Glen Carbon, IL Weather

Key Messages

- Warm days and crisp fall nights continue through the end of the coming workweek.

- Rain chances return for Halloween weekend.

Short Term

Issued at 200 AM CDT Sun Oct 25 2026

A persistent blocking pattern remains in place across North America and has led to our stagnant weather pattern for the last week or so. A trough over the East Coast and western Atlantic extends all the way south to Cuba. Another trough over the West Coast extends down to Baja California. In between, a high amplitude ridge extends from Texas to Hudson Bay. Highly amplified patterns like this tend to stick around for a while before finally breaking down. Our area is solidly under the influence of the ridge with broad surface high pressure stretching from Quebec to the Deep South. This surface high has kept Gulf moisture blocked well to the south, only making a northward push through west Texas and the High Plains.

The result for us locally will continue to be warm days and cool crisp nights. With daytime heating we mix into the warmth within the ridge with temperatures reaching the 70s to near 80. But the dry air, calm winds, and longer fall nights allow for nocturnal cooling into the upper 40s to near 50. The only recent addition to this pattern has been some increased high level cloud cover as the ridge axis has shifted a bit further east and high level moisture streams over the top of it. These clouds will have some minor effect by decreasing the extreme diurnal temperature ranges. Areas that get a little less sun will have a tougher time reaching 80 degrees despite the warm ridge. But that same blanket of clouds will dampen the nighttime cooling as well, with fewer locations dropping into the 40s.

Long Term

Issued at 200 AM CDT Sun Oct 25 2026

By in large the stagnant pattern continues for us locally through the upcoming week, though there will be some movement to cut back on the extreme blocking. Just as there's some easing in the eastern trough, portions of the western trough eject northeast around the ridge, cutting back on its intensity and amplitude. The most significant is on Monday when the core of the western US trough exits through Colorado into Minnesota by Tuesday. Guidance is in good agreement that this passes far enough to our north to avoid any significant impacts locally aside from increased mid and high level cloud cover. There is a cold front behind this trough, but it gets hung up over Iowa Wednesday into Thursday keeping us warm and dry south of it.

There is some restructuring of the pattern from a high amplitude ridge to a broader, flat topped ridge over much of the central and eastern US with the jet near the Canadian border. Out west, although one trough ejects, it's soon replaced by a new one dropping south out of Alaska into the Northwest US. While this general idea is agreed upon, guidance varies wildly in how this next trough progresses late this week and into the following week. Locally, though, we transition to more of a southwesterly upper level flow with better moisture transport through our region. While the precise details vary, this does translate to increased rain chances eventually, just in time for the Halloween weekend. These chances peak Saturday into Sunday when at least one shortwave trough is expected to pass near the region. Among the 00Z ensemble guidance, 80 to 90 percent of members produce measurable rain across our entire forecast area in the 24 hours ending at noon on Sunday. This indicates strong confidence that at least some rain will fall locally this weekend. Latest NBM PoP peaks at only 60 percent due primarily to timing uncertainty (will it fall Saturday, Saturday night, or Sunday?).

Looking beyond this weekend, there's a general consensus that the ridge fully breaks down and some flavor of troughing arrives early next week. But whether this represents a return to more "normal" temperatures or something even cooler is not well agreed upon.

Aviation

Issued at 1214 PM CDT Sun Oct 25 2026

Dry and VFR are forecast to continue through the TAF period. Patchy river valley fog cannot be ruled out Monday morning, but coverage should be less than previous mornings and confidence is lower in any TAF site impacts. Winds will remain light, becoming more southwesterly during the day Monday.