Marine Weather and Tide Forecast for Zion, IL

DISCUSSION
Issued at 251 PM CDT Fri May 15 2026

Through Tonight:

Into this evening, it will be breezy and pleasant with temperatures gradually falling from the 70s into the 60s.

Our main focus tonight is the likelihood of a weakening MCS (showers and embedded storms) to move across the area overnight tonight, with gusty outflow on its leading edge. Thunderstorms initiating by evening from the Omaha area to western Wisconsin will likely evolve into one or more linear clusters. Given the dry and mixed profiles today, not expecting any surface based instability overnight. On the other hand, a MUCAPE reservoir of up to 1500-2000 J/kg will expand across the MS River in tandem with a stout EML, which may maintain sufficient organization in the MCS (albeit still on an overall weakening trend due to unfavorable diurnal timing) to yield some strong to locally damaging wind gusts on the leading edge outflow boundary. The best chance for this appears to be in the northwest 1/3 or so of the CWA (ie. near and west of a Waukegan to Peru IL line). A relatively greater wind threat may present itself just north of the IL/WI state line due to the west-southwesterly flow orientation aloft.

Since the complex will be rooted above the boundary layer, effective bulk shear will be fairly modest. Can't completely rule out isolated large hail in any more robust cores behind the outflow if effective shear ends up a bit stronger than anticipated.

Saturday through Friday:

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 probably southern portions of the forecast area (though the exact position is of low confidence from current guidance runs). A trailing weaker convectively modulated short-wave and lingering warm air advection/ascent may keep scattered showers and isolated embedded thunderstorms going past daybreak into our southeast 1/3 to 1/2 of the CWA or so.

Into Saturday afternoon, subsidence in the wake of the morning short-wave(s) entail nebulous at best forcing mechanisms. Have maintained some chance pops (30-50%) mainly south of I-80, where a bit better potential of additional diurnal showers/isolated 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/around 80F east/far south to the low- mid 80s farther north/west would be attainable with some afternoon sunshine. As an additional minor note on temperatures, a weak cold front may slip into far northern Illinois and enable an onshore wind shift and more quickly cooling temps towards sunset near the far northeast Illinois shore.

Forcing mechanisms for any additional convection remain nebulous Saturday night, as forecast mid-level height rises cast uncertainty on any areas of focus. A northeastward propagating short-wave may conceivably support isolated to perhaps widely scattered showers and storms (20-40% PoPs/highest near and west of I-39) as the stalled front/outflow boundary lifts back north as a warm front. A broad level 1 of 5 severe threat in the current outlook may be a bit aggressive for the progged setup, (decreasing mid-level lapse rates, sub marginal effective bulk shear, and an again unfavorable diurnal timing).

Most recent guidance continues to indicate 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 (favoring near/north of I-88) departing and decreasing cloud cover in the warm sector, high temperatures in the mid-upper 80s appear likely at this distance. The exception may be near the Lake County IL shore, as a just east of south component to the winds could cap highs in the 70s later in the day. Breezy and mild conditions look to persist Sunday night, with lows generally in the mid to upper 60s.

Thunderstorm chances then return Monday and Tuesday, as a Central Plains surface low pressure 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 southerly 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 (as well as some threat for flooding given high column moisture), though forcing mechanisms remain unclear within the broad warm sector making convective timing/evolution of low confidence at this distance. Monday's overall messy look also lends uncertainty to how much instability can be realized at peak heating, particularly if morning activity ends up fairly widespread.

On Tuesday, in the wake of probable continued overnight/early day convection, the aforementioned cold front will approach.
It's not uncommon in these setups for renewed storms and any associated strong to severe threat ahead of the front to focus primarily near/southeast of I-55, though time will tell in this regard. For the Monday-Tuesday period, SPC days 4-5 outlooks continue to extend severe probabilities into our area, which appear reasonable. 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.

Castro/Ratzer

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/
Issued at 630 PM CDT Fri May 15 2026

Key Aviation Messages for the current TAF period:

- Showers and embedded thunderstorms are expected overnight prior to daybreak at area terminals. Lingering showers over the region through mid morning. Locally damaging winds are possible with the strongest storms.

- Lower confidence on wind direction trends in the wake of the thunderstorm threat

VFR conditions under mostly cloud-free skies are expected for the remainder of the evening. Winds remain out of the south with isolated gusts around 20 knots. Wind gusts should continue to diminish through the evening.

Showers and storms have already developed over western Iowa, southern Minnesota and northwest Missouri. Those storms are expected to track eastward toward the Chicago Metro Area overnight. Current projections has it arriving over KRFD around or just after midnight, before progressing to Chicago in the early morning hours. With increasing confidence in the threat for storms, the PROB30 was converted to a TEMPO with refined impact timing. Localized damaging winds may be possible with stronger storms (higher potential around and near KRFD) with a sharp change in wind direction from the southwest to the northwest as the storms pass by.

As the storms depart to the east/southeast, lingering showers are expected through mid-morning. Winds will become more variable in the wake of the storms, lowering confidence in trends. Models continue to suggest a brief period of southeasterly winds (prior to 15Z) before becoming southwesterly through most of tomorrow. With lingering moisture available, there is a non-zero chance for a shower in the afternoon, but with weak forcing confidence was too low for any precipitation mention in the TAF tomorrow afternoon. Winds will eventually diminish after 00Z and become more southeasterly.

DK

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.