Air mass thunderstorms: How they develop and why
Running into a pop-up thunderstorm that seemingly comes out of nowhere and rapidly falls apart after passing is not uncommon during the hot summer months. Over the past couple of days, these types of thunderstorms have been present in Central Missouri.
Although they haven't been severe, they can still impact your afternoon.
These are known as air mass thunderstorms and are typically seen in the hot and humid summer months. What makes these different from regular thunderstorms is that these do not usually form due to a front or an interaction with some lifting mechanism. Typical thunderstorms will form because the front/lifting mechanism will enter an area where unstable air (or air that is warm and contains moisture) and proceed to lift that air into the upper levels of the atmosphere. As it rises into the upper levels, the air begins to expand, and this expansion causes the thunderstorms to continue growing.
Air mass thunderstorms mostly form due to all the incoming solar radiation from the sun heating up the surface. This heating leads to more instability if paired with moisture, and the warmer air becomes less dense. The less dense air is more susceptible to rising, and as this rising motion happens, brief thunderstorms begin to form.
The reason these storms are typically brief is that these atmospheric setups, paired with air mass thunderstorms, have little shear, or winds changing in speed/direction with height. This shear is what allows regular thunderstorms to have a longer life cycle. Thunderstorms need those changing winds in order to keep a separate updraft and downdraft. The updraft is how the thunderstorm ingests the unstable air at the surface, and the downdraft is the storm's way of exhaling/releasing the energy it has built up.
When these regions aren't able to keep separation, like in an air mass thunderstorm setup, the updraft begins to take in the more stable air that the downdraft is letting out. This keeps the thunderstorm from taking in the energy it needs, and this is what causes the storm to fall apart. The lift from the heating is enough to get the thunderstorm going, but it can't sustain the growth needed to continue producing activity.
The lack of shear is also why air mass thunderstorms typically do not become severe. The shear allows storms to get taller and gain more strength as they take in more energy from the surface. Air mass thunderstorms do not typically get tall enough to produce any significant severe weather. They can, however, briefly cause downbursts, heavy downpours, and in rare cases, hailstones close to an inch in diameter.
This is like the weather setup that is in place for central Missouri on Monday. Temperatures were in the low 90s this afternoon, dew points were in the upper 60s, and wind shear was on the low end. This is part of the reason why isolated storm activity picked up during the peak heating hours of the day this afternoon.
