2020 has been a wet year, so far! The Louisville climate site is almost 10" above normal for precipitation through September 15th. A big contributor to the high rainfall totals has been an abundance of slow moving thunderstorms developing in abnormally moist environments. It's pretty clear the slow moving storms producing heavy rainfall have caused precipitation totals to add up this summer.
However, The National Weather Service in Louisville found one interesting byproduct of the wet summer has been a "hail drought" across southern Indiana and central Kentucky.
So far in 2020, the NWS Louisville office has seen the fewest amount of 1" or greater hail reports since 2009. Also, we have seen about half the usual amount of hail days/events than a more typical year. Below is a graph showing the amount of 1" or greater hail reports from 2010 to present in 2020. In addition, you can see the amount of hail days/events each year. Note that 2020 does seem to be experiencing a bit of a hail drought so far in 2020, but why?
The Science Behind the Change
The first reason is that in many cases, a moist or near tropical air mass isn't quite as unstable as a typical thunderstorm air mass. In other words, thunderstorm updrafts aren't as strong, and the weaker updrafts can't hold hailstones up long enough to grow. Another reason is that moist air masses are typically warmer in the mid levels of the atmosphere than drier air masses. Therefore, a hailstone is more likely to melt faster and grow slower in a more moist thunderstorm air mass. Some of your best large hail days have very cold and dry air in the mid levels! Overall, a hail stone will melt the fastest if it is small and in a rain shaft. In contrast, a large hail stone that is falling outside of a heavy rain shaft will melt much slower.