Why specific storm threats and the frequency of storms change during the summer

As we approach the summer season in Missouri, you might notice a familiar trend from year to year: storms, especially particular types of severe storms, become less and less frequent as summer heat takes over. This is because, although one of the main drivers of severe weather is consistently higher during this season, the other ingredient needed to sustain storms and encourage the development of specific severe threats, such as tornadoes, has been pushed out of the region.

This feature, which provides the extra support needed for all severe modes, is the polar jet stream. During the spring season, this feature frequently moves over or near Central Missouri, and the faster winds it brings mean there is a greater change in wind speed and direction with height.

This change in wind speed and direction with height is known as shear, and it is essential to ensure that a storm's updraft and downdraft can stay separated for longer periods of time. While this separation is occurring, the storm is able to continue ingesting warm, moist, unstable air, which is the fuel it needs to keep its strength.

The sustained updrafts in these scenarios are exactly what a storm needs to produce large, damaging hail. As long as that updraft continues, the hail stone will continue to be lofted into the air until it becomes too heavy for the updraft to lift, allowing it to grow to more impressive sizes compared to if the separation between the updraft and downdraft were not there.

This shear is also important for the formation of tornadoes, as the change in wind speed and direction makes the perfect case for rotation to occur in the storm. As that rotation extends from the storm itself to the surface, the tornadic potential increases significantly.

Without these greater wind speeds and differences thanks to the polar jet stream, the rain-cooled air from the downdraft begins to overtake the unstable air the storm is trying to ingest from the updraft, and the storm ultimately falls apart. This is what typically happens in the summer.

This is because as Missouri transitions from the spring to the summer, the polar jet stream begins to migrate northward, meaning winds in the upper levels of the atmosphere are consistently weaker. This means summer storms aren't getting that same support and atmospheric setup needed to thrive, so they aren't consistently able to sustain themselves enough to cause widespread or prolonged impacts.

So, the main thing that helps us with storms is the fact that during the summer, Missouri receives its most direct incoming solar radiation. This drives up temperatures as Gulf moisture lingers, which gives us the highest values of instability during the year. Although not as frequent, lifting mechanisms that do move through the area kick this very unstable air into the upper levels of the atmosphere, allowing for brief strong storms, but the minimal shear ensures storms cannot sustain themselves for long enough to cause widespread impacts.

This trend is shown in the storm report data from 2025 in the state of Missouri. For the spring season (March-May), 406 unique wind reports, 439 hail reports, and 85 tornado reports were submitted. For the summer season (June-August), 388 unique wind reports, 23 hail reports, and 14 tornado reports were submitted.

The main things that need wind shear to consistently be a threat, that being hail and tornadoes, fell off significantly during the summer due to the lack of wind shear. Wind reports were impressive, still getting up to almost 400, and that is because the types of storms that are most frequently observed during the summer season meet a different set of criteria that commonly pose a damaging wind threat.

An example of this would be a pulse storm during the afternoon hours. With storms like these, they initially obtain a strong updraft, but due to the lack of separation from the downdraft, the heavy rain and hail begin to cool the air in the storm, which builds up in the middle of it. The air eventually becomes much more dense than the surrounding air, and so, a strong surge of air from the storm hits the ground, spreading in all directions. This is known as a downburst, and although they are very unpredictable, they commonly happen in the summer just before the storm begins to fall apart, easily reaching wind speeds of 60+ mph as the air reaches the ground.