Summer Weather in the Tennessee Valley

It's almost that time of year. We're halfway through May, and we are starting to head into the warmer months of the year. We will get our first big taste of that this weekend with highs in the mid 80s. Coming are the hot, humid, lazy days of summer in the South, and they do present their own forecast challenges.

Surprisingly, one of the hardest forecast issues we face here during the summer months is the elusive scattered pop-up afternoon thunderstorm. As we get into June and especially July and August, we will deal with these on almost a daily basis. Temperatures warm in the afternoon, often into the 90s, and with the humidity semi-permanently in place, the atmosphere grows unstable. As that happens, we reach what is called the convective temperature. That is the temperature where a parcel of air can rise freely in the atmosphere without anything holding it back. When that happens, isolated to scattered pop-up thunderstorms fire almost at random during the afternoon hours and then last into the evening until they lose enough heating of the day to lose their fuel to keep them going. By time time of the year, the stronger jet stream winds have retreated northward, and that means these thunderstorms mostly move in slow and erratic paths. The struggle for forecasters is that we can't tell you the day before or the morning of a particular day exactly who will or will not get rain that afternoon. There are still things we just don't understand about the atmosphere, and that is one of those things that is just beyond the current capabilities of the science. It's just best to keep the rain gear in your car every day just in case. If you see a thunderstorm on summer afternoons, they fortunately usually do not last too long.

With our relatively close proximity to the Gulf of Mexico, we also have to worry about the remnants of tropical systems moving through the area. The Atlantic hurricane season runs from June 1st through November 30th. Gulf coast landfalling systems that move through the area can often bring heavy rains and flooding threats, gusty winds, and even the risk of spin-up tornadoes. MOST of these tornadoes are small, quick spin-up tornadoes found in mini supercell structures in the outer rain-bands of tropical systems, but occasionally even a strong or long-tracked EF2-EF3 tornado can form from these tropical systems. Because of the small and shallow nature of these storms, they can sometimes even happen before a tornado warning can be issued.

During the summer months, we still have a severe weather threat. We shift gears from tornadoes (except for tropical related tornadoes mentioned above) to complexes of thunderstorms that produce widespread straight-line wind damage. We've already seen two such creatures earlier this month. We rarely go through a summer without several threats like this, and they can sometimes be widespread and significant.

So, even though summer weather here is known for being hot and humid and usually not too many big big changes, sometimes that is not the case. Even in the summer months, it is still important to pay attention to forecast information and be aware of weather conditions.