Temperatures could drop to upper 30s this weekend in northern Illinois

It’s going to get cold Saturday and Sunday night, with northern Illinois temperatures expected to dip well into the 40s — and perhaps into the upper 30s.

Going west to east, both Carroll and Whiteside counties in the northwestern part of Illinois are expecting some of the lowest overnight temps over the weekend, according to the National Weather Service.

What about your garden? How will it fare when the mercury dips?

“Pick what vegetables you can,” says Judy Holesinger, one of the Master Gardeners with the University of Illinois Extension in Whiteside County. “Since weather is cooler, there isn’t much ripening.”

When it comes to flowers, Holesinger says they can be covered by a light fabric in cooler nights.

“Or cut them and enjoy them while you can,” she said. “Not a lot you can do when Nature comes around and says, ‘It’s time to go.’”

The NWS seven-day forecast does show a gradual drop in overnight temps in the coming days. It will be in the 60s on Wednesday, into the 50s by Friday and then solidly into the 40s by Monday morning.

The Chicago area may see a low reach down to 41 and chance of precipitation east of I-55 starting Friday night.

Extended forecasts show DeKalb and Crystal Lake with lows of 41 on Saturday night. Joliet and La Salle have milder forecasts, with temps on Saturday and Sunday closer to 44.

The NWS released its fall freeze historical data, and freezes can hit anytime in September to November. Northwest Illinois has had a freeze as early as Sept. 30, 1991. Twice summer extended until Nov. 12 before the first freeze. Those late freezes came in 1946 and 1882.

But records from 1980 to 2010 show that the first light freezes or the first hard freeze will usually take place in the first three weeks of October.

The NWS posts frost advisories, freeze warnings and watches and hard freeze warnings and watches through Oct. 20. Here is what they mean:

Frost advisory: Temps are less than or equal to 36 degrees. Frost coverage is patchy. The service advises that plants be covered before the sun sets so it can help retain heat.

Freeze warning/watch: Temps in 29-32 degree range. There is little that can be done to protect plants outside. Move sensitive plants inside.

Hard freeze warning/watch: Temps are 28 degrees or colder. Hard freeze warnings might be issued days in advance. This freeze will kill plants unless they are moved inside.