textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms are expected at times through this evening. These storms will be capable of locally heavy rainfall which may lead to flooding, particularly if they stall or redevelop over areas hardest hit from last weekend. - The strongest storms will also be capable of gusty to locally damaging wind gusts this afternoon and evening, greatest threat south of I-80.

- Mainly dry and warmer weather is expected this weekend into early next week.

DISCUSSION

Issued at 342 AM CDT Thu Jul 9 2026

The earlier complex of thunderstorms approaching from eastern IA/southwest WI has mostly eroded with only a few areas of rain remaining. Not currently expecting any additional lightning through the remainder of the overnight hours with this activity but will still have to keep an eye on the remnant MCV moving into northwest Illinois but it should move out over the lake already by 7-8 AM. Earlier warm advective showers southwest of the metro managed to produce a few lightning strikes but have since decreased in coverage. Still can't rule out a few spotty showers south of the Kankakee River Valley through daybreak.

Attention then turns to a second MCV moving across central Iowa. This feature is expected to progress east toward the area through mid morning. Concerns are that this could lead to an earlier "first round" of isolated to scattered showers and thunderstorm development from mid morning through early afternoon (30-40 percent chance). Forecast soundings show a largely uncapped environment here locally already after 9-10 AM so any forcing for ascent (such as with an MCV) would support thunderstorm development. Model guidance has been struggling with handling these smaller scale mesoscale features and have based the first 12 hours of the forecast largely on radar, satellite, and observational trends.

As the aforementioned MCV begins to shift southeast out of the area toward mid-late afternoon, additional thunderstorm development may then occur within the vicinity of a residual outflow boundary or convergence axis situated across northern and northeast Illinois. Exactly where this boundary ends up as well as what the coverage of showers/storms will be remains lower confidence, at least initially given a potentially slower arrival of the parent mid-level vort/wave (which is still out over northern NE/southern SD). Suspect that coverage may remain more isolated to widely scattered during the afternoon until the wave arrives leading to increasing coverage during the evening (potentially still over portions of the Chicago metro) before gradually shifting south and east and out of the area overnight.

The primary hazard with any thunderstorms today (from mid morning through this evening) will be localized heavy rainfall. PWATs in the upper end of climatology (2"+) will be supportive of instantaneous rainfall rates in excess of 1-2"/hr. Given the increased susceptibility for flooding in many areas after last weekend's multiple rounds of heavy rainfall, will continue to message the localized threat of flooding. In spite of generally weaker shear, precip loaded downbursts will remain possible with the strongest storms which could produce strong to locally damaging wind gusts (40-60 mph). The greatest threat (and level 1 out of 5 threat for severe weather) is greatest for areas well south of I-80 but can't fully rule it out areawide.

In the wake of our showers and storms a backdoor cold front is expected to move inland off Lake Michigan late in the evening. This may bring a period of fog and/or very low clouds across much of the metro through early Friday morning, some of which could end up locally dense. Something to keep an eye on with later forecasts.

Looking ahead to Friday, additional convectively augmented shortwaves are expected to move across the Mid-Mississippi and Ohio Valley regions. Think that the majority of the shower and storm coverage with these features will remain largely south of the area but have maintained 20-40% chances for areas south of a Peru, IL to Rensselaer, IN line in case they end up a bit farther north.

While a few diurnally driven showers can't be ruled out on Saturday, upper level ridging expands across the central CONUS over the weekend into early next week which should lead to mainly dry conditions into the first part of next week with temperatures in the upper 80s to around 90. Given we will be on the northeast periphery of the ridge we will have to monitor the potential for thunderstorm complexes rounding the ridge across the broader region toward midweek.

Petr

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 642 AM CDT Thu Jul 9 2026

Forecast concerns for the terminals...

- 20-30% chance for isolated to widely scattered showers and perhaps an isolated storm this morning.

- Scattered showers and thunderstorms expected this afternoon and evening which will be capable of locally heavy rainfall resulting in reduced visibilities.

- MVFR ceilings to develop late this evening and linger through Friday morning. Fog is also possible after midnight and could become locally dense especially near Lake Michigan.

A convectively enhanced low pressure center is drifting across eastern IA this morning and has started to develop some isolated showers along its eastern periphery. This feature is expected to continue eastward across northern IL and eventually into northwest IN through midday and may continue to produce isolated to widely scattered shower and perhaps even a thunderstorm (15-20% chance of a storm) as it does so. While the environment is supportive of showers/storms, this forecast is lower confidence since coverage of showers/storms will be more hit and miss. That said, opted to introduce PROB30s for SHRA into the TAFs for a couple hours this morning to account for this potential.

Heading into this afternoon, any morning showers/storms will exit the area and likely give us a 1-3 hour break in the weather before additional showers and storms start to develop along an outflow boundary that is forecast to be draped over northeast IL. The afternoon shower/storms should be of a higher coverage, but timing as to when exactly things will initiate makes this forecast too a bit lower confidence. Despite the low to medium confidence in timing, have introduced VCTS starting in the early afternoon through mid-evening to account for the uncertainty in timing with more targeted TEMPOs for the 3-4 hours confidence is highest in storms occurring at each terminal. Given the humid air mass in place, storms today will be capable of locally heavy rainfall which will lead to reduced visibilities in the IFR range. There is also the potential for locally gusty winds in the strongest storms where gusts in excess of 30-40 kts could occur.

Showers and thunderstorms should gradually push south and east of the terminals through the evening with most sites starting to dry out around 03z. However, there is some indication that some showers could linger through midnight (or even later). Behind the shower/storms, ceilings will lower to MVFR where they are expected to remain through the rest of the TAF period. In fact some IFR ceilings cannot be ruled out especially near Lake Michigan. Additionally, fog is forecast to develop over the lake this evening and ooze inland overnight some of which may become locally dense. With the uncertainty in how late showers/storms will linger, confidence is lower on how dense fog will become so have opted to keep 6SM BR going in the TAFs for now. Any fog that does develop tonight will erode shortly after daybreak on Friday.

Finally, winds today will generally be out of the west-southwest with speeds around 10 kts but directions will likely be variable in and around thunderstorms. Winds will become northeasterly this evening as the aforementioned outflow moves through and will remain northeast through the rest of the period with lighter speeds around 5-7 kts.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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