textproduct: Chicago

KEY MESSAGES

- Additional periods of storms are expected at times through mid-week. Some of these storms could be severe and lead to additional flooding.

DISCUSSION

Issued at 300 AM CDT Mon Aug 10 2026

Forecast remains complex through mid-week, as a nearly stationary frontal boundary lingers across the forecast area beneath an active west-northwest flow pattern aloft. Periodic shower and thunderstorm chances will persist with somewhat low confidence in timing and coverage, ultimately modulated by individual short waves and daily diurnal heating and instability trends. Episodes of severe weather and locally heavy rainfall remain possible.

In the near-term, latest surface analysis depicts a nearly stationary frontal boundary stretching from an area of low pressure over western KS northeast into western IA, then east along roughly the I-80 corridor across northern IL/northwest IN. A very warm/humid airmass is in place along/south of the boundary, characterized by temps in the upper 70/around 80 and dew points in the mid-70s at this late night hour. Aloft, seasonably fast west-northwest flow was in place across the region, supplying a track for a series of mid-level short wave disturbances of varying amplitude. One such wave is currently tracking from IA into northern IL, supporting a southwesterly 35 kt low-level jet atop the low-level frontal zone into northeast IL/northwest IN. Several clusters of strong thunderstorms had developed in the enhanced warm/moist advection regime associated with this jet last evening, with a small MCS remaining across our southern tier of cwa counties as of 2 AM on the nose of the jet in the vicinity of the surface frontal boundary. Given ML/MUCAPE values in the 3000-3500 J/kg range, 35-40 kts of effective shear and precipitable water values near 2 inches, severe weather and locally heavy rainfall remain a concern with these storms for the next hour or so, before they eventually slide southeast of the area toward/just after daybreak as the low-level jet gradually veers more westerly and weakens with the passage of the mid-level wave.

Once these storms exit the area, there should be a bit of a lull in convection in the wake of the departing short wave and during our diurnal instability minimum. Can't completely rule out some isolated elevated shower or thunderstorm development however, due to the presence of the front/baroclinic boundary and weaker but still persistent warm/moist advection. Chances should increase again at least a bit toward midday and into this afternoon however, as another weak wave or MCV (currently approaching the Omaha vicinity) tracks east during a more favorable diurnal time. CAM guidance loosely depicts this, though differences in coverage and timing details are evident. Additional development also appears possible later this evening, as the nocturnal low-level jet modestly increases atop the frontal zone. There does appear to be a somewhat consistent signal however, amongst both CAM and global guidance, for an MCS to develop over NE/SD later tonight which would approach the forecast area around/after daybreak Tuesday. This supports the highest pops (60-70%) during Tuesday, and have limited other mentions to just chance (20-40%) later today and tonight. Given otherwise similar instability/shear and moisture parameters, we remain in SPC/WPC slight risks for severe weather and excessive rainfall during these periods.

No significant change in the pattern is evident into mid-week (Wednesday, possibly Thursday), though confidence gradually decreases based on impacts of any preceding convective/MCS outflow impacts. Global ensembles continue to show a signal for our frontal boundary to shift a little farther south Thursday night and Friday, which may decrease our shower/thunderstorm potential briefly late in the week, though this signal is weaker than earlier runs and there are indications that the front may lift back into the area next weekend. Thus, it appears that our seasonably warm/humid and periodically stormy pattern may persist for much of the forecast period.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 707 AM CDT Mon Aug 10 2026

Key Messages:

- MVFR ceilings (initially IFR at RFD) likely to persist through this morning.

- Thunderstorms are likely to affect the terminals late this afternoon into tonight. Some potential exists for a prolonged period of TSRA impacts at/near the terminals during this time frame.

- Another round of thunderstorms is likely towards the end of the current 30-hour TAF period for ORD and MDW.

MVFR stratus has developed across northern Illinois and northwest Indiana and is likely to persist at the terminals through most, if not all, of this morning. Additionally, IFR ceilings and MVFR visibilities are being observed at RFD at press time, but these conditions are expected to improve by mid- morning.

Another round of thunderstorms is expected to develop in the area late this afternoon into this evening as a convectively- enhanced disturbance originating from the central Plains moves into the area. Confidence in the timing and location of this thunderstorm threat has increased since the last TAF issuance, and have introduced VCTS mentions and TEMPO groups for TSRA at the Chicago metro terminals for the late afternoon through early evening time frame. Maintained a PROB30 group for this time frame at RFD as it appears that the main surface boundary that convection will initially focus along should remain south of RFD, thus resulting in lower confidence in storms occurring near RFD during this time window.

Additional thunderstorms may develop later this evening into the overnight hours as another upper-level disturbance enters the region and a low-level jet provides ascent over the top of outflow boundaries left behind by the earlier storms. Exactly where these boundaries and storms will focus remains uncertain, but have this potential highlighted in PROB30 groups for all of our TAF sites. Wherever these storms do develop, it is possible that they will train over the same locations and result in prolonged thunderstorm impacts for several hours. Any storms, either with this round or the earlier round, will have the potential to produce strong downburst winds, erratic wind shifts, hail, and sharply reduced visibilities.

One additional round of storms is likely to occur sometime tomorrow morning through the early-mid afternoon, so right towards the end of the current 30-hour TAF period for ORD and MDW. Given the existing uncertainties at this time over timing and exactly where these storms will track, opted for PROB30 groups for TSRA at the very end of the ORD and MDW TAFs, but some refinement to this will likely be needed as forecast and observational trends become clearer. In the worst case, a mature line or bow may plow through the local airspace tomorrow and produce severe to potentially significant severe wind gusts, but it is not yet clear whether that will happen.

Lastly, confidence in wind direction trends for today is fairly low, partly due to potential thunderstorm influences and partly because synoptic flow will be fairly light. A weak lake breeze does look like it will result in easterly winds being observed at ORD and MDW and northeasterly winds at GYY this afternoon.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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