textproduct: Chicago

KEY MESSAGES

- Low chance (20-30%) for scattered showers and thunderstorms tonight, mainly across northwest IL. If storms make it into the area, then locally gusty winds and heavy downpours would be possible.

- 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 /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1253 PM CDT Fri Aug 7 2026

Aviation forecast overview for 18Z:

- Diurnal Cu field expected to return this afternoon across the region.

- Frontal boundary expected to drop into the region overnight, monitoring trends for potential showers in KRFD area this evening ahead of the front and MVFR ceilings area-wide with the front.

- Southerly winds turn northwesterly through the period, with a potential lake breeze developing Saturday afternoon.

South-southwest winds this afternoon will continue to mix sfc moisture into the low levels. This will expand the diurnal cu field that is currently situated north and west of the metro terminals. Expecting SCT-BKN, VFR ceilings for majority of this afternoon ahead of an approaching frontal boundary. Storms currently located across southeast Minnesota are forecasted to drop south along the axis of greatest instability near the Mississippi River Valley this evening. Decided to continue the PROB30 group for RFD this evening, due to close proximity to the axis. The frontal boundary is then expected to track through the region early Saturday morning, bringing lowered ceilings...however dry conditions are anticipated as the front moves through.

Southerly winds, less than 10kts, will continue this afternoon into the overnight before veering more westerly early Saturday morning. After fropa Saturday morning, winds become northwesterly by late morning, lasting through the afternoon. Model guidance is suggesting a lake breeze develops midday Saturday, impacting GYY and then MDW by mid-afternoon. Uncertainty exists with the westward extent of the lake breeze, therefore left mention out of ORD at this time.

Baker / Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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