Hail in Missouri over the past 15 years: cause, frequency, and impacts
Hail is consistently one of the costliest severe weather events, causing billions of dollars in damage annually to cars, homes, and crops in the United States. One of the places it commonly occurs is right here in Central Missouri.
Over the past 15 years, there have been over 5000 reports of hail from unique storms in Missouri. The majority of reports come from Kansas City, St. Louis, and Springfield, and this is due to the fact that, since there is a denser population, reports are more frequently sent in compared to areas where not as many people may be readily available to observe the hailstones. In this data, 2011 had the highest number of reports, nearing 800, whereas 2021 had the lowest number of reports at under 200. This is because of the differences year to year in the presence of ingredients needed for the production of severe hail.
The first important ingredient is convective available potential energy, otherwise known as CAPE. This is the amount of fuel the atmosphere has to use for storm production, so less CAPE generally means weaker storms, whereas more CAPE typically means stronger storms. The amount of CAPE present also drives the strength of the storm's updraft, which is essential for hailstones to be suspended in the storm long enough to reach a large or damaging level.
The other main ingredient needed is wind shear. Simply put, it is the change of wind speed and direction with height, and when there are increasing wind speeds with height that are changing direction, a turning or twisting of the air occurs. This circulation of air is what allows for the generation and maintenance of storms during the severe weather season.
Whenever these two ingredients, CAPE and shear, get together during the initiation of storms, the hail begins to form and get carried up and down inside the storm cloud by the updraft. These hailstones grow by colliding with supercooled water droplets in the cloud itself, and finally, when the hailstones are too heavy for the updraft to keep them up, they fall from the storm cloud to the surface.
Over the past 15 years in Missouri, the month that has had the most amount of hail reports is May, coming in at just over 1400 total reports. But, the entire spring season sees an exponential increase in reports before exponentially decreasing throughout the summer, and that is because the two ingredients that are needed for hail production, CAPE and shear, find a sweet spot to generate a prime severe weather environment for the spring season.
This sweet spot is found because CAPE and shear commonly have an inverse relationship when it comes to when they are present. Shear is commonly maximized during the winter season, decreasing as the warmer months approach. This is because of the presence of the polar jet stream near Missouri during the winter season, allowing for stronger winds in the upper levels before it moves north for the spring and summer season. CAPE is commonly maximized during the summer months because that is when Central Missouri gets its most efficient heating from the sun. As the air gets warmer and more energetic in the warmer seasons, the amount of CAPE that is readily available increases as well. So, during the spring, before shear has reached its average minimum and CAPE is becoming more present, the best conditions are met for the generation of storms and large hailstones.
Of course, hail can occur any time of the year, but for Missouri, the best ingredients and most frequent occurrence of hail events are likely to be during the spring season.
