textproduct: Chicago

KEY MESSAGES

- Scattered showers and thunderstorms will drift into portions of the area this evening, with the greatest coverage mainly across northwest Illinois and areas along and west of I-39.

- Active weather pattern to get underway Sunday with periods of thunderstorms through the at least the middle of next week. Some storms could be severe, especially Sunday and Monday.

DISCUSSION

Issued at 235 PM CDT Fri Aug 7 2026

A subtle shortwave trough is progressing across the Upper Midwest with a cold front lagging behind the wave and stretching from central MN to eastern NE. Ahead of the front, a complex of thunderstorms has developed in southern MN and northern IA which has started to turn towards the south as of recent radar trends. The expectation is for this thunderstorm complex to continue to dive southward across eastern IA this evening and gradually weaken as it does so. Therefore, dry conditions should prevail locally with partly cloudy skies and temperatures in the mid-80s this afternoon cooling into the upper 60s to around 70 overnight. However, with only modest capping being noted in recent amdar soundings and the 18z DVN Raob; there is a low chance that a few scattered showers or storms could drift into northwest IL later this evening. If this was to occur, the main concern with any showers/storms would be lightning and locally heavy downpours but some locally gusty winds cannot be fully ruled out. For now have opted to maintain a 20-30% chance for showers and thunderstorms through our forecast area tonight as a precaution.

By daybreak Saturday morning, any showers/storms that survive into our area should be eroded leaving us with another tranquil and seasonably warm summer day. Winds though will be out of the northwest behind the aforementioned front, but temperatures are still expected to top out in the mid-80s Saturday afternoon. Otherwise, expect mostly sunny skies through Saturday evening.

Heading into Saturday night, a building ridge of over the southwest CONUS will help to deepen a broad trough over the Upper Midwest and northern Great Lakes. This will in turn generate stout northwest flow over northern IL and northwest IN and allow several shortwaves to dive through the region Saturday night through at least the middle of next week. The first of these shortwaves is forecast to move through the region Saturday night into Sunday and lead to the development of a complex of thunderstorms somewhere near the SD/MN border. Given that the highest instability axis should be stretching from eastern IA southeastward into central IL, this complex should traverse somewhere near or west of this axis Saturday night and thus keep the bulk of any storms west of us. However, if this axis is able to shift east then the storm complex could brush our western counties late Saturday night into Sunday morning. The favored scenario right now is that the storms stay west of our area Saturday night, but have opted to keep some 30-40% POPs in the forecast for our western half to account for some lingering uncertainty in the exact storm track.

Regardless on how Saturday night goes, a warm front is expected to lift into northern IL and northwest IN on Sunday which will surge a warm, humid, and unstable air mass into the area. Couple this with the lingering ascent associated with the shortwave aloft and conditions look to be favorable for additional thunderstorms to develop Sunday afternoon and persist into the evening. While guidance is in good agreement on thunderstorms occurring Sunday, there continues to be some uncertainty in storm coverage and the exact timeframe storms will occur. That said, with the large amount of instability (3000-6000 J/kg) and 40-45 kts of deep layer shear any storms that develop on Sunday could become severe and contain a threat for damaging winds, hail, and maybe even a threat for tornadoes. With the aforementioned uncertainties the latest Day 3 outlook from SPC does have our area in a marginal (level 1 of 5) risk for this potential.

An additional period of thunderstorms are forecast for Monday as another shortwave pivots across the region. Similar to Sunday, the environment will be warm, humid, and unstable which will support a fair coverage of thunderstorms especially during the afternoon and evening hours. However, confidence on coverage and the exact timing of storms is low and will likely depend on how storms evolve on Sunday. Nevertheless, any storms that materialize on Monday will also have a chance to become severe and thus SPC has put our area in the 15% (slight risk) outlook for Monday.

Beyond Monday the forecast through the middle of next week looks to favor more periods of showers and thunderstorms as shortwaves continue to round the trough overhead. While there is a chance that as repeated rounds of storms occur that the outflows from storms push the better storm environment south of the area and give us a break, confidence on when or if this will occur remains low. So for now our official forecast will keep 40-60% chances for storms around through much of next week, but suspect changes to these will occur as trends present themselves. Otherwise, expect seasonably warm temperatures to prevail with highs in the 80s and overnight lows in the 60s.

Yack

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1233 AM CDT Sat Aug 8 2026

Key aviation forecast messages for the 06Z TAFs:

- Isolated SHRA/TS possible prior to daybreak, though probability is too low for TAF inclusion.

- Period of MVFR ceilings possible around/after daybreak.

- Lake breeze shifts winds to NE/E for GYY and MDW this afternoon. Less confidence in boundary reaching ORD.

Surface cold front stretches from northeast WI through eastern IA at issuance time. Modest warm/moist advection ahead of the front in association with a weak mid-level disturbance will maintain about a 20% chance of isolated SHRA/TS through about daybreak across the terminals, though too low for mention in TAFs. Will tactically amend if anything develops which might directly impact one of the airfields. Otherwise, the cold front will push east across the area early this morning, moving through the Chicago terminals after daybreak. While mainly VFR cloud layers are expected, there remains the potential for a short period of MVFR ceilings along/just behind the front this morning before cloud cover quickly diminishes.

Northwest surface winds around 10 kt are anticipated behind the front once the boundary layer mixes after sunrise, though high- res model guidance suggests winds over/near Lake Michigan will turn more north-northeast this afternoon. This is most likely to affect GYY by early afternoon, and likely MDW by mid-late afternoon as well. Current HRRR/RAP runs keep the boundary east of ORD through the afternoon, before eventually weakening and shifting winds south-southeast this evening at all sites.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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