As the hay bale decor and gourd displays cropping up on front porches suggest, Tuesday, the autumnal equinox, marks the first day of fall: The three-month period ending in late December that in Chicago means the ushering in of declining temperatures.
But this year, long-range forecasters suggest, the season won’t be quite as cold as usual.
There is a slight indication the swath of northern Illinois that includes the Chicago metro area will feel just a little warmer than a typical year, said Matthew Rosencrans, a meteorologist with the Washington D.C.-based Climate Prediction Center. Since it’s the same conclusion as last year’s seasonal outlook, however, residents with short memories may just conclude it feels the same as 2019.
“Nobody would notice a half-degree difference on any given day but when you take it over a whole autumn, it means there’s just a handful more slightly warmer days than in an average year,” he said. “You’re not going to say, ‘Wow, it was a really warm autumn,’ but for things like utilities, energy consumption, even agriculture, it may make a slight difference.”
The normal average temperature for the season, which goes from the autumnal equinox to the winter solstice on Dec. 21 and is an average of all temperatures recorded (overnight lows to daytime highs and everything in between), is 37.6 degrees. When the climate prediction center determines a seasonal forecast, there is about a one-third probability of having what forecasters consider above normal, below normal or normal temperatures. Again this year, instead of the typical 33% probability of warmer temperatures, the center says there is a 40% chance of warmer temperatures.
That doesn’t mean every day or even a portion of each day will be warmer than usual. Rather, on the whole, the 37.6-degree average may be bumped up by about half of one degree, for an average of about 38.1 degrees, Rosencrans said.
The center is predicting “equal chances” of above normal, below normal, or normal precipitation for October, November and December, he said. Rosencrans said there was not a strong signal that would push the odds in any direction.
Anthony Artusa, another meteorologist at the climate prediction center, previously has explained that “equal chances” should not be mistaken as meaning precipitation is expected to remain on par with averages for that period.
“Equal chances means it doesn’t tilt the odds in any direction, we essentially can’t tell if any of those three categories outdoes the other. It’s an educated way of saying, ‘I don’t know,’ because we want to work off the data rather than take a guess,” he said.
Some areas can expect moderate drought conditions, Rosecrans said. The climate prediction center also last week released a map of forecast drought conditions that shows the area around Chicago shaded to represent a locale where “drought persists.”
The easiest way to explain where drought conditions are possible are locations that were not hard-hit by the summer’s heavy rains, said weather service meteorologist Brian Leatherwood.
“Overall, it’s looking to be an average fall according to the climate center,” Leatherwood said.
Looking specifically to November and December, Rosencrans said there emerges a slight tilt toward more precipitation than usual, which at that point in the year could fall as either rain or snow.
“There’s a slight tilt toward wet conditions,” he said. “As it relates to drought, snow, if it doesn’t melt until spring, won’t relieve the drought conditions immediately.”
The precipitation outlook for winter months is not yet complete, Rosencrans said, but early indications for the Chicago area suggest more snow than usual.
“Right now our precip outlook starting in December and January … does have an above normal outlook for the core winter months. We are actively researching to do a seasonal snow outlook,” he said. “Stay tuned.”
kdouglas@chicagotribune.com
Twitter @312BreakingNews
