A perfect balance of light: The equilux explained

What is the Equilux?:

While the fall equinox marks the official start of the astronomical autumn season, it is not the day where we receive exactly 12 hours of daylight and 12 hours of darkness. That distinction belongs to what's known as the equilux, which means "equal light". 

The equilux is the day when sunrise and sunset are separated by almost exactly 12 hours, resulting in an equal balance of day and night. Because of the Sun's size and the way Earth's atmosphere bends sunlight, the equinox and equilux do not occur on the same day. For instance, Rockford's spring equilux took place on March 17th, while the autumn equilux is scheduled to occur tomorrow, September 26th. Our sunrise is at 6:47AM and sunset is at 6:48PM, giving that equal split in day and night. Once the equilux passes, daylight hours continue to shrink, allowing nights to grow longer. 

Dry air gains ground as rain chances across northern Illinois fade

Shower Chance Early:

Over the next few days, we'll continue to watch a bit of tug-of-war unfold across the region as dry air anchored over the Great Lakes battles increasing moisture trying to push in from the west. Right now, moisture is winning out, leading to sprinkles, and even an isolated shower to start the day.  

Fortunately, any rainfall over northern Illinois will remain light, with no heavy rain expected. In fact, the best chance for any rain will be during the morning hours, primarily before 9-10AM. After that, dry air will take over as high pressure inches west, allowing conditions to gradually dry out and skies to transition to a mix of clouds and sun. Temperatures remain on the cool side, topping out in the upper 60s, while a light easterly wind keeps the early-fall feel. 

Dry Weekend Ahead:

High pressure will continue to inch closer to the area through the weekend, allowing dry air over the Great Lakes to win out. As a result, rain chances remain slim and very limited, with Sunday presenting the only weekend opportunity for rain in the form of a brief sprinkle. Otherwise, the persistent push of dry air from the east will make it difficult for moisture to successful spill into the region. Temperatures remain comfortable, hovering around 70 degrees. 

 

 

Active pattern brings more frequent rain chances to close out September

 After a very wet weekend, the last few days had been a nice break from the rain. But another active pattern will bring us better chances for rain again toward the middle and end of next week, right in time for the flip of the calendar into October.

It all starts with a shift in the upper level pattern over the next week. This coming weekend, blocking between high pressure over the Great Lakes and a strong Nor'easter will keep rain chances to a minimum. But once that pattern releases, a stronger jet will draw more frequent low-pressure systems our direction, beginning Tuesday and Wednesday.

The stronger Southwest direction to the winds near the surface and aloft will work to draw in that much more moisture. As a result, it will be much more abundant compared to this weekend, resulting in better coverage and intensity of rain. Rain chances will begin Tuesday afternoon and could continue on and off through the end of the week. A few thunderstorms may also be possible, with stronger dynamics in place.

The active pattern may continue into the first few days of October as well. The Climate Prediction Center gives Northern Illinois around a 50-60% chance to see above average precipitation between September 30th and October 4th, likely owed to the more active jet stream mentioned above. Looking further ahead to mid-October, we typically get at least one or two shots of chilly fall-like weather, with the first potentially set to arrive around October 5th-8th.

Weather pattern blocked into the weekend

 

It has been nice to get a break from the thick overcast skies the last several days, allowing a little more sunshine to help warm temperatures into the low 70s both Wednesday and Thursday. But the clouds have moved back in Thursday evening which will leave our skies cloudy through the night.

Temperatures will be slow to fall this evening, dropping only into the upper 50s overnight. Light rain well to our west will slowly inch close to the Stateline Thursday night and Friday morning but will be battling - at least initially - drier air from high pressure over the eastern Great Lakes. Early Friday - after 3am or 4am - a few sprinkles and/or light rain showers may start to move into northwest Illinois, spreading east through daybreak. The highest rainfall totals are expected to remain west of the Mississippi River with enough rain to wet the sidewalks locally, adding up to perhaps a tenth of an inch at most. A few sprinkles may last through Noon, but throughout the afternoon the cloud cover should begin to break thanks to drier air settling in from the east. This will help clear skies Friday night allowing temperatures to fall around 50 degrees Saturday morning.

High pressure will hold strong through the weekend keeping the highest moisture to our west. This will result in a mostly dry, and comfortable, weekend with temperatures in the upper 60s and low 70s.

This blocking pattern is the result of what will be a strong low-pressure system developing off the east coast this weekend. Through Saturday afternoon the center of the low will remain just off the coast, but by the evening it'll begin to shift back west. This will help hold high pressure in place through Monday afternoon across the Great Lakes. 

There could, though, be a few showers that try to sneak into our air space late Saturday night and Sunday as an upper-level disturbance moves through the Upper Midwest. But with the drier air in place, any rain is expected to remain on the lighter side.

  

Pleasant days, limited rain chances ahead

Pleasant Thursday:

Yesterday provided a welcome break from the recent stretch of heavy rainfall across the Stateline. While a few clouds drifted overhead at times, temperatures remained comfortable, topping out in the upper 60s and low 70s, making for a pleasant start to the fall season. More of the same weather is on tap today as high pressure remains firmly in control across the Great Lakes and southeastern Canada. 

Shower Late Tonight:

High pressure will keep our weather dry through the night, but forecast models continue to show a subtle increase in moisture occurring after midnight. That may be just enough to spark an isolated shower in a few spots, though some of the precipitation could fall as virga, which is precipitation that evaporates before reaching the surface due to a layer of dry air that is situated beneath the cloud base.   

Dry End to the Week:

The best chance for any precipitation will come before 9 to 10AM Friday as a weak disturbance slides through the Stateline. Even then, rainfall looks spotty, with totals remaining under 0.10". Most locations closer to I-39 will remain dry with forecast trends pushing the better chance for measurable rain to the west of the Mississippi River. Clouds will hang tight, though a few peeks of sun cannot be ruled out with highs topping out in the upper 60s.

 

 

Cool pattern with limited rain chances through the weekend

 If it feels like we haven't seen the sun in a long time, the data backs it up. Rockford has not seen under 50% cloud cover in over a week and a half! There was a little bit of sunshine Wednesday, but not enough to break the streak of mostly cloudy days.

Not much broad sunshine expected in the coming days, despite higher pressure remaining nearby. But the cooler East wind sticks around, blocking most of the abundant moisture from reaching our area. We may see a few isolated showers or even sprinkles Friday and Saturday, but many will stay dry with high pressure limiting precipitation. Further East, a strong low pressure system will bring coastal flooding and heavy rainfall to parts of the East Coast.

Eventually, the pattern breaks down and sends a stronger cold front our direction toward the middle of next week. This also comes with broader chances for rainfall, as hinted by the Climate Prediction Center's outlook. Expect a wetter pattern toward early October with more frequent chances for precipitation.

Autumn arrives with a break in the rain across northern Illinois

Very Wet September:

Yesterday's rainfall may not have been a blockbuster by any means, but every little bit counts. Rockford picked up an additional .21" of rain on Tuesday, pushing the monthly rainfall total to 6.43". That puts September 2026 ranked among the top 10 wettest on record through the 22nd of the month. To compare, Rockford typically sees 2.79" of rain from the 1st to the 22nd, meaning we've received more than double our average rainfall so far. 

Break from the Rain:

Rain chances will become much more limited over the next few days as high pressure settles over the northern Great Lakes. While a brief sprinkle can't be ruled out today, most of the day should remain dry with a mix of clouds and sun. Similar conditions are expected Thursday, with dry weather likely lingering into week's end. With a cooler fall air mass in place, temperatures will end up just below late-September standards, in the upper 60s each day.