textproduct: Central Illinois

KEY MESSAGES

- Calm weather today across the area with light winds, highs in the mid 80s, and comfortable humidity levels.

- Moderate to heavy rainfall and localized flooding concerns Friday and Saturday as a wave of widespread showers and thunderstorms spreads in. The NBM probability of exceeding 2 inches is highest (50-60%) NW of a Havana to Peoria line.

- Below normal temperatures from clouds and precipitation on Friday and Saturday will transition to more seasonable warmth and humidity by early next week.

DISCUSSION

Issued at 235 AM CDT Thu Jul 30 2026

*Calm Conditions Persist Through Tonight*

A 1017 mb surface high is currently meandering eastward across northern/central Illinois, promoting clear skies, light E/NE winds, and comfortable conditions. Efficient radiational cooling is allowing temperatures to cool into the low 60s (some spots could locally dip into the upper 50s by daybreak). At the same time, GOES-19 nighttime microphysics imagery reveals some patchy shallow fog has been developing in Vermilion and Edgar counties, though is transient in nature and poses minimal risk for notable visibility reductions. Any rural fog will quickly burn off as the sun rises this morning.

As the anticyclone drifts into Indiana today, winds will veer to become southeasterly by midday. Dew points near 60 degrees and afternoon highs in the mid 80s will keep conditions comfortable. As the next weather system begins to approach, thin cirrus clouds will begin to spill into central Illinois this morning, gradually thickening into the afternoon, filtering out more sunshine.

*Heavy Rain and Localized Flooding on Friday and Saturday*

A mid-level shortwave traversing across Montana and the Dakotas atop the ridge today will become further enhanced by convective/MCS activity in the northern Plains before digging southeastward through the Midwest. This system is a bit unusual for late July, with a sfc low pinwheeling its way through the area Friday and Saturday, accompanied by a wave of showers and thunderstorms. The chance of thunderstorms is highest Friday night as elevated instability (MUCAPE of 1000-1500 J/kg) overspreads central and southeastern Illinois. SPC SWODY2 keeps the risk for severe weather just to our southwest, as the main instability axis will reside in Missouri Friday afternoon. However, will have to monitor the speed of the sfc low, as a quicker progression could push this risk more into western Illinois. On Saturday, lingering showers and storms underneath the low may have the tendency to produce a few cold air funnels (mainly across SE IL), given the vorticity-rich environment in place.

The precipitation associated with this system could result in heavy rainfall and flooding in some areas, given PWATs in the 1.9-2.2 inch range and deep column moisture. WPC Day 2 (Friday) ERO has a slight (15-39%) risk of excessive rainfall across much of the area, mainly west of I-57. On Saturday, WPC maintains a marginal (5-15%) risk of flooding east of the I-55/I-155 corridor. Slow storm motions and training of convective cells is the main concern. Broader precipitation shield along the northern side of the low will promote steady light-moderate rain rates. There remains some disagreement among guidance on the exact track of this system (GFS/NAM weaker & further south, ECMWF stronger & further north). In the scenario of a stronger low, a more well-defined mid-level dry slot could slightly reduce rainfall totals across central IL, keeping the bulk mainly north, though confidence in this is low. The initial push of showers is expected Friday AM from west to east and should conclude sometime late Saturday. While there will be some lingering low-level dry air across central IL ahead of this, top-down saturation should occur rather quickly. The global models suggest the potential for a deformation zone along the NW side of the system, which would keep rainfall going into the evening, so this will have to be monitored.

*Seasonable Temperatures Return by Next Week*

Widespread clouds and precipitation on Friday and Saturday will put a lid on temperatures, with afternoon highs as low as the mid 70s both days. On Friday, our southeastern counties should see the rain hold off until the early afternoon, allowing a few extra hours for temperatures to climb into the low-mid 80s. Some clouds may linger on Sunday, but surface high pressure will begin to build back in and clear skies out through Tuesday. Each day, high temperatures will climb an additional 1-2 degrees (to the mid 80s), with dew points in the mid-upper 60s.

By Wednesday, a piece of a cutoff mid-upper low over TN/KY may drift back up into the area as it joins the jet stream, bringing a low (20-30%) chance of showers across our southeast. Otherwise, a cold front hung up in the Midwest should remain northwest of the ILX CWA, though may begin nosing into the area mid-week.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1152 PM CDT Wed Jul 29 2026

VFR conditions expected to prevail through the period. Main concern is with potential for shallow ground fog in eastern Illinois toward daybreak. May be more of a factor just north of KCMI/KBMI where winds will be mostly calm, but any visibility reductions should be short lived.

ILX WATCHES/WARNINGS/ADVISORIES

None.