textproduct: Chicago

KEY MESSAGES

- A powerful cold front is rolling southward through the area today with a notable temperature drop behind it.

- Thunderstorms are likely to develop near the front this afternoon, with the highest chances of thunderstorms near and south of I-80. There is a level 3 out of 5 severe threat in the region with large, possibly destructive hail and damaging winds the main threats.

- Conditions will turn much colder Friday before a warming trend develops over the weekend.

DISCUSSION

Issued at 1227 PM CDT Thu Mar 26 2026

Today and tonight...

A sharp cold front, enhanced by the cold waters over the lake, is rolling swiftly southward across northern Illinois early this afternoon. By about 2 PM, this will be in the vicinity of a Rochelle to Joliet to Valparaiso line, and by 4-5 PM near a Pontiac to Rensselaer line before finally getting shoved south of the entire forecast area through 6-7 PM. For the locales south of I-80, the hourly temperature drop as the front passes may be near 20+ degrees as the front will be arriving during peak heating.

A series of diffuse mid-level waves, coincident with with enhanced moisture above 14 kft and steep 500-400 mb lapse rates will continue to support a threat for isolated to widely scattered high-based showers and storms through mid afternoon prior to the "main show". Overall, the severe threat with these is probably a level 1 out of 5, with steep mid-level lapse rates supportive of some marginally severe hail.

Through about 4 PM, a modest 700 mb shortwave that's currently pushing east across Iowa will shift across northern Illinois. This will likely be the feature that finally helps lift/erode lingering capping (both for surface based and elevated parcels), leading to the development of initial robust thunderstorms through 4 to 6 PM, both on the leading edge of the cold front (which will be across our very far southern CWA) and atop the front itself up through about I- 88. It remains a bit unclear what the coverage of elevated convection will be during this window, with recent hires guidance backing off a bit. However, given the look on forecast soundings and the ever-increasing (albeit modest) large scale ascent, suspect we will see at least widely scattered elevated convective cores developing through 6 PM. Coverage should then generally expand through the early evening hours as additional moistening takes place above the frontal inversion.

If a storm or two manage to fire well ahead of the cold front in the 4 to 6 PM timeframe in the vicinity of US-24, there would be a brief tornado threat, but this window will close rapidly as the boundary surges southward. Large hail--perhaps with a few instances of very large 2+" diameter hail--remain the main threat, with an eventual transition to bowing segments/clusters with an attendant localized damaging wind threat. Based on the latest guidance, the severe threat in general has shifted about 20-30 miles south from yesterday, with the main MUCAPE reservoir setting up near and south of I-80. The main overlap of the very strongest ascent and most significant instability looks to occur south of about I-80 and east of I-55, and this is where we expect the severe threat to maximize into this evening.

Broad southwesterly 700 mb flow will continue to overtop the front well into this evening which will likely lead to several rounds of elevated convective development. After 9-10 PM, any lingering severe threat should transition well south of I-80, with the severe window ending completely by midnight-1 AM.

A localized training/heavy rainfall threat will exists through this evening. Given the ongoing drought/dry conditions, expectation is for any flood threat to remain isolated and thus have refrained from issuing a flood watch at this time.

Friday and beyond...

Much colder conditions are expected in the wake of this system tonight into Friday. Highs Friday should mainly top out in the low-mid 40s, except holding in the 30s near the lake. A shortwave trough embedded within the broader amplifying northern stream trough is expected to dig southeastward into the western Great Lakes Friday. Most guidance suggests air mass will be too dry to support any precip, though there is some signal for sufficient low-level moistening/0-3 km CAPE to support some rain/snow shower activity Friday afternoon over far NE IL. These models are likely too moist and opted to keep the forecast dry for now, thinking that the better precip threat will remain farther north across WI and MI.

After a cool start, Saturday looks like a nice day as the core of the surface high crests overhead. Initially light and variable winds will become south to southwesterly during the afternoon as the high shifts off to our east. Conditions will be quite dry with afternoon RH values likely dropping into the 20-30 percent range (if not lower than this). Depending on how high we mix, could be a slightly elevated fire danger across our westernmost locales during the afternoon.

The surface pressure gradient will tighten up a bit more on Sunday as the next surface low gets going east of the Continental Divide. A low-amplitude shortwave is forecast to transit the region during the afternoon which may end up tossing some increase mid and high-level cloud cover our way, possibly tempering insolation some. If this cloud cover ends up remaining more scattered in nature, temperatures will rise into the mid to upper 60s, with afternoon RH values possibly falling near or under 30 percent. With somewhat breezier southerly flow, there may be a slightly increased fire danger on Sunday as a result.

During next week, medium range guidance generally suggests that the dominant upper ridge that's been in place this past week will transition to broad longwave troughing. This in turn will establish more of a west to southwesterly flow aloft regime across the central CONUS which will support a period of above average temperatures along with unsettled weather conditions at times.

An initial area of low pressure is forecast to develop northeastward into Iowa/Minnesota Monday night into Tuesday which will drag a warm frontal zone northward through the region. An associated mid-level wave accompanying this feature will support a period of showers and thunderstorms in the region, particularly Monday evening and overnight. While mid- level shear will be modest to strong, a general dearth of instability currently looks like it'll limit the strong- severe potential in the area. An associated cold front is slated to shift through the forecast area Tuesday night into Wednesday with additional shower chances.

Model spread increases during the Wednesday-Thursday timeframe, with solutions varying from cool and dry post-frontal/easterly flow across the area, to the main baroclinic zone stalling overhead with additional disturbances riding northeastward along it.

Carlaw

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 631 PM CDT Thu Mar 26 2026

- Ongoing shower and embedded TS activity tapers off in a north to south manner later this evening.

- Strong and gusty northeasterly winds continue into tonight, decelerating into Friday morning. A north to northwest breeze builds into Friday afternoon.

- Roughly 1,000 ft Fuel-Alt MVFR ceilings expected to arrive as the rain showers continue this evening, with a brief window of IFR cigs too. Cigs lift closer to 1,500-2,000 ft later tonight, then lift/scatter to predominantly VFR for Fri afternoon.

Ongoing rain showers and embedded thunderstorms will continue over the next few hours, gradually tapering off in a north to south manner into tonight. The additional rainfall should be accompanied by a decline in ceiling heights (likely to Fuel-Alt MVFR, and even IFR levels for at least a brief period). Strong north-northeasterly winds with gusts to around 30-35 knots continue through the evening, with some crosswind issues for W-E runways remaining possible. Winds will make a gradual decline after midnight tonight through dawn Friday morning. Ceilings gradually lift back to MVFR levels late tonight into Friday morning.

Ceilings lift and scatter to predominantly VFR levels through Friday, though the exact timing remains uncertain. Most guidance suggests predominantly VFR by early Friday afternoon. The northeasterly breeze continues into Friday morning, backing north to northwesterly towards midday Friday. Areas east of I-57 might hang on to due north or northeasterly winds Friday afternoon (roughly half of the available guidance reflects this scenario, with the other half allowing northwest winds to spread in).

Sheppard