Area Forecast Discussion

KEY MESSAGES

- A line of severe thunderstorms is forecast to produce damaging winds and several embedded tornadoes across central Illinois this evening (risk level 4 of 5). The window for the most intense storms will be 4-10pm.

- While the risk is lower (level 2 of 4), there could be some localized flooding with these storms as well, especially north of I-72 and west of I-57 where many locations received several inches of rainfall yesterday.

- More showers and thunderstorms are likely (60-80% chance) Saturday night, with a level 1 of 5 (marginal) risk for severe weather and excessive rainfall.

UPDATE

Issued at 835 PM CDT Thu Jun 11 2026

A squall line of severe storms was racing eastward around 60 mph across central IL late this afternoon and early evening. Currently the line of storms was along and just south of I-72 and still producing severe storms with several counties having severe thunderstorm warnings and going into west central and nw Indiana where some tornado warnings were in effect. This squall line had a history of widespread damaging winds, and some large hail and tornadoes over central and northeast IL. This line of storms is moving quicker and will likely stay severe as it moves into southeast IL late this evening. Very unstable air mass still ahead of these storms with MLCAPES of 2500-3500 j/kg and increasing bulk shear values moving into southeast IL next few hours. The convection was se of a Bloomington to Jacksonville line and the flood watch will be allow to expire at 9 pm cdt. The latest HRRR has line of strong to severe storms moving into far southeast IL between midnight and 1 am and ending in se IL between 3-4 am. Lows overnight 55-60F from I-72 north (coolest in Galesburg) to mid 60s in southeast IL.

DISCUSSION

Issued at 230 PM CDT Thu Jun 11 2026

***** SEVERE WEATHER OUTBREAK THIS EVENING *****

At 230pm, temperatures and dewpoints across central and southeast Illinois were in the mid-upper 80s and mid 70s, respectively amidst southerly flow, evapotranspiration "ET" (mostly evaporation this early in the growing season), and sunshine. RAP mesoanalysis suggests this is contributing to MLCAPE values of 1000-3500 J/kg - a sharp gradient across the area with higher values to the south. Surface heating should continue to boost both temperatures and instability and reduce convective inhibition (CIN) throughout the afternoon ahead of an approaching cold front, which will sweep the area west-east during the mid to late evening hours. RAP/HRRR/NAM suggest MLCAPE will peak around 4000-5000 J/kg with 35-45 kt deep layer shear ahead of that front, which should provide sufficient forcing to maintain the ongoing storms across west-central IL and result in development further to the south. This highly volatile convective environment should favor organized convection capable initially of all severe hazards (tornadoes, large hail, and damaging winds), with an increasing wind and quasilinear convective system (QLCS) tornado threat as storms grow quickly upscale into bowing lines. PWATs approaching 2 inches will favor precip loading, especially in the taller cores which will feature melting hail, while steep mid level lapse rates and wide CAPE profiles will enhance potential for RIJ formation in bowing segments; this will increase the risk for high-end (75+ mph) and long-duration (20+ minutes) straight-line winds which would result in swaths of enhanced damage (trees down, shingle damage, power outages). SPC has issued a level 4 of 5, "moderate," risk for severe weather, driven by wind and tornadoes, which could be strong north of a Macomb to Bloomington line. Winds ahead of these storms may gust up to 40 mph from the south, as descending hydrometeors from anvils aid in momentum transport from a 40-50 kt LLJ (strongest over northern IL).

***** LOW RISK FOR FLASH FLOODING *****

With some locations north of I-72 and west of I-57 having received over 3 inches of rainfall yesterday (and multiple rounds of heavy rainfall in days prior), there is concern that flash flooding could occur with storms this afternoon and evening. This is especially true with the ongoing training storms north of a roughly Macomb to El Paso line. 3 hour FFG in some of these areas is under a half an inch, and with various CAMs advertising 1 hour rain rates >1 inch it makes sense there could be some issues, especially in urban and poor drainage areas. A Flood Watch was issued for Cass to Dewitt, and counties further north, in effect until 9pm.

***** LESS WARM AND MUGGY STARTING TONIGHT *****

Behind the cold front, conditions will turn much less humid and, with some clearing, temperatures are forecast to fall into the mid to upper 50s overnight west of roughly I-57. High temperatures tomorrow should only climb into the low 80s, making it feel much more pleasant heading into the weekend. Warm air advection (WAA) will bring a slightly more humid airmass into the region tomorrow night into Saturday ahead of another shortwave trough, but convection to our southwest should limit the northward extent of deep layer moisture - resulting in a low (albeit nonzero) risk for severe weather and heavy rainfall Saturday/night in our neck of the woods. Both SPC and WPC have us in marginal risks for each hazard.

Behind that second front, much cooler conditions return, with NBM only giving a 15-30% chance high temperatures reach 75 degrees on Sunday. Temperatures are slated to moderate gradually towards the middle of next week, as southwest flow returns ahead of the next upper level low pressure system.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 640 PM CDT Thu Jun 11 2026

Strong to severe thunderstorms are currently moving through the area. These storms are currently producing strong winds with recorded gusts up to about 50 knots or potentially greater. A tornado is also possible with this line of storms as it pushes through the area. These storms are moving through quickly though and will exit the area between 01z to 02z. Brief drops to MVFR ceilings is expected in the storms and potentially for an hour or so behind them as well. Visibility will drop to IFR levels within the storm. The good news is that quiet conditions will return in the wake of the departing thunderstorms. Winds will remain out of the northwest before weakening this evening while backing to out of the west. With calmer winds in place, there could be isolated patches of fog or low stratus. However, the probability of fog or low ceilings is low at a <=20% chance. Have kept this potential omitted from all TAFs at this time but trends will be monitored through the night. Otherwise, light west/northwest winds are expected for the rest of the TAF period as high pressure moves into the region.

ILX WATCHES / WARNINGS / ADVISORIES

Flood Watch until 9 PM CDT this evening for ILZ031-037-038- 041>043-047-048.