Marine Weather and Tide Forecast for Winthrop Harbor, IL

DISCUSSION
Issued at 313 AM CDT Tue Aug 4 2026

Seasonably strong west-southwesterly mid and upper-level flow will overspread much of the Midwest into the Great Lakes tonight into Wednesday as a large upper trough slides eastward across the north central CONUS. As it does, a north-northwest to south- southwest oriented surface cold front will gradually shift southeast from the Upper Midwest today, into the Great Lakes on Wednesday. This front will be the focus for a few rounds of showers and thunderstorms through the midweek period as forcing is augmented by several smaller scale impulses tracking east- northwestward along it. The threat for a few strong to severe storms cannot be ruled out in our area, particularly on Wednesday as the front shifts into IL and IN. However, potentially of more concern is the threat for an episode or two of very heavy rain producing showers and storms in and near our area given the axis of deep rich moisture pooling along the front (precipitable water values near 2").

There continues to be uncertainty with the exact timing of these waves of showers and storms in our area, owing largely to the timing of, and extent of convective enhancement to the individual smaller scale impulses translating through the larger scale trough. However, there continues to be a signal for convection to develop with one such wave near the frontal boundary later today into this evening west of our area across IA. This activity may then shift east-northeastward into parts of northern IL tonight into early Wednesday morning, though likely in a weakening state. Nevertheless, lingering cloud cover and rain showers Wednesday morning over parts of northern IL could keep afternoon heating/destabilization tampered.
Accordingly, this could potentially impact the favored placement for the next round of convection.

Another round of developing convection is probable into Wednesday evening/night in association with another impulse forecast to sweep eastward across the Mississippi Valley into the lower Great Lakes. If cloud cover and lingering rain from any early morning convection linger through the afternoon and tamper destabilization enough, it remains plausible that this next round of convection could become favored farther south (potentially south of I-88), though how far south this may be is unclear at this time. Nevertheless, wherever this evening round of showers and thunderstorms sets up, very heavy rainfall and possible flash flooding will be a concern, particularly if storms train northeastward over the same areas along and north of the frontal boundary.

While the Wednesday night convection could push the effective front south of the area into Thursday, forecast guidance suggests that another impulse will track into the region Thursday afternoon. If this wave is strong enough, it could result in the front lifting back northward and stalling over portions of my CWA If this occurs, an additional wave of heavy rain producing convection would be possible Thursday afternoon and evening. Confidence on this is low at this time, however.

Mainly dry and seasonably warm weather is expected late in the week through potentially most of the upcoming weekend. However, ensemble guidance favors the Hudson Bay low/trough to remain a dominant feature through the weekend before ejecting south- southeastward into the upper Great Upper Lakes early next week.
Accordingly, the presence of broad upper troughing across the Great Lakes amid subtropical ridging across the southwestern CONUS will support a belt of strong northwesterly mid and upper- level flow atop another southward shifting surface cold frontal boundary early next week. Building instability in the vicinity of a boundary will thus support increasing chances for convective episodes (some possible severe with more heavy rain)
in the area early next week.

KJB

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/
Issued at 1244 PM CDT Tue Aug 4 2026

Key messages for the 18Z TAF period:

* A system of showers and embedded thunderstorms progged for Wednesday morning, with another round possible in the late afternoon/evening

* Periods of MVFR possible with the rain on Wednesday, especially at RFD

SW winds up to around 10 kt this afternoon will subside for the night. A system of showers with possible embedded thunderstorms will work across northern IL on Wednesday morning. Favored time of arrival is after 09Z at RFD and after 12Z at the Chicago sites. Periods of MVFR vsby will be possible with the showers. A break in precip coverage is expected during the early part of the afternoon before a second round arrives during the late afternoon into evening. Additional thunderstorms and periods of MVFR will be possible.

At RFD, the favored signal is for MVFR cigs to settle overhead with the morning precip and remain through the afternoon leading up to the next round. MVFR cigs may find their way over Chicagoland as well, but confidence there is lower.

Looking ahead, showers and a couple of storms may linger through the night and into early Thursday with a signal for low stratus and fog possibly developing Wednesday night.

Doom

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.