 Spring is the season when severe weather kicks up again. In some areas, like the Central United States, it also means that tornado season will be starting up soon. And while this fact is common knowledge, the science behind why this season experiences such a noticeable uptick in severe weather events isn’t discussed as frequently. In this article, we will be exploring the key ingredients required for severe thunderstorms like supercells to form and how they come together at much greater rates during the spring months than at other times of the year in the mid-latitudes.  Ominous skies over eastern Colorado in May 2018. The key ingredients for supercell thunderstorms are: instability, a source of lift, moisture, and wind shear. Instability refers to the general alterations of atmospheric stability. For instance, if you were to draw out a parcel of air, said parcel would rise, fall, or remain in its current location depending on how stable (or unstable) its surrounding environment was.  Source: Columbia University The atmosphere can become more unstable via surface heating; whenever sunlight warms the ground, certain parcels at and near the surface will warm at a faster rate than their surrounding ones. It is these lighter, warmer parcels that will rise and will continue to do so until they enter a cooler environment. They themselves will then begin to cool and become heavier until they eventually sink back down to the surface where they can be heated up again and repeat this convective process. Source: Columbia University In order for storms to initiate, there must be a source of lift that allows for parcels of air to be vertically driven into the atmosphere. As such, the solar radiative heating that was recently described is one of many sources of lift that are available to prospective storms. If there is both sufficient heating and enough moisture over a localized area, then we can further expect for these parcels to vertically carry that water content along with them, leading to the vertical mixing of both heat and moisture across several layers of the atmosphere. These parcels will eventually condense and produce isolated showers and thunderstorms over those locations. Once they die, their outflow can help to produced localized areas of lift that can allow for a few more of these sorts of thunderstorms (oftentimes called garden-variety thunderstorms) to develop. These sorts of environments can be routinely found in tropical and subtropical areas such as Florida or the Caribbean, especially during the summer months. This makes spring unique for areas in the mid-latitudes in that it is around that season that plants and other forms of vegetation begin to grow and mature. In states such as Illinois and Indiana, which host large amounts of agricultural production, the added vegetation provides moist soil and added surface moisture content.  Source: Thomson Higher Education That being said, these types of thunderstorms are almost never severe; once the environment runs out of surface moisture to mix into the atmosphere by the mid-to-late afternoon hours, then any storms that did develop will soon die out. In fact, these sorts of thunderstorms tend to have life expectancies of just under an hour. In order for storms to maintain themselves, they require another ingredient that has not yet been discussed. We have already discussed one primary lifting mechanism: solar radiation. This process, however, only accounts for isolated thunderstorms to develop, given that such thunderstorms are limited to pockets where the air has become sufficiently unstable enough for parcels to rise and condense. If we take a look at North America, the return of spring re-introduces warmer air masses to the rest of the continent, while still experiencing cold spells whenever arctic air masses sink down into the lowe-48. The fronts that divide the two air masses themselves become large-scale lifting mechanisms that can also trigger thunderstorms. Moreover, geography also can play a significant role in providing sources of lift, as they are natural barriers that an air mass must overcome by rising on the wind side of the mountain before descending again on the leeward side. If the ascending air mass has sufficient moisture content and there is enough radiational heating, then the end-result can be oragraphically-induced thunderstorms.  Source: Thompson Higher Education Starting around the start of spring, low-level moisture tends to also be advected (transported) from the Gulf of Mexico and into the Great Plains of North America. This introduction of moisture into the environment allows for any prospective storms to have a greater source of moisture than they otherwise would, aiding in the production of very sharp contrasts between moist and dry air masses. It is these sorts of boundaries between the two air masses that lead to the development of a dry line, which commonly develops in the Great Plains when this sort of moisture advection occurs. This area of the world also harbors a unique topographical setting which further allows for the production of dry lines; the elevation of the Great Plains gradually increases from east to west given they cover an area that’s stretches from the Mississippi River Basin all the way to the front-range of the Rocky Mountains. As such, when Gulf Moisture moves across the Great Plains, it slows down and fills in the areas which host lower elevations. As it does so, this air mass then comes in contact with much drier air from the American Southwest, which descends into the Great Plains from areas of higher elevation. Given that this drier, denser air mass, is descending into a moister, lighter air mass, the end result is a source of lift that can enable for much larger areas of ascent that can cover large swaths of the Plains on a much greater scale than what is observed from isolated, tropical and subtropical thunderstorms.  Source: Kendall/Hunt Publishing We have already discussed three of the key ingredients that are required for severe thunderstorms: a source of lift, instability, and moisture. And as one would imagine, some of the most severe thunderstorms on the planet are a result of strong sources of lift, such dry lines. Nevertheless, the final ingredient required for supercells to initiate given that without it such a thunderstorm would not be able to maintain itself: wind shear.  Source: Earth Sky Wind shear is essentially the change in wind speed and/or direction with respect to height. In the case of isolated thunderstorms that occur in the tropics, this ingredient is nearly non-existent given that the air masses of such environments tend to be stable with the exception of the localized areas of enhanced heating where the storms were able to initiate from. As such, once the thunderhead reaches its highest level in the atmosphere, all of the parcels that have cooled and condensed will sink back down to the surface. This downdraft will then cut off any supply of warm, moist air that initiated the thunderstorm, and produce an outflow boundary that may help to produce a new one in its wake. Under conditions in which there is greater wind shear, however, such a thunderstorm would have a downdraft that sinks further away from the core of the storm. As such, these thunderstorms require sources of lift that can produce sufficient enough wind shear given that this difference in the location between a thunderstorm’s updraft and downdraft will then in turn allow for a thunderstorm to maintain itself for a much longer period of time, strengthen, and become a much more severe thunderstorm that can potentially produce hail, damaging winds, and of course tornadoes.  Late-afternoon supercell over Nebraska Given this requirement, supercells are most likely to be produced over areas with strong, well-defined air mass boundaries, such as surface fronts or dry lines. As such, the mid-latitudes, which include areas like the Great Plains of North America, are prime locations for such thunderstorms to develop in the spring. This season is when these boundaries begin to develop at greater rates across the continent while the reintroductions of vegetation, along with increased radiative heating, ensure that the environment is much more prime for supercells and the severe weather hazards that come along with them. To learn more about all things climate, please click here! ©2019 Meteorologist Gerardo Diaz Jr.
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