Marine Weather and Tide Forecast for Winthrop Harbor, IL

DISCUSSION
Issued at 336 AM CDT Fri May 15 2026

Early morning GOES vapor imagery depicts an upper level trough extending from a closed low over Manitoba southward across the upper Midwest. Deep (sub-990 mb) surface low pressure was stacked beneath the mid-level circulation, with a cold front trailing south-southwest across the Plains to another area of low pressure over southeastern CO. Broad southerly low-level flow was being induced downstream of the upper trough and enhanced mid-upper westerly flow aloft, with a 35-40 kt 850 mb low level jet oriented from OK and eastern KS into southeast IA.
Associated isentropic ascent into northern IL has resulted in saturation on the 305-310K surfaces near the base of 6.5-7 C/km mid-level lapse rates, allowing development of elevated convective showers/thunderstorms within weak (<200 J/kg) of MUCAPE which decreases with eastward extent per RAP soundings and mesoanalysis fields. More favorable conditions for elevated convection were located farther southwest of the forecast area across northeastern MO, where the combination of a subtle, compact mid-level short wave, greater elevated instability and stronger convergence on the nose of the 850 mb jet was evident in the development of scattered thunderstorms. High-res CAMs indicate these storms will spread east across central IL through daybreak. Gradual veering of the low level jet looks to keep the bulk of this activity south of the WFO Chicago cwa, though areas south of the Illinois/Kankakee river valleys will likely see some showers and potentially isolated thunder along the northern periphery this morning.

After the passage of these morning short wave trough axes, relatively quiet weather conditions appear in store for the forecast area the remainder of the day through this evening.
Increasing south-southwest flow ahead of the Plains cold frontal trough and surface low will support modest but persistent warm advection, which combined with decreasing cloud cover by this afternoon should support highs from the low 70s in the east to the mid-upper 70s west. South winds gusting 20-25 mph should prevent significant lake cooling, except along the immediate IL shore north of Chicago where a slight southeast wind component is expected. Breezy winds will persist into the evening, with temps slowly falling in the 60s.

Thunderstorm chances will increase west of the Mississippi River this afternoon and evening where daytime heating, returning low level moisture and convergence along the cold frontal boundary will be enhanced by cooling aloft and increasing mid-level winds associated with another mid-level short wave tracking east across the region. Thunderstorm which develop across parts of IA and southeast MN late this afternoon and early evening may evolve into one or more linear clusters, which would eventually move into northern IL toward/after midnight tonight. While surface-based instability will be weaker with eastern extent and loss of diurnal heating overnight, guidance suggests sufficient MUCAPE (~2000 J/kg) and deep-layer shear (~35 kts) to maintain sufficient storm organization to present a wind/marginal hail threat especially into our northern IL counties before decreasing with further eastward extent. SPC's new Day 1 outlook appropriately has brought the marginal (level 1 of 5) risk into the Chicago metro area and the slight (level 2 of 5) near the I-39 corridor for overnight tonight.

By Saturday morning the primary MCS from tonight is expected to move east of the area, though a combined outflow/cold frontal boundary is forecast to become nearly stationary across a portion of the forecast area (though the exact position is of low confidence from current guidance runs). Lingering warm advection/ascent may be ongoing atop the low-level boundary Saturday morning, which may keep the threat of showers and a few thunderstorms around especially across the southeastern cwa.
Have limited NBM blended pops but maintained some chance pops (30-50%) mainly south of I-80 Saturday afternoon, where better potential of additional diurnal showers/storms would more likely be focused near/south of the stalled boundary. Potential for lingering cloud cover and outflow-disturbed wind field could limit high temperatures somewhat, though current model low-level thermal fields suggest afternoon highs from near 80 east/far south to the low-mid 80s farther north/west would be attainable with some afternoon sunshine.

Guidance is in good overall agreement in rising heights across the western Great Lakes region late Saturday/Saturday evening as the upper pattern begins to evolve into a western-CONUS long wave upper trough and a downstream upper ridge developing downstream to the east. This induces renewed surface low development over the High Plains, with a corresponding increase in southerly return flow into the Mississippi Valley and Great Lakes region. Also depicted is a northeastward-propagating short wave later Saturday night into early Sunday which may support additional shower/thunderstorm development with the stalled front/outflow boundary lifting back north as a warm front.
Again, diurnally less-favorable timing would tend to limit surface-based instability, though guidance forecasts of 1500-2000 J/kg of MUCAPE and 30-35 kts of deep-layer shear could support storm organization and a marginal wind threat with stronger storms. SPC Day 2 outlook includes our area in a level 1 of 5 (marginal) risk for Saturday night/early Sunday.

Current guidance indicates breezy (30-35 mph gusts) south winds will develop Sunday afternoon, after the warm front lifts north of the area. With any morning showers/storms departing and decreasing cloud cover in the warm sector, high temperatures in the mid-upper 80s appear likely at this distance. Breezy and mild conditions look to persist Sunday night, with lows generally in the mid-60s.

Thunderstorm chances then return Monday and Tuesday, as the Central Plains surface low slowly lifts northeast through the northern Great Lakes and eventually trails a cold front through the forecast area. Ahead of the cold front, the forecast area will reside in breezy warm south flow, characterized by daytime temperatures in the 80s and increasing low-level moisture return (surface dew points potentially upper 60s-to near 70F) from the western Gulf. Resulting instability, combined with mid-level flow near 40 kts would potentially support strong/severe thunderstorms, though forcing mechanisms remain unclear within the broad warm sector making convective timing/evolution of low confidence at this distance. SPC Days 4-5 outlooks continue to extend severe probabilities into our area. Quiet and cooler conditions are then expected by mid-week, as surface high pressure builds across the region in the wake of the Tuesday cold frontal passage.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/
Issued at 1213 PM CDT Fri May 15 2026

Gusty S/SSW winds (occasionally up to 30 kts) will continue this afternoon before easing this evening. Wind directions may trend more towards 170 at the Chicago-area terminals this evening.

A thunderstorm complex will likely track out of Iowa late this evening and overnight. This should gradually weaken with eastward extent, but has the potential to produce a period of strong wind gusts--particularly INVOF RFD. Have converted the PROB30 at RFD to a TEMPO group given higher confidence in timing and thunderstorm coverage. At the Chicago-area terminals, confidence is a bit lower, with an increased potential for outflow to work its way ahead of thunderstorms leading to a gradual weakening trend. Suspect we will need to convert to TEMPOs eventually as timing confidence increases.

In the wake of convection, winds could turn SEly for a period before turning SW through Saturday morning. The chance for additional thunderstorms Saturday afternoon is too low for a mention in the current extended ORD/MDW TAFs.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for the IL and IN nearshore waters.