You’re standing on the banks of the Fox River. One minute, it’s a serene scene of shimmering water and pontoon boats. The next? You’re checking the basement for seepage because the sky just dumped three inches of rain in two hours. That’s the reality of the weather in McHenry IL. It isn't just a daily forecast; it's a personality trait for those of us living in the Chain O' Lakes area.
Honestly, if you don't like the temperature, wait twenty minutes. Or just drive five miles east.
McHenry sits in a weird geographical pocket. We get the brunt of "clippers" coming down from Canada and the humidity that creeps up from the Gulf of Mexico. It makes for some wild swings.
The Four Seasons (And the Three Hidden Ones)
Most people think of spring, summer, fall, and winter. In McHenry, we actually have: Deep Freeze, Mud Season, Humidity Hell, and The Two Weeks of Perfection.
January is historically the coldest month here. We’re talking average highs of 29°F, but that doesn't tell the whole story. January 2026 has already kicked off with some brutal stretches. Wind speeds often average around 17.8 mph this time of year, making the "real feel" much lower than the thermometer suggests. If you're out by the Stratton Lock and Dam, that wind whips off the water and cuts right through a parka.
Then there’s the snowfall. We average about 37 inches a year. But it’s never "average." Some years you're shoveling 21 inches in a single 24-hour blizzard—like the 2011 "Snowpocalypse" that buried McHenry County—and other years, it’s just a dusting that turns into slush by noon.
Breaking Down the Temperature Spikes
- July: The absolute furnace. Highs average 83°F, but the humidity is the real killer. Relative humidity can sit at 71%, making it feel like you’re breathing through a wet towel.
- October: This is the sweet spot. Highs near 62°F. It’s perfect for the McHenry Outdoor Theater or hitting the pumpkin patches.
- December: Dark and damp. We only get about 3.6 hours of actual sunshine a day. It’s basically the "big sleep" for local hikers.
Why the Fox River Changes Everything
You can't talk about weather in McHenry IL without talking about the water. The Fox River is the lifeblood of the city, but it’s also our biggest weather-related stressor.
When we get heavy spring rains or a rapid "Big Thaw" in March, the river doesn't just rise; it creeps. The 1938 flood is still the stuff of local legend—it took out thirteen bridges and cost the county a fortune during the Great Depression. Even recently, in 2025, we saw localized flash flooding in August that turned residential streets into temporary canals.
The Fox River acts as a thermal regulator too. In the fall, the water holds onto summer heat a little longer, which can sometimes delay the first hard frost for houses right on the shoreline. Conversely, in the spring, that cold water keeps the immediate riverfront a few degrees chillier than the rest of town.
The Snow Squall Phenomenon
Have you ever been driving down Route 12 or Route 31 and suddenly you can't see your own hood? That’s a snow squall.
These aren't full-blown blizzards. They’re short, intense bursts of snow and wind that can drop an inch of accumulation in fifteen minutes. The National Weather Service in Chicago has been getting better at issuing "Squall Warnings," and for good reason. Just this past December 2025, a series of squalls caused multiple pile-ups because the road surface temperatures dropped so fast the salt couldn't even melt the ice.
Basically, if the sky looks purple-grey and the wind picks up, get off the road.
Predicting the Unpredictable: 2026 Trends
So, what are we looking at for the rest of this year?
Historical data from the Illinois State Climatologist shows our wettest month is typically June. We’re expecting more of those "training" thunderstorms where one cell follows another over the same area. This usually leads to the "No Wake" orders on the river, which frustrates the boaters but saves the piers.
Interestingly, the 2025-2026 winter season started early. We had over 15 inches of snow by early December, the quickest start since 1978. It suggests we’re in for a more active "clipper" pattern through February.
Surviving McHenry Weather Like a Pro
If you're new to the area or just visiting, here is the "insider" strategy for dealing with our climate.
1. Respect the River Gauges
Check the USGS gauge at the McHenry Dam religiously if you live in a low-lying area. Anything above flood stage (usually 4.0 feet at the tailwater) means you should probably start moving the patio furniture.
2. The 20-Degree Rule
Always dress for 20 degrees colder than the forecast says if you’re going to be near the water or out in the open fields toward Woodstock. The wind is a different beast out here.
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3. Summer Storm Prep
June and August are our "Derecho" months. These straight-line wind events can top 70 mph, which is enough to flip a heavy trampoline or drop an old oak tree onto your roof. Keep your gutters clear; McHenry's "moderate" flood risk score of 35 is mostly due to drainage issues during these high-intensity bursts.
4. Sump Pump Maintenance
Don't wait for the spring thaw. Test your sump pump in February. When that snow melts and the ground is still frozen, that water has nowhere to go but your basement.
The weather in McHenry IL is a mix of Midwestern toughness and scenic beauty. It’s why we put up with the sub-zero January mornings—because we know those July sunsets on the river are worth it.
To stay ahead of the next big shift, set up localized alerts for McHenry County specifically, rather than just "Chicago," as our conditions often vary significantly from the city. Make sure your backup generator is fueled before the spring storm season begins in late March.