textproduct: Central Illinois

KEY MESSAGES

- There is a Marginal Risk (level 1 of 5) for severe weather through tonight. The highest probability for thunderstorms with damaging wind gusts will focus across the Illinois River Valley from late evening into the overnight hours.

- Windy and very warm conditions are expected Sunday and Monday as highs surge well into the 80s.

- The next significant chance for convection will come into the picture Monday into Tuesday as a slow-moving cold front approaches from the west.

DISCUSSION

Issued at 250 PM CDT Sat May 16 2026

*** Scattered Thunderstorms Tonight ***

An outflow boundary created by early morning convection across north-central Illinois became the primary forcing mechanism for additional showers and thunderstorms along the I-72 corridor midday today. This activity has subsequently weakened and shifted just south of I-72...with 19z/2pm radar imagery showing showers focused along a Quincy to Terre Haute line. Due to extensive cloud cover, MLCAPEs across much of central Illinois are currently 1000J/kg or less. Meanwhile further south toward the I-64 corridor where sunshine has been more prevalent, MLCAPEs range from 1000-2000J/kg. Scattered cells are beginning to form in the moderately unstable/weakly sheared environment from Saint Louis eastward to Evansville where a few strong storms with gusty winds will be possible into early this evening.

The greater focus for additional convection will be further west across Missouri into southern Iowa where an even more unstable airmass characterized by MLCAPEs of 2000-3000J/kg exists. As the nocturnal low-level jet strengthens from northern Texas to southwestern Iowa later this evening, thunderstorms will develop west of the Mississippi River. Some of this activity will spill into the Illinois River Valley as it lifts northeastward, posing a gusty wind risk from roughly 11pm to 5am. Have therefore focused the highest PoPs (60-70%) across the Illinois River Valley tonight. Further east, several CAMs suggest a few cells forming along the outflow boundary as it lifts northward overnight. Since this is even further away from the low-level jet energy, am not expecting widespread or organized convection along/east of I-55.

*** Severe Weather Risk Monday into Tuesday ***

All models show central Illinois firmly in the warm sector on Sunday, as S/SW winds gusting 25-30mph push afternoon highs into the middle to upper 80s. Things get more interesting during the Monday/Tuesday time period as a significant trough and its associated cold front come into the picture. The 12z May 16 models are in much better agreement concerning the timing of this process...with both the GFS and ECMWF showing FROPA Tuesday afternoon/evening. Given this trend, confidence is growing that significant thunderstorm chances will likely hold off until Monday night at the earliest. While the current forecast features likely to categorical PoPs Monday afternoon, think these will eventually be lowered...as the primary focus for widespread storms will initially be much further west across eastern Kansas/western Missouri immediately ahead of the front. This convection will spill into the Illinois River Valley Monday night, potentially posing a damaging wind risk. The storms will likely decrease in intensity overnight as they push eastward across the I-55 corridor. The most recent SPC outlook highlights areas west of I-57 with a 15-30% chance for severe.

With a slower FROPA expected, the likelihood of convective re-development across parts of central Illinois on Tuesday continues to grow. While it is still too early to pinpoint the exact development zone, most models and conventional wisdom suggests it will be along/east of I-55. SPC shows a 15-30% chance for severe along/east of the Illinois River, and this seems reasonable until minor timing discrepancies and mesoscale details are better resolved.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 108 PM CDT Sat May 16 2026

Showers and thunderstorms that developed along an outflow boundary this morning continue to percolate along/just south of the I-72 corridor early this afternoon. The convection has been steadily diminishing due to an overall lack of instability, so the risk for thunder at the terminals for the remainder of the afternoon is low. Main aviation forecast challenge will be the potential for convective re-development this evening into the overnight hours as the boundary lifts back northward. Model consensus suggests the greatest areal coverage of showers/storms will focus across the Illinois River Valley...but scattered activity could develop further east along the boundary as well. As a result, have included a TEMPO group for thunder at KPIA between 07z and 11z...with PROB30 at the remaining terminals. The boundary will lift well northward out of the area by Sunday morning, resulting in a return to dry conditions across the board. Winds will initially be southerly with occasional gusts of 15-20kt this afternoon. Stronger gusts of 20-25kt are likely on Sunday as central Illinois punches into the warm sector.

ILX WATCHES/WARNINGS/ADVISORIES

None.