textproduct: Chicago

KEY MESSAGES

- Severe thunderstorms Monday with all hazards possible, large hail, damaging winds and tornadoes. Heavy rain and localized flooding also possible.

- Potential for fog across primarily northeast Illinois late tonight into Sunday morning.

- Seasonable and dry conditions Sunday, but cooler temps near Lake Michigan.

DISCUSSION

Issued at 249 PM CDT Sat Apr 25 2026

While this morning's expansive stratus quickly eroded for a vast majority of the area, a more prominent axis of status/fog over Lake Michigan continues to advect onto the southern Lake Michigan shore this afternoon. Satellite trends today combined with the guidance that has best handled the stratus suggest that stratus will redevelop/expand farther inland this evening and overnight under a lowering, shallow and strong inversion. An easterly veering low-level wind profile in response to high pressure passing to the north should focus the stratus over northeast Illinois, with less confidence on the western extent before sunrise Sunday morning. While the lowering inversion has the potential to erode cloud cover from top to bottom, it would also result in very low ceilings that could transition to fog late tonight into Sunday morning. Low stratus should then quickly erode during the morning, though the stabilizing effect of Lake Michigan may allow stratus to persist well into the afternoon along the shore. Otherwise, temps Sunday will be similar to today with highs in the lower 70s well inland to around 50 near the shore.

Focus remains on a potentially significant severe weather event in the region on Monday, with our forecast area remaining on the eastern periphery of the greatest risk at this time. Conceptually, the synoptic set-up with a negatively tilted trough swinging across the Missouri River Valley, associated surface low track across Iowa, and incoming strong mid/upper- level jet dynamics all point to a concerning severe weather set- up. The biggest questions for our area regarding both coverage and intensity of convection revolve around 1) the amount of warm sector contamination/longevity from any morning convection associated with an incoming EML and 2) the potential convective mode with eastward extent across the Mississippi River.

A low/mid-level wave tracking well ahead of the main trough Monday morning will likely induce an area of elevated convection fueled by steep mid-level lapse rates on the edge of the EML. This convection will likely affect at least the northwest CWA, with any stronger embedded cells capable of producing damaging hail. There is remaining uncertainty with how far southeast this convection extends into the remainder of the CWA, which could delay erosion of existing inhibition while also focusing renewed elevated convection on the north side of any effective outflow boundary.

Conditional on a "clean" warm sector well northeast into Illinois Monday afternoon, the noted strong forcing should erode capping to the west and force discrete convection around the I-35 corridor in Iowa and Missouri. Convection would likely congeal with eastward extent across the Mississippi River, favoring primarily a QLCS set-up late Monday afternoon and especially Monday evening. ENE-pointed deep-layer shear vectors suggest that convection could remain discrete or semi-discrete through at least west-central Illinois, with any congealing line likely containing supercell elements across much of the remainder of Illinois. A narrowing higher theta-e axis with northward extent may limit the supercell/QLCS tornado threat far into northern Illinois, but the the embedded tornado threat could also increase/expand in connection with increasing low- level flow (60+ knot LLJ) during the early evening.

Will also need to monitor for a flooding threat, especially north of I-88 where rivers levels remain elevated, if the morning convection persists/redevelops through the day.

Behind Monday's system, a transition to longwave troughing over the eastern Great Lakes and eastern Ontario/Quebec will favor a cooler end to April and start to May with perhaps a couple chances for showers later in the week. Nighttime frost potential will also increase late in the week, specifically for interior northern Illinois.

Kluber

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 632 PM CDT Sat Apr 25 2026

Forecast concern for the terminals...

- Period of IFR to LIFR ceilings and fog late tonight into Sunday morning, especially at GYY.

A mid-level ridge (area of high pressure) continues to reside overhead which is allowing mostly clear skies across much of northern IL and northwest IN this evening. However, there is a plume of low stratus and fog over southern Lake Michigan that is oozing inland at GYY and MGC. Recent satellite trends do show the back edge of this stratus and fog gradually eroding which should allow the ceilings and visibilities at GYY to improve by 01z, but confidence is low as to whether or not the clouds will fully scatter out. Regardless, the low clouds are forecast to redevelop over Lower MI this evening and pivot westward overnight as winds become more easterly. This should result in a more widespread period of IFR and LIFR ceilings at all the TAF sites after midnight through at least mid-morning on Sunday. In addition to the low clouds, there is also a signal for some of the low clouds to build down into fog as the inversion overhead intensifies. While visibilities should generally be in the 3-5 SM range, some locally lower visibilities (as low as 1 mile) may occur near the lake (15-20% chance).

Once the low clouds and fog erode Sunday morning, expect another day of partly to mostly clear VFR skies. Winds will remain out of the east around 10 kts with some occasional 15-20 kt gusts possible during the afternoon.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 10 PM CDT this evening for the IL and IN nearshore waters.