Marine Weather and Tide Forecast for Winthrop Harbor, IL

DISCUSSION
Issued at 248 PM CDT Sat Jun 14 2025

Through Sunday Night:

Northern Illinois and northwestern Indiana are on the northern periphery of a slow-moving upper-level trough drifting east- northeastward along the Ohio River beneath a strengthening ridge along the US/Canadian border. Closer to the surface, a broad high pressure system is centered over southern Ontario, leading to a slight onshore breeze around the Lake Michigan basin. The near annual maximum sun angle quickly eroded the overcast skies that started this morning, giving way to mostly sunny to partly cloudy skies. Aside from somewhat humid air lingering in the region, it's a fairly nice mid June afternoon out there.

Tonight, the relatively relaxed surface pressure gradient along the southern flank of the surface high to our north will lead to light winds. Decreasing humidity levels (thanks for incoming air from Lower Michigan) will set the stage for a comfortable night with lows falling into the upper 50s to around 60.

Tomorrow, a couple of convectively invigorated upper-level shortwaves will meander from the northern Plains and into the Midwest. With Canadian surface high continuing to feed a steady supply of dry low- to mid-level air into the Lower Great Lakes region, do have to think that any convection or convective cluster that attempts to work southeastward toward northern Illinois will struggle and ultimately decay. All things considered, opted to lower the inherited 20% PoPs to 10% and convert mentionable rain to sprinkles, altogether along and west of I-39 tomorrow. Otherwise, tomorrow is shaping up to be another pleasant mid-June day with partly cloudy skies. Lower humidity levels tomorrow should allow for highs to be some 5 degrees warmer than today and in the low to mid 80s. The exception will be along the Lake Michigan shoreline where onshore flow will hold temperatures in the upper 60s.

Borchardt

Monday through Saturday:

As we approach Friday's summer solstice, the weather will certainly be summer-like this upcoming workweek (very warm and humid with occasional rounds of storms), likely followed by hot and humid conditions next weekend. The main timeframe of concern from a convective standpoint is Tuesday into Wednesday.

The week will start on a mainly dry note with shallow moisture progged and very weak flow. A weak remnant MCV crossing the MS River late in the day could kick off some isolated diurnal pulse-type convection primarily to the west of I-39. Highs will reach the mid to upper 80s inland, with moderately humid feeling dew points (60s). A lake breeze will keep highs near the lake in the 70s, or perhaps lower on the immediate shore.

On Tuesday, thunderstorm chances will uptick in the afternoon- evening (30-50% PoPs) as another MCV or convectively modulated mid- level impulse progresses eastward. Marginal deep layer shear suggests generally pulse-type to perhaps loosely organized multi-cells with downpours, localized gusty winds, and small hail. Highs will be in the upper 80s to around 90F, with ~65-70F dew points yielding peak heat indices in the low-mid 90s.

Later Tuesday night, there's a rather strong signal for a MCS moving across the mid-MS Valley. Overnight timing is on the late side to support an appreciable severe threat given weakening mid-level flow (marginal bulk shear) locally. However, PWATs pushing near 2" (150-200% of normal) and modest steering flow may result in a localized flash flooding threat if thunderstorms aren't on a rapid decaying trend.

The signal for severe storms on Wednesday has become a bit less apparent on the last couple model cycles. Ongoing convection from the overnight MCS and resulting debris cloud may delay/limit stronger destabilization. In addition, while some previous cycles suggested a stronger, more amplified trough ejection across the region, recent runs have trended toward a weaker, positively tilted trough and weaker surface low over the area. There may even be a play for suppressed convective coverage during the afternoon if subsidence is strong enough behind the morning MCS. With all this being said, we'll still need to monitor this period closely. Deep layer shear of 35 kts or more and the presence of frontal boundaries and remnant outflow could yield a severe threat contingent upon sufficient destabilization.

Very dry mid-level air and height rises in the wake of Wednesday night's cold front passage will result in primarily dry conditions Thursday-Friday. Thursday's highs will be near seasonable in the lower 80s, along with lower dew points/less humid conditions. A warm front will lift north on Friday, bringing a return to very warm (mid-upper 80s) and humid conditions. More amplified mid-level ridging over the eastern US next weekend (500 mb heights in the 590s DaM) will carry 850 mb temps into the lower +20s Celsius and 925 mb temps in the mid- upper 20s C, supporting highs around 90F/lower 90s on Saturday and likely even warmer on Sunday. Heat indices could flirt with 100F amidst dew points in the 70s. If the more amplified ridging comes to fruition, convective chances will likely be minimal due to strong capping.

Castro

AVIATION /18Z TAFS THROUGH 00Z MONDAY/
Issued at 1237 PM CDT Sat Jun 14 2025

Only forecast concern is mvfr cigs early this afternoon. Mvfr cigs across the Chicago terminals and areas south of the terminals will lift to low vfr over the next few hours and also begin to scatter out, dissipating this evening. Increasing high level clouds are expected tonight into Sunday.

Northeast winds around 10kt will diminish this evening and are expected to become light and variable across most of the area tonight, becoming northeast again Sunday morning.

There is a small chance for a few showers across northwest IL and RFD Sunday morning. Confidence is too low to include any mention with this forecast. cms

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