Living in the Metro East means you kinda just accept that the sky has a personality of its own. If you’re looking up the weather for O’Fallon Illinois right now, you’re likely seeing the classic mid-January shuffle. It’s 45°F outside today, Friday, January 16, 2026. Honestly, that feels almost decent for this time of year, but the 11 mph wind coming off the west is doing that thing where it bites right through a light jacket.
The "feels like" is sitting at 39°F.
It’s cloudy. It’s grey. It’s very... Illinois.
But if you’ve lived here long enough, you know this 40-degree air is a total tease. By tonight, we’re dropping down to 24°F. Tomorrow? Don't even get me started. We’re looking at a high of 27°F on Saturday with wind gusts that’ll make you regret leaving the house.
The Wild Swings of O'Fallon's Climate
Basically, O'Fallon sits in this weird geographical sweet spot—or sour spot, depending on how much you hate humidity—where Arctic air from the north and Gulf moisture from the south constantly duke it out. We call it "humid subtropical," but that sounds way more tropical than it actually feels when you're scraping ice off your windshield at Scott Air Force Base.
The numbers tell a story of extremes.
👉 See also: Fitness Models Over 50: Why the Industry is Finally Paying Attention
- January: The absolute coldest month. Lows average 25°F.
- July: The heat dome. Highs average 90°F with humidity that feels like a wet blanket.
- May: The wettest month, averaging nearly 5 inches of rain.
Right now, we are smack in the middle of the "cold season," which usually runs from late November through February. If you're looking at the weather for O’Fallon Illinois for the upcoming week, keep your snow boots handy. While today only has a 20% chance of a stray flake, next Saturday and Sunday (January 24-25) are showing much higher potential for actual snow showers and light snow. Temperatures are going to bottom out around 12°F.
Severe Weather is the Real Conversation Starter
You can't talk about O'Fallon weather without mentioning the "T" word. Tornadoes.
Just last year, on May 16, 2025, a massive EF3 tornado tore through the St. Louis metro area. It stayed on the ground for 27 minutes. While O'Fallon itself dodged the worst of that specific cell, O'Fallon Park in St. Louis saw brick townhouses literally collapse. It’s a sobering reminder that our "moderate risk" days aren't just background noise on the news.
Most people around here have a weather radio. If you don't, you're doing it wrong. The National Weather Service office in St. Louis (LSX) is basically our North Star during the spring.
Why Humidity is the Silent Killer
In the summer, the dew point is everything. Once that number crosses 70°F, O'Fallon becomes a sauna.
✨ Don't miss: Finding the Right Look: What People Get Wrong About Red Carpet Boutique Formal Wear
July 23 is historically the muggiest day of the year. During those weeks, the air is so thick you can practically chew it. It’s why our local HVAC companies are the busiest people in town from June to September. If your AC goes out in August, you aren't just uncomfortable; you're in a legitimate health crisis.
What to Expect for the Rest of January 2026
If you're planning your week, here's the lowdown on the weather for O’Fallon Illinois according to the latest station data from Scott AFB:
Monday, Jan 19, is going to be the "bright" spot—literally. It'll be sunny, but brutally cold with a high of only 24°F and a low of 12°F. It’s that deceptive winter sun that looks warm through a window but freezes your nose hairs the second you step out.
By Wednesday, Jan 21, we swing back up to 50°F.
That’s a 26-degree jump in 48 hours.
🔗 Read more: Finding the Perfect Color Door for Yellow House Styles That Actually Work
This is exactly why O'Fallon residents keep a parka and a light hoodie in the backseat of their car at all times. You never know which version of Illinois you're going to get after lunch.
Surviving the O'Fallon Elements
Most experts suggest a "layered" approach to life here. Not just clothes—life.
- Check the Dew Point: In summer, ignore the temperature; the dew point tells you if you'll actually be able to breathe outside.
- The West Wind: Our weather usually comes from the west/northwest in the winter. If the wind is kicking up at 15+ mph like it’s forecast to do this coming Friday, Jan 23, expect the "feels like" temp to be 10 degrees lower than the thermometer says.
- Frost Dates: If you're into gardening, don't get cocky. The average last frost isn't until April 10. Every year, someone plants tomatoes in March because of one warm week, and every year, the Illinois frost gods smite them.
Honestly, the weather for O’Fallon Illinois is a test of patience. We deal with the grey Januaries and the oppressive Julys because the Octobers are perfect.
Actionable Next Steps:
Check your tire pressure this evening. With the temperature set to drop from 47°F today to 13°F tomorrow night, your "low pressure" light is almost guaranteed to pop on. Get ahead of it now so you aren't shivering at the gas station air pump tomorrow morning.