Falling leaves and creative costumes signal the end of October — but the weather leading up to Halloween seems to tell a different tale for Chicago.
Monday highs, forecast to reach the upper 60s, will kick off a warming trend of summerlike temperatures the next few days, reaching the high 70s to low 80s on Tuesday and Wednesday. Both of those days will also be very windy. But the warmth will only last until Wednesday night, when a cold front and widespread, soaking showers will pass through.
“We’re really looking at a pretty unprecedented warm spell,” said Gino Izzi, senior meteorologist with the National Weather Service office in Chicago.
Normal average temperatures for this time of year in the area have historically remained between 48 and 49, and the normal maximum is 56 to 57.
If Tuesday’s high reaches above 78, it will break the daily record for Oct. 29 set in 1999. Wednesday temperatures are expected to be “just shy of the record” high for Oct. 30, Izzi said. But the weather service is forecasting a low of 67 early Wednesday, which would break the date’s record for warmest low temperature so late in the fall, Izzi added.
Thursday’s temps, forecast to hover in the low 60s, likely won’t come close to breaking the warmest Halloween record of 84 degrees, set on Oct. 31, 1950.
Wednesday’s rain might provide some relief from a moderate drought that has affected most of central and northern Illinois, including some parts of Cook County that are experiencing more severe impacts. The warm weather on Tuesday will elevate grass fire and wildfire dangers.
“We had that rain a few days ago, but for the most part, there are a lot of fuels on the ground, like grass and leaves and such,” Izzi said. “It does look like (Wednesday’s) could be a beneficial rain. … Each one of those will start to chip away at the deficit of rainfall that we’ve had for the past couple of months.”
A real fall treat, however, will come Thursday night.
Midweek rains should clear up by midmorning and cool temperatures to 45 to 50 degrees in the evening, just in time for trick-or-treating.
Over the previous three nights, the weather will range from the upper 40s to the high 60s. But even the cool Thursday evening temperatures will be a bit unseasonably warm as climate change warms evenings faster than it warms days.
According to climate scientists, fall temperatures have warmed by 2.5 degrees on average across the United States since 1970. In Chicago, October nights have gotten hotter by 3 degrees in the same timespan — from 42 to 45 degrees — which is consistent with an average 3.3-degree increase in nighttime temperatures across 216 U.S. cities.
On Oct. 30, 1974, Chicago broke a record warm low for that date with 62 degrees — “which seems very likely to be broken” with the low of 67 early Wednesday, Izzi said.
For context, people don’t need cooling to be comfortable indoors when outside temperatures are under 65 degrees, according to weather experts.
Higher nighttime temperatures can lead to insufficient or poor sleep, which experts say can in turn compromise the immune system, increase the risk for cardiovascular disease and diminish cognitive performance.