textproduct: Central Illinois

KEY MESSAGES

- A Tornado Watch is in effect until 10pm CDT for locations along and south of a Macomb to Chenoa line. All severe hazards are possible...with the greatest probability for strong tornadoes focusing south of a Springfield to Paris line.

- Quiet weather will prevail Thursday through Saturday...with the next chance for thunderstorms arriving by Sunday.

DISCUSSION

Issued at 316 PM CDT Wed Jun 17 2026

19z/2pm mesoanalysis shows an outflow boundary left behind by the morning convection along a Quincy...to Litchfield...to Salem line. This is considerably further south than previously thought: therefore the most significant risk for widespread severe weather later this afternoon and evening has shifted south as well. As such, the new Tornado Watch coordinated with SPC does not include the far northern KILX CWA from the Peoria area northward.

MLCAPEs are currently under 1000J/kg: however, as the boundary mixes slowly northward this afternoon, the RAP suggests MLCAPEs will increase into the 1500-2000J/kg along/southeast of a Quincy to Champaign line over the next couple of hours. Impressive wind fields aloft are contributing to very strong 0-6km bulk shear of 60-80kt...while 0-1km shear values remain in the 30-40kt range. Given marginal instability and extreme shear, am expecting supercell thunderstorms to rapidly develop along/ahead of a slowly advancing cold front across the Illinois River Valley imminently. All severe hazards will be in play...including damaging wind gusts in excess of 70mph, hail larger than golf balls, and tornadoes. The Significant Tornado Parameter (STP) is currently maximized closer to the outflow boundary from central/northern Missouri across south-central Illinois, but is progged to lift northward later this afternoon/evening. At this time, it appears the highest probability of potentially strong tornadoes will focus along/south of a Springfield to Paris line.

Thunderstorms are currently developing in a band from Galesburg/Rushville eastward to Pontiac/Champaign. CAMs have generally sped up the development process, as well as the gradual S/SE movement of the band. As a result, think the primary severe risk will be focused along the I-72 corridor between 4pm and 6pm, then will sink toward I-70 by 7pm to 9pm. The primary line of convection will exit the far SE KILX CWA around midnight: however, a few showers may linger further north throughout the evening into the first part of the overnight.

A short-wave trough will bring a period of mid-level cloudiness to central Illinois Thursday morning, with forecast soundings suggesting the potential for a few light rain showers mostly along/south of a Rushville to Robinson line where slight chance PoPs (20%) are warranted. Once this wave departs, a return to cool and mostly dry weather will be on tap for Friday and Saturday. The next significant chance for precipitation will hold off until late Saturday night into Sunday as a stronger short-wave trough traverses the Plains and approaches from the west. Given better deep-layer moisture and strong forcing, think showers and thunderstorms will become widespread. The severe weather risk remains uncertain at this time, with several machine learning algorithms such as Colorado State and NSSL keeping the greatest probabilities further south. A few NCAR members do however suggest 15-30% probs northward into central Illinois, so will need to keep an eye on mesoscale details as the time nears.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1035 PM CDT Wed Jun 17 2026

Quiet weather with VFR conditions are expected for the 06z TAF package. Earlier convection has cleared the terminals as a cold front slowly moves into the region from the N/NW. Winds will be steady from W overnight around 8-12 kts, shifting to the northwest after 13-16z and increase to 10-15 kts. Lingering debris CU around SPI, DEC, and CMI will clear out by 8-10z, followed by diurnal CU developing after 13-15z with bases around 5-7kft. Friday night, skies will clear with winds relaxing to to 4-7 kts from the NW.

ILX WATCHES/WARNINGS/ADVISORIES

None.