textproduct: Central Illinois

KEY MESSAGES

- No major weather systems are expected to affect central and southeast Illinois through at least the middle of next week. Temperatures will remain above normal, with highs in the low to mid 80s into early June.

DISCUSSION

Issued at 229 AM CDT Fri May 29 2026

The upper level pattern remains amplified, with an Omega block expected to be the dominant feature into the middle of next week. Early Fri morning, IL was situated directly beneath the upper ridge axis, with a weak, elongated trough present over the central Plains. Some cirrus clouds continue to stream over Illinois, but the thicker cloud cover (and precipitation) associated with this trough extend from the Dakotas to northern MS/AL. 06z/1am sfc analysis reveals a 1022-mb sfc high remains positioned over the Great Lakes region, resulting in continued NE flow and advection of a drier airmass into central IL. Sfc dewpoints as of this writing range from the low 60s in SE IL to mid 40s and low 50s north of I-72. Deep layer moisture is also low, with RAP-analysis depicting a gradient in PWAT values from SW to NE across the CWA, ranging from 0.4" to 0.7". An axis of higher moisture content is present ahead of the aforementioned trough, with values climbing to 1.5-2" over portions of MO/KS/AR.

The trough over the central Plains will gradually nudge east today, resulting in increasing cloud cover and mostly cloudy skies. The NBM high temperature forecast was too cool in previous days, but given the increasing cloud cover opted not to adjust temps higher today. The NBM still has temps above normal today, with highs in the low to mid 80s. There is a low chance (10-30%) for showers along and southwest of a Rushville to Lawrenceville line this evening into Saturday morning, but precip chances remain higher to the south and west of the ILX CWA.

This weekend, the upper ridge begins to strengthen and a renewed sfc high pressure pushes south into the Great Lakes. Despite this, low precip chances linger at times throughout the weekend, owing to the weakening but persistent shortwave just west of the area, as well as additional weak waves emanating off the primary upper low over the Rockies. Still, most locations in our CWA have less than a 30% chance of seeing rain at any point this weekend.

The Omega block persists into early next week, but guidance shows signs of weakening by Tues, followed by a transition to a more zonal flow regime for the latter half of next week. This more progressive pattern could offer some better rain chances, although the zonal flow regime appears short-lived with both GFS and EC ensembles depicting a return to upper ridging over the Plains next weekend (around June 7th). CPC 8-14 day outlooks (valid June 5-11) feature a 50-60% chance of above normal temps. Normal highs for this time of year are in the low 80s. The NBM suggests a 15-30% chance for highs to climb into the 90s each day June 5th-7th. For comparison, despite the forecast calling for above normal temps today through early next week in response to this Omega block, the probability of highs climbing above 90F is very low (less than 10% each day).

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 450 AM CDT Fri May 29 2026

VFR conditions will prevail through the period. Cirrus coverage will increase through the period, while some diurnal cumulus could develop around 5 kft. There is a low chance, less than 10%, of a shower at KSPI tonight, but precip should stay to the south and west. Winds will remain light, less than 10 kts, turning to southeasterly during the day, then backing to northeasterly overnight.

ILX WATCHES/WARNINGS/ADVISORIES

None.