Johnsburg, IL Weather

Key messages

- Periods of showers and/or drizzle are expected at times through the weekend, though the highest chances/coverage is expected tonight into Saturday morning.

- Additional periods of wet weather are likely next week.

Discussion

Issued at 100 PM CDT Fri Oct 30 2026

There is a mid-level trough over west central Texas (via SPC Mesoanalysis) that is expected to gradually move northeastward over the southern Plains today/tonight. Additionally, a stout 100+ knot upper jet is located over southeastern Ontario, placing northern Illinois in the right entrance region of its location and therefore in an area increased upper level divergence. For know, a near surface dry airmass has been winning the day keeping all activity on KLOT radar currently showing up in the western portions of the forecast area to mainly virga, to maybe a few sprinkles. As a plume of increased moisture and warm air advection moves into the area this evening and in the overnight, there is a better chance for increased coverage for light showers into Saturday morning. Given the unfavorable time of day, thunder is not expected. Although slightly steepening low/mid-level lapse rates keeps the chances non- zero, but below formal forecast mention.

As the upper level jet streak weakens and moves east tomorrow morning, the mid/upper level trough is expected to move over the western Great Lakes Saturday afternoon and evening. With weaker forcing and a lingering dry boundary layer air mass, confidence is lower for precipitation tomorrow evening. Slight chance PoPs were maintained for tomorrow night for the isolated chance for a light shower, if not just sprinkles/drizzle during any trick-or-treating. Certainly not any kind of washout, some areas will probably remain completely dry. But enough of a nuisance to keep precip mention in the forecast through Sunday morning.

A pseudo-stationary upper level cutoff low west of Seattle, WA is projected to finally move eastward over the intermountain west on Sunday as another short wave currently near Baja California moves northward, merging with the low, further amplifying the trough early next week. While specific details still need to be ironed out, models are showing fairly good agreement that this trough will induce surface cyclogenesis east of the Rockies and that will move over the Great Lakes in the middle of next week bringing better chances for widespread rain. Behind this system, a cooler airmass will drop temperatures down to make it feel a little more autumn-like.

DK

Aviation

Issued at 635 PM CDT Fri Oct 30 2026

The primary forecast concern this period is the chance of showers and timing.

Scattered showers south and southwest of the terminals currently are expected to slowly drift north tonight into Saturday morning and guidance has been showing an overall decrease in shower coverage overnight, with perhaps the best chances for showers across northwest IL and at RFD. Opted to continue only prob mention for the rest of the terminals and primarily for Saturday morning. Isolated showers will remain possible Saturday afternoon with perhaps another chance of showers late Saturday evening through Sunday morning.

While cigs may lower into low vfr or perhaps briefly into high mvfr with any periods of showers, confidence is very low for mvfr cigs and expect mainly prevailing vfr cigs through the period.

Easterly winds at or below 10kt will continue tonight, becoming more southeasterly Saturday and possibly south/southeast Saturday evening. cms

Watches, warnings and advisories

IL: None. IN: None. LM: None.