textproduct: Chicago

KEY MESSAGES

- An unseasonably warm and humid pattern will arrive on Sunday and persist through at least the middle of the workweek, accompanied by periodic showers and thunderstorms, and a threat for severe weather Monday night through Wednesday.

DISCUSSION

Issued at 245 PM CDT Sat Apr 11 2026

Our Saturday began with sunny skies, but blowoff cirrostratus from a decaying MCS along the IA/MO state line is now filtering our sunshine for the afternoon. Temperatures are a few degrees warmer than this time yesterday sitting in the 50s across the area as of 2 PM. That decaying system will survive as a cluster of showers as it progresses over our area later today and tonight. Showers look to approach the I-39 corridor toward the end of this afternoon and work eastward through the evening, primarily across areas north and west of the I-55 corridor. Deep layer moisture and marginal instability provide a potential for a few heavy downpours and possibly a couple of embedded storms. If any surface-base storms get going, they may attempt to organize given an impressive 0-1 km shear profile, but parameter spacing for organized convection looks pretty messy outside of this, so the potential is low. These showers will move away from the area late tonight.

A strong warm advective regime will spread into the region on Sunday sandwiched between a strong high over the Mid-Atlantic and deepening Plains cyclone. This will bring both unseasonably mild temperatures and windy conditions to Sunday. A northern Plains low will lift its warm front north across the area Sunday night and temperatures should begin warming prior to daybreak. Middle and upper 70s are expected by the afternoon. Strong SSW winds will allow for such a warmup and gusts to 35 to 40+ mph are forecast for the late morning and afternoon.

At least the first half of Sunday looks dry with gusty winds and maybe a few breaks in the clouds. Suppose an isolated shower or two can't be ruled out earlier in the day amid the deep moist upglide. Precip will blossom out to our west during the afternoon out ahead of a sharp, moisture-laden shortwave impulse ejecting out of the southern Plains. Scattered showers could get going in our western CWA as early as early-mid afternoon with precip coverage expanding across the CWA into the evening as the wave lifts through the area. Despite the warm surface temps on Sunday, relatively warm air aloft will generate very marginal instability. For this reason, not expecting anything in the way of severe convection on Sunday. Enough instability should be around to certainly offer a chance for some embedded thunderstorms, though thunder coverage may be fairly slim. The biggest concern with storms later Sunday would be pockets of very heavy rain given the deep layer saturation. Most of the rain, including the heavy rain and thunder potential, should move away from the area prior to the start of Monday, but a few showers could linger into the morning.

A drier airmass will work into the region behind the wave clearing out precip by Monday morning. That aforementioned low pressure center will progress across the upper Midwest Sunday night, but its cold front will get hung up to our northwest. This will then serve as the warm front for a deepening low spinning up over the Plains. This will lock us in an open warm sector and keep the low level warm advection going into Monday. However, the middle and upper levels will cool in the wake of the departing wave leading to steep mid- level lapse rates and high instability aloft on Monday. Throughout the day, though, any apparent convective triggers will remain outside of the CWA. There's disagreement on how much capping we'll maintain in the warm sector during the day, but the boundary layer looks to remain markedly capped at least into early afternoon and guidance generally isn't resolving any free convection going up in our area. So thunder coverage during most of the day looks likely scarce, if any, but certainly can't rule out the potential for some freely convecting storms in the PM hours given that instability and a *potentially* uncapped boundary layer.

The primary window of concern for Monday would be during the evening and mainly across our north. Guidance likes the idea of an MCS charging east across the upper Midwest as a strong shortwave impulse interacts with the surface warm front. There is uncertainty on how far north into WI the front will set up and the MCS will track. A majority of medium range camps including the GFS, NAM, and RDPS track storms Monday evening across northern or north-central WI which would greatly limit or altogether prevent thunder coverage down into IL. However, the Euro has consistently been parking the front just north of the state line which would give us a better shot at getting in on some of that evening convection, especially across our northern CWA. The latest day 3 outlook from the Storm Prediction Center lines up well with this thinking having a Slight Risk (Level 2/5) along and north of the WI state line with a Marginal Risk extending south to about I-80. Any storms that do develop in our area on Monday would have the ingredients necessary to become severe.

Additional opportunities for severe weather in the region exist on Tuesday and Wednesday, although uncertainty grows after Monday. Anomalously warm, moist, and unstable airmass will remain in place into Tuesday with highs progged in the lower 80s away from the lake. Convection is anticipated to blossom east of that aforementioned Plains low during the PM hours. Guidance is struggling with where to track this low and the position/orientation of the warm front which will have implications on convective chances locally. But simply given the ambient environment at hand being strongly supportive of deep, robust convection, will want to maintain a close eye on forecast trends during this period. The center of the low looks to work across the upper Midwest on Wednesday or Wednesday night driving a strongly forced cold front through the area, but again with lots of uncertainty in the timing and path. While not quite as unstable, a convectively favorable environment will be found ahead of this front and whatever is lost in instability may be made up for by a strengthening shear profile and forced ascent along the front. A lot of details are obviously yet to be ironed out, so stay up to date on the forecast for Wednesday as well. Continued warmer-than- normal conditions are favored during the latter half of the week.

Doom

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1216 PM CDT Sat Apr 11 2026

Generally east to southeast winds and gradually thickening mid to upper-level cloud cover will prevail through the afternoon as a surface high pressure system centered on the Great Lakes starts shifting eastward.

This evening and overnight, an MCV currently drifting eastward along the IA/MO border will shift into the Lower Great Lakes. With a pocket of dry air in place across the region, the expectation is that precipitation will begin to wane as the circulation approaches the area this evening. Nevertheless, will maintain the inherited PROB30 groups for VFR SHRA focused on the 00-06Z timeframe. Renewed shower and thunderstorm development is expected overnight as a low-level jet develops and intersects the remnant circulation. While CAM guidance favors renewed convective development across Wisconsin overnight, do have some concern eventual development could end up a hair further south (near RFD and possibly even DPA/ORD) given the center of the MCV is further south than depicted in guidance. Did consider adding additional PROB30 groups for the early morning hours for this potential, but felt the probability is a hair below 30% at this point in time. This will be something to monitor for later TAF packages. Winds will turn southeasterly to southerly overnight.

After daybreak, winds will turn southwesterly and increase in magnitude as mixing builds into the base of the LLJ. Gusts may quickly ramp up to 30-35kt by the end of the TAF package.

For the last 6 hours of the 30-hour TAF window at ORD/MDW, increasing low-level moisture and warm-air advection will support the development of convective instability by midafternoon. As a convectively-invogorated shortwave approaches the region by evening, isolated to scattered convection may develop within the airspace. As a result, opted to introduce PROB30 groups for TSRA for the last portion of the outgoing TAF.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

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