Record-breaking warmth giving way to rapid cool down

It didn't take long for temperatures to break a 48-year-old Chicago weather record Thursday with a mark of 57 degrees.

Once the clock struck midnight, it already was two degrees warmer than it had ever been before on a Dec. 26 in Chicago, according to an alert from the National Climate Data Center.

The thermometer at O'Hare International Airport dipped into the low 50s during the early morning hours, but rose again later during the mostly sunny day where temperatures ultimately topped out in the lower 60s, according to the National Weather Service.

Joggers in nature preserves spotted mosquitoes, while other insects also re-emerged during the warm spell.

Entomologists at Chicago's famed Field Museum said a small number of insects appearing in late December doesn't portend anything dire for the insect population during the more traditionally warmer months.

“Insects are very diverse,” said Maureen Turcatel, collections manager of insects at the museum. “I don't think temporary mild temperatures will affect these insects, especially pests. Whatever was active these last couple of days is only a fraction of the local insect population, so I wouldn't be too concerned about it.”

The anomalous warm trend was due to a stalled jet stream that pushed warm air from the southwest and kept it in place for several days, National Weather Service meteorologist Lee Carlaw said.

But a return to normal was on the warm front's heals. A cold front was due to arrive overnight in the suburbs and was expected drop temperatures into the 30s, Carlaw said.

Late Thursday morning, that air mass had settled over Dubuque, Iowa, just west of Illinois where the temperatures were below freezing and the area experienced “freezing fog” while the Chicago area was still enjoying the last heat wave of 2019. That cold front will affect the West and Northwest suburbs the most, Carlaw said.

Highs Friday should be around 40, with showers moving into the area for the weekend.