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 through late this morning.

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

DISCUSSION

Issued at 157 AM CDT Sun Apr 26 2026

Primary forecast concern remains severe thunderstorm and heavy rain potential Monday into Monday evening.

Low clouds across southern lower MI and IN will continue moving west early this morning with another mostly cloudy start expected for much of the eastern cwa. There may also be fog that develops as these lower clouds arrive. Confidence is low for how widespread the fog may become and how low visibilities may drop. Current fog trends appear on track for now, first developing across northwest IN and then spreading west/northwest across northeast IL. Assuming fog does form, the best potential for dense fog appears to be over Lake Michigan and possibly along the north shore toward UGN, which may persist through midday. The low clouds are expected to lift and scatter through late morning/early afternoon with increasing high clouds into this evening.

The models are now trending faster with a leading round of showers and thunderstorms Monday morning, possibly entering the western cwa before daybreak Monday morning and possibly holding together while moving across at least most of northern IL and perhaps the entire cwa through midday. Some of this activity could be marginally severe, as well as producing locally heavy rain. While there remains some uncertainty with this time period and how widespread the precipitation may become, it may not have a large impact on the overall severe potential for Monday afternoon into Monday evening with the overall setup still quite favorable for severe thunderstorms. Initial supercell development west/southwest of the area is expected to eventually evolve into a squall line as it moves across the local area. Where/when this happens is still uncertain and as noted in the Day 2 outlook, the morning convection could allow an outflow boundary to move back north as an effective warm front. Where exactly that occurs is uncertain, but it would serve as the focus for further tornado potential. Thus, despite the increasing expectation for morning convection, the overall severe trends remain largely unchanged, all hazards are possible, though perhaps the largest hail potential may be favored across the western half or so of the cwa.

Heavy rain remains a potential with a slight risk for the day 2 excessive rainfall now across the entire cwa. No change among the models showing precipitable water values into the 1.5 inch range. There remains the usual uncertainty from model qpf forecasts, though there does seem to be a gradual increase from swaths of 1-2 inches to perhaps swaths of 2+ inches, though there also remains uncertainty for where these may materialize. It remains a bit early for a flood watch, but if these trends continue, a flood watch may be needed, especially if these qpf amounts begin to more consistently appear over the Rock, Fox and Des Plaines River basins.

Outside of any convection, prevailing south/southeasterly winds on Monday may gust into the 35-40 mph range until late afternoon, and then diminish some as they shift southwest Monday evening.

There may be some lingering showers Monday night into Tuesday morning across northern IL, but this potential seems to be fading with the latest model runs. Another system lifting north across the Ohio River Valley Tuesday afternoon into Tuesday night could bring showers and possibly a few thunderstorms to areas mainly south of the Illinois and Kankakee Rivers Tuesday night.

A period of below normal temps is expected for the end of the week with lows possibly in the mid/upper 30s for at least parts of the area Thursday, Friday and Saturday mornings. Frost potential will need to be monitored, which from this distance appears Saturday morning would be the coolest with the best chance for frost. cms

AVIATION /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1131 PM CDT Sat Apr 25 2026

The airspace remains under the influence of a large high pressure system centered in southern Ontario, which is leading to persistent easterly low-level flow. As a result, the expectation is for a bank of IFR to MVFR cigs currently across Ohio and far eastern Indiana to continue spreading westward toward the terminals overnight. As the stratus bank moves over Lake Michigan, cloud bases may build toward the ground leading to either fog or LIFR cigs. Based on the current speed of the stratus deck, continue to favor the early morning hours (generally after 10Z) for cigs or fog to rapidly expand across the terminals. So, will maintain the inherited TEMPO groups, albeit with modest adjustments in time. Of course, will allow trends to dictate AMDs through the night.

Whatever fog and stratus develops overnight should erode by mid-morning. The remaining TAF period then looks quiet as easterly winds gradually turn southeasterly and upper-level clouds thicken.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.