Much like last year, 2019 featured a wet summer that saved its greatest heat for late summer and early fall.
Although not quite as hot as last year’s all-time hottest September, we still finished the month over five degrees above normal. If it hadn't been for the brief cool snap in the middle of the month we would have likely surpassed last years record.
It's strange how patterns repeat themselves like that sometimes. I know there is a meteorological explanation for it if you dig deep enough, but predicting it is quite another thing. Meteorologist Joe Bastardi of WeatherBell, however, nailed it. He called for "never-ending summer" for the eastern U.S. back in late August, although the heat was tempered in the northeastern states.
One major pattern that locked in our heat last month was the train of tropical systems that moved across the Atlantic from Africa. This tends to keep all the troughs and ridges in the upper levels locked in place until the storms stop coming across. At least eight tropical storm systems came across the Atlantic basin during September. Once this pattern breaks, you can look for changes to happen fairly quickly.
Last year, the heat finally broke during the second week of October, while this year it was the first week. We also had our fifth driest September on record, which combined with the heat to put us in slight to moderate drought. This drought exists despite us being over eight inches above normal for the year. So what does the hot and dry September portend for the rest of fall and winter? It certainly looks right now that October will finish warmer than average, but I do expect a cool-down, maybe a big one, for November. Autumn rainfall may continue to be a little on the short side, but not nearly as dry as September.
While it is still a little early to develop a final winter outlook, there are several important signals that we can point to that will point us in a direction. The water temperatures in the equatorial Pacific are the most important driver of our winter (and sometimes summer) weather here. We know that warm waters (El Nino) generally bring a little cooler and wetter winters, while cooler water (La Nina) brings generally drier and warmer winters.
But what happens when the water temperatures are in the "normal" range? Simply put, the outlook becomes much more sketchy. Other factors may then either work in concert with each other or one of them may become the dominant feature. The North Atlantic Oscillation (NAO) is another player in our winter weather that can become a dominant feature but is much harder to predict beyond a couple of weeks. Also known as the "Greenland Block," a negative NAO features high pressure over or near Greenland that forces arctic air further south than normal, especially into the eastern states. A positive NAO usually features a greater chance of milder temperatures in our area. We haven't had a winter with a dominant negative NAO since 2012-13, which for us brought a mild December and January and a cold February and March. So the "correlation" doesn't always work.
Ocean temperatures in both the northern Pacific and Atlantic can also influence where high and/or low pressure tends to set up. For instance, abnormally warm water in the Pacific south of Alaska tends to favor an area of high pressure that can force the jet stream up into the Arctic that can then dive southward into the U.S. bringing cold air.
So this year we have a large blob of warmer than normal water in the north Pacific, a neutral signal in the equatorial Pacific and low (vs. the norm) sea ice in the Arctic. If Siberian snowfall increases through the rest of October as fast as it has started, then all those factors taken together point to at least an average if not below average winter in temperature. I have seen several winter forecasts for en extreme winter for the South. It's possible, but likely too much sensationalism. However, there may be a grain of truth to them as most all of them have a general winter pattern of a ridge in the west and trough over the eastern states. While I like that general idea, I have a feeling it won't be quite so simple. More next month.
Weather averages for September 2019: Avg. low: 65. Avg. high: 89. Lowest: 54. Highest: 96. Mean: 77.1 (+5.1). Rainfall: 0.94" (-3.79"). 2019 total to September 30: 46.58" (+8.67").
Mark Jenkins is Madison County’s cooperative weather observer.