenter of tornadic activity, we must look at the favorable geography that resides in the Plains region of the U.S. First, we focus our attention to the south, to the Gulf of Mexico. This body of water plays an integral part in setting the stage for any type of severe weather event. It is because of this large basin of 80 degree water, that moist, tropical air masses can be transported northward across the nation's mid-section, given the right pattern. Looking to the north, cool and dry air masses are commonplace over central Canada. These types of air masses are therefore readily available during the spring and summer seasons. Lastly, we focus our attention to the west, where the Rocky Mountains lie. The Rocky Mountains act as a sponge for any moist air masses that come in off the Pacific Ocean. As Pacific-based systems head east across the Rockies, any moisture that hasn't already been squeezed out, is done so here before the systems enter the Plains. Because of this, the mid to upper levels of the atmosphere tend to be very dry across the Plains. To recap, humid, Gulf air is readily available to the south, bone-dry air masses routinely come off of the Rocky Mountains, and cool and dry air masses are easily tapped into over central Canada.

It is the juxtaposition of these opposing air masses that provides the energy needed for strong-to-severe storm development.  However, the presence of atmospheric energy alone will not produce thunderstorms. A well-defined front, dry line, or low pressure system (or any combination of them) is also needed in order for this to happen. When low pressure systems emerge in the western Plains states, southerly winds develop ahead of them, drawing warm and humid air masses northward from the Gulf of Mexico.  At the same time, these systems tap into the much cooler air masses found in Canada, that eventually build in on the north and northwest sides of the low. Lastly, dry air off the Rockies is transported eastward. The dividing lines between these varied air masses are what meteorologist's label as cold fronts, warm fronts, and dry lines. These boundaries serve as catalysts for storm development. Given the right combination of ingredients along one of these boundaries, strong thunderstorms will develop, along with the potential for tornadic activity.

The image below shows where tornadoes have touched down between the years of 1950 and 2015. Notice that every state has had at least a few tornadoes, but there is a clear focus for tornadic activity over the central and southern Plains states.

Image Courtesy: SPC (Storm Prediction Center)

The position of the jet stream dictates where the strongest fronts and low pressure systems will track. During early to mid-spring, the jet stream has a tendency to be the strongest over the central and southern Plains states. This is why severe weather is more common in the southern Plains during the months of April and May. By the mid to late summer months, the main belt of the jet stream typically shifts to the north, thereby shifting the heightened risk for severe weather to the northern Plains region. The image below indicates the probability of severe weather (tornadoes, winds over 58mph, or hail greater than 1 inch diameter) for May 20th (based on data from 1982-2011). During this time, the highest likelihood for severe weather resides in a bullseye over central Oklahoma.

The next graphic plots the same data, but for June 24th. Notice how the heightened risk for severe weather has drifted northward by early summer. This change is driven by the expected northward migration of the jet stream by this point in the year.

The spring and summer seasons are an ideal time to enjoy the great outdoors. Due to the added volatility in the atmosphere, it is also an important time to have a solid plan of what to do when severe weather strikes. Make a habit out of checking your local forecast from Weatherology or on the Weatherology app for iPhone or Android users. You can also get a solid idea of where the greatest risk of severe weather lies each day by checking the severe storms forecast. This plot highlights where the greatest risk for severe storms is for the current day, or the next few (by clicking on Day 2 or Day 3). It is also a great idea to purchase a battery-powered NOAA Weather radio, so you will always be alerted with storm warning information no matter what time of day it hits. By being prepared and keeping an eye on the forecast each day, you can keep you and your family safe from the dangers that each severe weather season brings. Have a safe and happy spring/summer season!


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