textproduct: Chicago

KEY MESSAGES

- Showers and a few storms will move across the area tonight. No severe weather is expected tonight.

- There is a threat for a regional severe weather outbreak tomorrow, with a threat for strong tornadoes between 4 and 9 PM CST, especially along/west of I-39.

- There is another threat for severe weather Friday and Saturday, though the threats are conditional on placement of boundaries and system arrival timing.

- A transition toward a quieter, but cooler, pattern is on track to arrive early next week.

UPDATE

Issued at 859 PM CDT Wed Apr 1 2026

This evening, low hanging stratus is overspreading northern IL in advance of a warm front draped across south-central IL. The center of low pressure tied to this warm front is currently positioned over eastern KS. Meanwhile, just upstream of our CWA, scattered showers with a few embedded thunderstorms are lifting into western and northwestern IL. Over the next several hours, this precip will propagate into our area and we should see an overall increase in coverage as the storm's low level warm conveyor strengthens and noses into northern IL. Highest precip coverage during the night will be favored across areas roughly northwest of I-55 where the greatest moisture advection and kinematic forcing will focus while more scattered coverage is expected farther east. A couple of isolated thunderstorms will be possible, but thunder coverage locally should be low, if any, as the ample instability will largely remain outside of our CWA through the night. However, deep saturation in an environment categorized by moderate deep layer forcing and weak mid-upper level flow could promote periods of soaking downpours and localized flooding.

In other news, 950mb winds staying up near 30 kt will keep winds on the breezier side through the night. The warm front will charge northward overnight eventually reaching our southern CWA by around dawn. As a result, temperatures will remain pretty steady here for the rest of the night, and may even warm a couple of degrees in spots south of I-80. The front will continue lifting north across the CWA through the morning hours. Areas of fog may be found north of the advancing front late tonight into tomorrow morning, some of which could be locally dense.

The going forecast is very much on track through the night with only some hourly refinements made to PoP trends. For more on the developing severe potential for tomorrow, refer to the full discussion below.

Doom

DISCUSSION

Issued at 123 PM CDT Wed Apr 1 2026

Tonight through Thursday Morning:

A recent surface analysis placed a low-level frontal zone from the Kansas/Oklahoma border eastward along the Ohio River Valley ahead of a vigorous upper-level shortwave quite evident in water vapor imagery crossing the southern Rocky Mountains. As the shortwave digs into the Southern Plains late tonight, lee cyclogenesis will lead to the development of a low pressure system in western Kansas and induce pressure falls in the Upper Midwest (on the order of 6 to 9 mb/6hr through the overnight hours). As a result, the expectation is for a gradual increase of southwesterly low-level flow across the Mississippi River Valley tonight (925-850mb flow progged to increase toward 35-40kt by daybreak Thursday). The corresponding increase in a deep layer of isentropic ascent (35 to locally 50kt of upglide upon the 280 to 300K theta surfaces) as well as rising thickness within the warm air advection regime to force a weakly coupled upper-level jet structure will lead to the blossoming and northward spread of showers through the overnight and morning hours of Thursday. Put plainly, the aforementioned frontal zone will shift northward into our area tonight into tomorrow morning accompanied by showers and a few thunderstorms.

While upright instability will be relatively weak tonight through tomorrow morning, relatively strong forcing amidst PWATs climbing above 1.25" will support "warm-rain" process healthy downpours within the northward-moving shield of showers, especially where low-topped convective processes can take place. The progressive northward movement to the front should tend to limit the opportunity for the heaviest rain rates to train through the night, but will have to watch for any persistent convective echoes south of Interstate 80 and especially near the US-24 corridor where 1 to locally 3 inches of rain has fallen in the past 24 hours. Did collaborate the introduction of a Level 2 threat level for flooding with WPC valid for tonight south of I-80 to account for any worsening of ongoing ponding in low lying areas.

While heavy cloud cover and warm-air advection scattered showers and storms may very well continue across parts of the area through noon, the intensifying southwesterly low-level wind field forced by the tightening low-level pressure gradient ahead of the approaching surface low (surface gusts poking north of 35 mph) will be able to force temperatures upward into the upper 60s to lower 70s. Will note that there is a play for southwesterly winds to gust as high as 40 to 50 mph if mixing is especially efficient tomorrow particularly southeast of I-55, though the upward effects of warm-air advection usually limits the ability for mechanical mixing to bring the strong low-level wind field down to the surface. This will be something for later shifts to assess (to inform any decision on whether a Wind Advisory may be needed for part of the area).

Thursday Afternoon and Evening:

Focus then turns to the threat for severe weather. During the afternoon and evening hours, the core of the upper-level shortwave will eject northeastward into the Upper Mississippi River Valley as the surface low lifts from near Kansas City, Missouri to La Crosse, Wisconsin. Steepening mid-level lapse rates by virtue of differential cyclonic vorticity advection atop the continued moistening boundary layer (surface dew points rising toward the mid 60s) will support the development and northeastward advection of an uncapped instability plume characterized by MLCAPE >1000 J/kg into the Mississippi River Valley by mid-aternoon. At the same time, a 500mb speed max will arc northeastward toward the Great Lakes atop the already strong low-level wind field. Conceptually speaking, the synoptic pattern including the expected path of the surface low across eastern Iowa during peak heating matches analogs for significant severe weather events, including tornado outbreaks, across the region.

At this point, there appears to be two windows for severe weather tomorrow afternoon. The first window will be during the early afternoon hours (call it noon to 4 PM) as any residual elevated warm-air advection convection, or newly forced convection along a prefrontal trough, attempts to become rooted to the eastern edge of northeastward-moving instability axis. Often, this can lead to supercell structures that are nearly but not fully surface-based owing to relatively cool, or at least not primely unstable, storm-relative inflow originating off the main instability axis. Should a supercell manage to become surface-based during the early afternoon window, a threat for tornadoes could emerge. Though in most cases, these kinds of early-day supercells do struggle to latch onto the surface and largely present a threat for lightning and downpours. (The hail threat should be mitigated by strong low-level storm relative inflow beneath weak shear above 3 km).

The more concerning time window for severe weather will be during the late afternoon and evening hours (4 PM to 9 PM) ahead of the cold front where low-level moisture will be greater, mid-level lapse rates will be steeper, and synoptic-scale forcing will be strongest. With semi-orthoginal orientations between deep-layer shear vectors and the cold front, initial storm mode will favor discrete supercells with strong mean-flow supporting east-northeasterly storm motions of 45 to 55 mph. The impressively sheared low-level environment characterized by 0-1km shear/SRH of nearly 40kt/275 J/kg, moisture-laden instability axis supporting LCLs beneath 1000m, and strong southwesterly flow to support frictionally-generated turbulent near-surface vorticity streams will lead to an environment uniquely favorable for strong and long-track tornadoes. Based on the current arrival time of the instability axis, the favored area for such a threat for tornadoes appears highest from eastern Iowa through northern Illinois extending toward I-39, though perhaps as far east as I-55 in the most aggressive destabilization scenarios. Some upscale growth into clusters may transition the primary threat to damaging winds toward Lake Michigan and northwestern Indiana with time. And, the strong low-level storm relative inflow beneath weak shear above 3 km should tend to limit the threat for damaging, or at least giant, hail. So, the main threat tomorrow could very well be tornadoes.

As is often the case with severe weather set-ups, a regional tornado outbreak tomorrow is hardly set in stone. One very easy to envision failure mode is for morning and early afternoon shower and afternoon coverage to be expansive, limiting the ability for meaningful destabilization in the northeastward- moving instability axis by late afternoon. In such a scenario, new thunderstorm development along/ahead of the cold front could be kept to a minimum or lead to low-topped "stringy" convective elements that struggle to deepen within the strongly sheared low-level environment. Or, thunderstorms that develop upstream in Iowa could fall apart while moving into northern Illinois if the northeastward movement of the instability axis is impeded. This is all to say, the occurrence of severe weather in our area tomorrow is not a guarantee.

When put altogether, the Storm Prediction Center has upgraded much of our area to a Level 3/5 threat level for severe weather (driven by tornado probabilities). Tomorrow will be a day to stay very weather aware, especially if west of I-39, between 4 and 9 PM.

Friday into Saturday:

Yet another upper-level shortwave is poised to dive into the central Plains Thursday night into Friday, causing the cold front Thursday evening to stall somewhere in the Ohio River Valley Friday. A surface pressure col will move over the area Friday morning before pressure falls in the Midwest allow for southerly flow to redevelop and the warm front to lift back northward. Exactly how far northward the front will retreat is an item of low confidence, and will be important to the forecast for temperatures by mid-afternoon. In scenarios where the front gets hung up somewhere across central Illinois and Indiana, many areas may remain in the 50s for much of the day. Meanwhile, any areas south of the front will have the potential to jump back up into the 70s.

Friday afternoon, forecast soundings depict minimal capping along the warm front leading to concerns for the development of convection. With the upper-level wave remaining well to our west, shear profiles along the front will be modest and largely driven by low-level directional changes across the front itself. Taken together, the conceptual pattern appears supportive of low-topped supercell structures with a threat for funnel clouds and weak tornadoes along the warm front. At this point, the front is favored to be somewhere south of I-80, and perhaps even south of US-24. Pin-pointing the location of the front will be a priority in future forecast packages.

The upper-level shortwave is expected to shift eastward into the Great Lake Friday night into Saturday likely accompanied by a band of showers and thunderstorms. The unfavorable diurnal arrival time of the peak forcing suggests the severe weather threat should be low in our area, though any slowing of the wave would be cause for concern for one last window of severe thunderstorms in our area (This stretch from mid-March through now has been relentless!). At any rate, Saturday will feature nearly steady temperatures in the 50s to around 60 as cold air advection works into the region behind the cold front.

Borchardt

Sunday Onward:

In the wake of this system, a period of drier weather is expected to close out the weekend as a surface high moves into the Mississippi Valley. However, the upper trough is expected to stall over the Great Lakes which will result in nearly constant northwest flow resulting in seasonably cool high temperatures in the 40s to lower 50s for Sunday and again on Monday.

The upper trough is forecast to start pushing east of the Great Lakes on Monday as an upper ridge builds over the central CONUS. As a parting gift a final shortwave is progged to round the backside of the trough Monday night which could bring us one more period of rain showers during this timeframe, That said, there is a lot of variability in guidance as to how much moisture will be available Monday night as the wave moves through so there is a chance that the rain avoids northern IL and northwest IN all together. For now though will advertise a 20-40% chance for rain. After Monday night, the upper ridge will be moving into the Great Lakes which will promote dry conditions for the middle of next week with temperatures moderating towards more seasonable values in the mid to upper 50s.

Yack

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 714 PM CDT Wed Apr 1 2026

Key aviation forecast messages:

- Widespread rain expected overnight into Thursday AM with a low chance of TS (<30%).

- A period of low-end IFR to possibly LIFR VSBYs/CIGs may develop Thursday AM prior to south wind shift toward midday.

- Thunderstorms are expected Thursday afternoon into the evening, some could be severe, especially between 21-3Z.

An arc of showers is currently lifting northeast toward the area this evening and are expected to reach the terminals in the ~3-4Z timeframe with associated MVFR VSBYs and IFR CIGs developing overnight through early Thursday morning. A few lightning strikes can't be ruled out toward daybreak Thursday though confidence is not high enough to include with this update.

As the initial batch of heavier showers lifts northeast after daybreak a period of fog and low stratus may try to develop prior to the arrival of the warm front and south wind shift. Have maintained low-end IFR conditions to account for this for now but there remains a signal for a brief period of LIFR CIGs and VLIFR VSBYs which will need to be monitored closely for future inclusion.

CIGs and VSBYs will improve in the wake of the front with SSW winds gusting into the 30 kt range by mid afternoon. Thunderstorm coverage is also expected to increase markedly by mid afternoon given a largely uncapped environment, some of which could be severe with all hazards possible (damaging wind, hail, and tornadoes). Have opted to capture this with a broader 6-hr PROB30 group but know this will need to be converted to more focused TEMPO groups as confidence in the time window of strongest storms increases. At RFD, waves of showers and storms may occur through most of the morning and afternoon.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 AM CDT Friday for the IL and IN nearshore waters.