Recent reports of flooding rains in Tennessee and the torrential downpours from Hurricane Ida, and its remnants, brings the topic of flash flooding to IQ Weather.
September is the midpoint of hurricane season in the northern hemisphere.
Of course, hurricanes and tropical storms are very heavy rain producers, and there is always a threat for flash flooding with them. That is because they form over warm ocean water and, as a result, contain massive amounts of water vapor that can turn back into rain. Plus, tropical storm systems move rather slowly, so it takes a while for them to move over any one area. The slow movement increases the potential for rainfall dramatically.
It’s not uncommon for a tropical storm or hurricane to produce 20 to 30 inches of rain over any one location. In 2001, Hurricane Allison produced up to 40 inches of rain!
Water is a non-compressible fluid. It can produce enormous pressure on anything in its path. For example, water moving at 25 mph has the pressure equivalent of wind blowing at 790 mph, faster than the speed of sound! You can find more examples of the power of moving flood water here.
Since most coastal locations do not have good drainage systems, that much rain has nowhere to go. It does not take long for streams to swell and come out of their banks. If the soil is already saturated from previous rains, the rapid rise in water is even quicker. Walking or driving through rising water is extremely dangerous, and should be avoided!
One of the reasons that you generally see more flash flooding situations during the warm season is that warm air has the ability to hold a lot more water vapor than cold air. In fact, air’s ability to hold water, doubles every 20 degrees. For example, 70° air can hold twice as much water vapor as 50° air. And remember, tropical storms and hurricanes are warm core storms…the air temperature gets warmer as you get closer to the center of the storm! They contain a lot of saturated air…and when those storms move over land and start to dissipate, they release all of that water in the form of torrential rains.
Similarly, summertime storms across the country can also produce heavy rains due to their moisture content. In some situations, numerous storms can track over the same location, causing flash flooding potential to rise. This is a phenomenon called “training”. Think of how the railroad cars on a track travel over the same location. If each of those cars were a thunderstorm, you can imagine how much rain they would deposit.
“Training” storms are most frequent in the warmer months than cold months due to a complex set of reasons. In colder months, storm systems tend to move quicker as the jet stream pushes them along at higher velocities…therefore limiting the threat for flash flooding.
Flash flooding causes more weather-related deaths than any other type of situation. They may not be as interesting as tornadoes or hurricanes, but they are very dangerous, and can often catch you off-guard!