textproduct: Chicago

KEY MESSAGES

- A soaking rainfall is likely for most or all of the area on Sunday. Some potential for thunderstorms and flooding also exists, though whether these threats will materialize here remains uncertain.

DISCUSSION

Issued at 320 AM CDT Sat Jun 20 2026

Through Tonight:

Modest isentropic ascent within a low- to mid-level frontal zone activated a pocket of modest elevated instability over our forecast area a little while ago, allowing for a few storms to develop near and just north of the I-80 corridor. This activity has since dwindled as it has progressed to the east-southeast, but with the isentropic upglide expected to persist across our southern CWA for a while longer, scattered showers and perhaps a few additional rumbles of thunder may continue to be observed there through a little after sunrise this morning.

Otherwise though, expect another dry day today with plenty of sunshine, comfortable humidity levels, and high temperatures similar to or just a touch warmer than yesterday's highs. A lake breeze will also develop once again and keep some of our northwest Indiana locales slightly cooler relative to elsewhere. This lake breeze is likely to eventually make a push inland into northeast Illinois as well, but likely not until the breezy west-northwesterly synoptic gradient flow there today starts to relax with the approach of sunset.

Sunday:

An upper-level shortwave trough will eject eastward out of the central Rockies today, spurring the development of a surface low beneath it while also inducing widespread convection across the central Plains this afternoon and evening. The exact track and strength of this disturbance and associated surface low is of relevance for our area as this will largely dictate whether a threat for thunderstorms (including ones that may be severe) and flooding could materialize in our forecast area on Sunday amidst 1.25-2" precipitable water values. Generally speaking, a more northerly track for these features and a deeper surface low would result in a greater threat for thunderstorms and flooding in our area, while a more southerly and weaker solution would tend to keep these threats suppressed to our south.

Compared to 24 hours ago, there does appear to more support in general across the latest CAM and ensemble suites for a more southerly solution that keeps the surface low, an attendant warm front, richer boundary layer moisture, and appreciable instability south to potentially well south of our forecast area. With the expectation that at least one southeastward- propagating MCS will come out of the central Plains convection tonight, it appears more likely than not that convection will just continue to regenerate along the associated outflow into tomorrow morning and afternoon, which would prohibit the aforementioned warm front from advancing northward much, if at all, tomorrow. Thus, do believe that these more southerly model solutions have some credence to them and that the threat for thunderstorms and the greatest threat for flooding on Sunday could very well remain to our south. That said, support for this solution is not unanimous across the broader spectrum of model guidance, and if the aforementioned morning MCS doesn't materialize, decays much quicker than presently expected, or is delayed by at least a few hours, then that would open the door for the warm front to surge northward towards our southern CWA tomorrow.

Regardless of which scenario pans out, it is expected that most or all of our forecast area will receive a soaking rainfall on Sunday given the abundance of both forcing support aloft and moisture through the tropospheric column (our northernmost counties could still potentially miss out on this soaking rainfall if one of the more extreme southerly solutions were to play out, which is not out of the question at this point). There has also been a consistent model signal for a pronounced deformation band to develop along the northern flank of the surface low. How efficiently this band is able to crank out rainfall will depend on the strength of the surface low and the associated frontogenesis, but most model guidance suggests that a corridor of 2-4" rainfall totals will be observed somewhere in or just south of our forecast area when all is said and done. With many area creeks, streams, and rivers still a bit more elevated than usual from our recent rainfall, amounts like these do raise concern for additional river rises and flooding and possibly areal flooding in some locations as well, so will issue a Hydrologic Outlook to give greater awareness to this possibility.

Monday through Friday:

After Sunday's system clears the region, quieter and drier conditions should settle into the area for the beginning of the upcoming work week. However, there remains a fairly consistent signal in ensemble and deterministic guidance for another upper- level disturbance to arrive in the region mid-week and bring a renewed likelihood for precipitation to our area. Late Tuesday into Wednesday still appears to be the favored time frame for precipitation associated with this system to occur in our forecast area. Thereafter, guidance indicates that there could be a couple of follow-up shortwaves embedded within the northwesterly flow aloft that track through the Midwest during the latter half of the week in this lead disturbance's wake. This may result in additional showery periods beyond Wednesday into the following weekend amidst continued near to below normal temperatures.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 614 AM CDT Sat Jun 20 2026

Key Messages:

- Breezy westerly winds (gusts 20-25 kt) develop later this morning and continue into the afternoon.

- Lake breeze wind shift likely by early (late) this evening (afternoon) at MDW/ORD (GYY).

- Easterly winds Sunday with rain developing by midday.

While there are no significant aviation weather concerns anticipated through the period, breezy westerly winds (gusts 20-25 kt) will develop later this morning into this afternoon. Then, as the winds begin to abate into early this evening, a lake breeze is likely to shift the winds onshore at ORD, MDW and GYY around, or just before sunset. Wind speeds should ease quickly following the easterly wind shift, with winds likely going light (under 5 kt) and variable by mid to late evening as a surface ridge axis shifts overhead.

On Sunday, the morning will start out dry. However, our next weather system will quickly be approaching from the west. As it does, expect surface winds to settle from an easterly direction after daybreak. Rain will also develop across the terminal airspace late Sunday morning, with periods of a soaking rain and associated low MVFR to IFR CIGs and VSBYs expected Sunday afternoon into the evening. Given that the best rain chances and lower flight categories will come just beyond the current 30 hour ORD and MDW TAFs, we have opted to introduce a VCSH mention at the end of the period.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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