ST. LOUIS, Mo. – The approach to forecasting the fall months is the same as looking at the spring months. We are transitioning. Transitions in the atmosphere can always be rough, that’s why the Fall is called a “second severe weather season.”
September is actually never really a Fall month. During most years the first two weeks are always warm, much more like the summer heat and humidity. There will still be some 90’s early in September. That is good news for businesses who are dealing with Covid-19 and have to keep customers outside. More warm air for more outside time. But with that, we need to watch for storms. We are working into a weak La Nina pattern, so the later parts of September into October could develop rather stormy. The monthly temps will average above normal. The fall or vernal equinox is Tuesday, September 22 at 8:30 am.
We’ve had a very active tropical season so far, and that will extend into October. All tropical storm systems affect us in the Midwest, sometimes directly and sometimes indirectly. Since the La Nina signature for the Midwest is all about fast changes, look for a dramatic drop in temps during the month. There is an old weather saying that, “The first frost is 90 days after the cicadas start singing in the summer.” I heard cicadas in mid-July, so lets go with the signs in nature and say that the first threat of frost is mid-October. The fall leaf show has been getting later and later with each year. This year, lets look for late October to early November for the Fall colors. Monthly temperatures, like September, will average above normal. Some rain, but not more than normal.
We have had some wild Novembers over the last couple of years. Snow showed up early two years ago, and last year too. No reason to think that will be different this year especially with the weak La Nina pattern setting up. But, November could also swing the other way too. As the pattern sets up, the jet stream will be amplified more than normal, so wild temperature swings and strong storms will be the rule from late October through November. The overall monthly effect will be the average of extremes. And temps will average out near normal, with above-normal precipitation.