textproduct: Chicago

KEY MESSAGES

- Severe thunderstorms Tuesday afternoon and evening, with the potential for very large hail, damaging winds and tornadoes.

- Potential for heavy rain and localized flash flooding Tuesday afternoon into Wednesday morning.

- Possible very strong winds (40+ mph) Thursday night/Friday.

DISCUSSION

Issued at 250 PM CDT Mon Mar 9 2026

Through Tuesday Morning...

It's a mild early spring day out there today with temperatures in the mid 60s to around 70 early this afternoon. These mild temperatures were made possible by breezy SW winds which have been regularly gusting to between 20 and 30 mph. A reservoir of higher theta- e air and marginal elevated instability is advancing northward into central IL on the nose of a building LLJ and will extend into southern and southeastern portions of our CWA this evening. Guidance advects over 1,000 Joules of MUCAPE over our south and southeast. However, with the low levels being so warm, there will be appreciable capping to mixed layer parcels with little forcing around to break it. If parcels do poke through or precip initiates above the cap, very dry air aloft should help choke out updrafts. Putting this altogether, the forecast calls for a low chance for isolated showers primarily into parts of northwest Indiana this evening with a pulsey storm or two not out of the question.

Meanwhile, a cold front is dropping southeast across western WI and northeastern IA this afternoon. The front will advance into our north early this evening before stalling in the area and becoming a quasi-stationary front. Guidance favors areas of largely sub-dense fog this evening and overnight around the CWA. But a handful of high res camps track some dense fog immediately along and south of the front. Some fog may linger into the morning tomorrow before the inversion lifts mid-late morning. Fog is also favored over southern Lake Michigan this evening into tomorrow.

The front will drop south of I-80 during the morning before stalling again into the afternoon. Areas of drizzle may develop early afternoon where a saturated boundary layer will interact with some low level lift north of the front. Quite a spread in high temperatures is anticipated tomorrow on either side of the front. Middle to upper 70s are forecast in our far south while temperatures may be stuck in the 40s in our far north and especially closer to the lake in our north. The rain and thunderstorm potential, including expectations for severe weather, can be found in the focused discussion below.

Doom

The Rest of Tuesday into Early Wednesday...

Overview:

Weak surface low pressure of between 1000-1005 mb will be located near Kansas City, MO at midday on Tuesday. The surface low will then reach southeast Iowa in the early evening and only slowly track east-northeast through Wednesday morning, to southern lower Michigan by midday Wednesday. A backdoor cold front sagging southward tonight into Tuesday morning will be making inland progress through Tuesday afternoon before slowing/stalling somewhere south of I-80. This boundary will mark a very sharp divide between a much cooler and moist marine influenced air mass (temps in 40s and 50s and dew points in upper 30s to around 50F) and with a stable boundary layer to the north and an unseasonably warm and humid air mass (temps in 70s and dew points in 60s) to the south. Fog and drizzle may very well develop north of the slowly sagging boundary, particularly near Lake Michigan. We'll have to monitor for at least locally dense fog, similar to what happened last week.

Where the boundary stalls will ultimately dictate exactly where an appreciable 'all-hazards' surface-based severe threat (including strong tornadoes) will likely unfold along and to its south late Tuesday through early Wednesday. To the north of the boundary, the tornado threat will likely be essentially nil, though there will still be a threat for large, damaging hail, and isolated damaging wind gusts punching through the low level stability. With Lake Michigan water temperatures in the upper 30s, early spring climatology of the all important surface boundary position suggests a southward lean from the global ensemble depiction. This is where the high resolution guidance can often better resolve the farther inland propagation of the shallow but dense cool marine layer. And indeed, the 12z CAM guidance (to varying extents on the specifics), trended farther south with the surface front late Tuesday, which is the direction we nudged our temperature and dew point forecast.

Big Picture Convective Trends:

An impressive EML (elevated mixed layer) plume with very steep mid-level lapse rates will be advected east-northeastward tonight into Tuesday morning. The presence of an EML is typically quite important in terms of higher end severe potential (more on that later), but it also results in formidable capping needing to be eroded (whether it be MLCIN near/south of boundary or MUCIN near/north of boundary). There's meaningful spread in the capping magnitude across guidance solutions into Tuesday afternoon, owing to modest at best large scale forcing, with neutral mid-level height tendencies. It may come down to a subtle 700 mb impulse being able to cool and moisten the column sufficiently to erode CIN as early as 2-3PM or so.

Otherwise, the more expansive convective footprint should correspond with the arrival of stronger forcing by Tuesday evening and especially overnight. We kept our PoPs in the chance (30-50%) range by 5-7 PM and then dramatically ramp up Tuesday night. As a possible caveat here, the subtle nature of the forcing could keep open warm sector convection into Tuesday evening scattered or even widely scattered before higher coverage arrives overnight. Rain and embedded thunderstorms will persist near the surface low path through early Wednesday, with precip then ending northwest to southeast Wednesday PM behind the robust system cold front (details in the Wednesday onward discussion section below).

Severe Threat:

Seasonably strong wind fields will result in supportive effective bulk shear profiles for severe storms upon convective initiation (whenever that occurs Tuesday PM). Along and south of the surface front, the highly favorable deep layer shear profile will be enhanced by strong low-level destabilization (0-3km MLCAPE of 100+ J/kg) and a looped/curved low to mid level hodograph. Right moving supercells that remain south of the front will have the potential for tornadoes, some strong, and large, destructive hail continuing fairly deep into the night.

Pertinent to the boundary location discussion in the overview, there appears to be a realistic play for the surface-based severe threat to focus primarily near and south of US-24. However, with the inherent wiggle room and uncertainty in this element, plus the global ensemble guidance still holding farther north in its frontal position, SPC's updated day 2 outlook kept the 10% tornado probabilities just south of I-80. We'll have a much better idea on the higher end threat zone informed by observational trends on Tuesday.

To the north of the boundary, large MUCAPE due to the 7.5-8C/km or greater mid-level lapse rates and highly favorable effective bulk shear will present a threat for elevated supercells capable of large, damaging hail. In addition, isolated severe downburst winds will be possible even in the presence of a stable boundary layer, often aided by gravity wave propagation that is not predictable in advance. The surface low track Tuesday night will probably temporarily drag the surface front farther north and its associated surface-based hazards later Tuesday night, though by then storm mode should be quite a bit messier than just after CI.

Flooding Threat:

Corridors of heavy rainfall and associated flooding are a distinct possibility, if not probability. Deep, moist convection with 300% of normal PWATs and the slow system movement through Tuesday night will introduce the potential for swaths of the area that receive successive rounds of heavy rainfall (amounts of 2-3" or more possible in 6 hours). Where these corridors of training convection evolve will likely also be tied to the exact position of the surface boundary. If the boundary gets more quickly reinforced south by initial elevated convection late Tuesday- Tuesday evening, the training convection may tend to focus primarily south of I-80. While parts of central Illinois and Indiana are in a D3 drought, this is not typically a significant mitigating factor when it comes to areal flash flooding.

On the other hand, if the footprint of heavy rain is farther north and includes more of the Chicago metro, especially the city and flashy small stream basins in the south suburbs and northwest Indiana, urban flash flooding potential will commensurately increase. With the range of plausible outcomes generally suggesting locations near and south of I-88 having a higher threat for corridors of flooding rainfall, an ESF (Hydrologic Outlook) was issued this afternoon.

Castro

Wednesday through Monday...

The back edge of the double-barrelled surface low will work across the CWA Wednesday morning and drag the front along with it, moving off to our east by the end of the morning. The thunder threat should wrap up in our south early-mid morning with some post frontal showers possible into the afternoon. Profiles cool enough in the cold advection that some wintry mix or non-accumulating snow showers may scoot across northern portions of the area.

Another strong storm system is expected to move across the northern Plains and upper Midwest Thursday night and across the northern Great Lakes region Friday. Strong southerly winds will develop across the area Thursday evening ahead of a trailing cold front from this storm system which will move across the area Friday morning. Winds will shift west/northwest behind the front and continue to be strong on Friday. Still several days away but given the track and strength of this system, wind gusts 40+ mph are possible. The local area appears to be on the south edge of the precipitation, which could end as a rain/snow mix Friday morning.

No significant changes to the model trends for this weekend into early next week. It still appears active with a system moving across the area this weekend with the potential for a rain/snow mix then a pattern shift to colder early next week with perhaps additional snow chances.

Doom/cms

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1247 PM CDT Mon Mar 9 2026

Aviation forecast concerns for the 18Z TAFs:

- Area of MVFR/IFR stratus spreads toward/southeast of the Chicago terminals late tonight.

- Cold front sags into the area from the north toward morning, shifting winds NNE. Period of IFR ceilings and MVFR vis in light fog possible behind the front during the morning.

- Deterioration to IFR/possible LIFR later Tuesday afternoon for Chicago terminals with some drizzle/showers/fog developing.

- TSRA threat ramps up quickly toward early Tuesday evening, though greatest chances for TAF sites looks to be just beyond end of current ORD/MDW 30-hour forecasts (00Z onward).

Midday surface analysis places low pressure east of Lake Superior, with a slow-moving cold front trailing through central WI and into central IA. Breezy southwest winds gusting 20-25 kts were occurring within the warm sector across the forecast area and will persist (while slowly easing) through the remainder of the afternoon. Farther to the south, an area of IFR stratus was evident it satellite imagery spreading north-northeast across southern and east-central IL. Low-level mixing has slowed the northward progression somewhat after sunrise, though this area of low-level moisture is expected to continue to lift into eastern IL/northern IN later tonight and may provide a period of IFR ceilings for the Chicago terminals by/after midnight. GYY appears most at risk, with lower chances farther to the northwest.

The cold front to our north is then expected to eventually sag into the area toward daybreak Tuesday, providing a wind shift to the northeast as well as additional IFR ceiling potential in cooler air spreading in off Lake Michigan along with some MVFR visibilities in light fog. Guidance suggests ceilings would improve to MVFR by late morning/midday (though perhaps not right along the lake for GYY). By late afternoon however, with the cold front now stalled south of the area, forecast soundings depict deepening low-level moist layer and increasing large scale ascent which may lead to showers/drizzle and ceilings lowering back into IFR. In addition, thunderstorm potential also increases later in the afternoon along/south of the stalled front and eventually north of the front across the terminals just beyond the 00Z end of the current ORD/MDW 30-hour forecasts.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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