textproduct: Chicago

KEY MESSAGES

- Less windy and still mild today, with some isolated showers in spots.

- Showers and thunderstorms tonight will produce rainfall which may exceed an inch north of the Illinois and Kankakee River Valleys before tapering to showers and ending Friday morning. This could produce new rises on area rivers and creeks.

- Warmer, but more active weather pattern with periods of storms returns late in the weekend through early next week.

DISCUSSION

Issued at 327 AM CDT Thu Apr 9 2026

Surface low pressure ~1005 mb was analyzed north of Lake Superior early this morning, with a trailing cold front extending southwest through WI and northwest IL, then back across KS to another area of low pressure near the CO/OK/NM border. A couple of bands of showers were noted along/ahead of the surface front, within a ~45 kt southwesterly low level jet axis. Despite the strong southwest flow, low-level moisture is lacking per 00Z upstream SGF/TOP RAOBs and surface dew points only in the 40s. SPC/RAP mesoanalysis also indicates little in the way of MUCAPE within the frontal zone, thus little or no thunder is expected this morning as these bands of showers continue to move to the east-southeast ahead of the front. The front is progged to slow as it shifts south of the I-80 corridor later this morning/midday and eventually become nearly stationary in the vicinity of the U.S. 24 corridor (Peoria- Watseka-Kentland) later this afternoon. Forecast soundings continue to depict dry low-levels and little in the way of instability along the front, and CAMs generally have a dry look to them through mid-afternoon.

Farther to the west, another mid-level short wave emerging from the northern Rockies this morning is forecast to quickly translate eastward toward the upper Midwest through this afternoon, inducing renewed low-level southerly flow across the Plains. This increases forcing into the low/mid-level baroclinic zone north of the stationary front into IA/western IL toward evening, expanding showers and thunderstorms back into the region this evening/overnight. Stronger forcing and moisture transport is expected to result in greater coverage/intensity of convective rains north of the surface front, with guidance generally indicating a swath of ~1.00+" of QPF across roughly the northern 2/3 of the forecast area. Ensemble probabilities currently stand in the 20-40% range of reaching/exceeding an inch of rainfall. WPC has included this area in a marginal risk (level 1 of 4) their Day 1 Excessive Rainfall Outlook (ERO), and if confidence in 1.00"+ amounts continues to increase today may need to consider a Flood Watch for tonight into early Friday given recent heavy rains.

The aforementioned short wave eventually tracks east of the area Friday morning, with the front then pushing south as a cold front again in the wake of an associated surface low pressure wave. Shower coverage should decrease in intensity and coverage while shifting southeast of the area by mid-late morning. Seasonably strong (~1030 mb) surface high pressure then builds into the western Great Lakes region Friday afternoon and night, providing a brief period of dry and cooler weather.

Global models/ensembles remain in good agreement in the upper level pattern becoming more amplified as we head into the weekend, in the form of a deep upper trough and closed low developing over the West Coast downstream ridging across the Gulf/eastern CONUS. Locally, the surface high pressure over the area Friday night shifts east on Saturday, allowing low-level flow to become southeasterly during the day, with temperatures moderating back into the 60s (except where onshore winds maintain cooler temps along the IL Lake Michigan shore). Broad warm/moist advection, tapping low-level moisture from the western Gulf, then ramps up Saturday night into Sunday downstream of surface low pressure which lifts from the Plains into the upper Midwest. Warmer (70s/80s temps) and more humid (dewpoints into the 60s) weather, combined with a series of mid- level short wave disturbances riding the fast southwest flow downstream of the western CONUS upper trough look to provide periods of active stormy weather into early next week.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 628 AM CDT Thu Apr 9 2026

Key Aviation Messages for the current TAF period:

- Widespread chances for rain are expected across the region tonight. Embedded thunder is possible, but the chance is too low for mention in the TAF.

- Wind shift to the northeast late this afternoon/evening, but confidence in timing is low.

- IFR cigs possible with the heaviest downpours after 05Z, then lingering into Friday morning.

Lingering showers are passing over the airspace ahead of a front. However, with the amount of low level dry air, it is no surprise that no obs are reporting precip. Still, sprinkles are possible through mid morning as the front sags south and stalls, but no impacts are expected. Otherwise VFR conditions and west southwest winds up to 10 knots are forecasted through the early afternoon.

As a wave moves northeastward out of the Plains, the stalled front will begin to lift northward. There is a chance for an initial wave of rain around or just before 00Z, then rain chances increase as more moisture surges north overnight. The threat for thunder is a bit more limited. Models are keeping the MUCAPE access south of the TAF sites. Embedded thunder is possible with the rain overnight, but the probability remains less than 30 percent and therefore left out of the TAFs. One bit of uncertainty during this renewed rain threat tonight are wind directions. Previous model runs suggested a lake breeze developing (and switching winds to the northeast) around or after 22Z, but have since drifted away from that projection. However, the large scale wind directions are expected to switch to the northeast with the heavier rain after 00Z. For now, the TAFs kept a wind shift to the northeast with the initial wave of rain, but confidence is low with better confidence in northeast winds after 02Z. Lastly, as the heavier rains move in overnight, IFR conditions are expected with heavy rain rates and lower cigs. Guidance has been far more bullish recently on the threat for IFR cigs lingering through Friday mid-morning as rain tapers off, before lifting to MVFR levels.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 7 AM CDT this morning for the IN nearshore waters.