Marine Weather and Tide Forecast for Zion, IL

DISCUSSION
Issued at 303 AM CDT Mon Apr 21 2025

Through Tuesday:

A 1001 mb sfc low was located over western WI at 08z with cold front trailing south along the Mississippi River between IL and IA, then southwest across Missouri. This low and the associated vigorous upper trough will move northeastward into Ontario by later this afternoon with trailing cold front moving east of our CWA by mid morning.

Cooler and less humid air will filter into the area today in the wake of the front. Post frontal stratus should keep skies pretty cloudy through midday followed by gradual clearing from the southwest this afternoon. Gusty westerly winds are expected today, especially this morning when a few gusts of 35 to 40 mph are possible. Winds will ever so slowly begin to ease this afternoon, followed by a much more rapid decrease in winds toward evening.

Narrow surface ridge axis will move across the region tonight with southerly flow becoming reestablished on the backside of the ridge Tuesday. The southerly flow should allow temps to rebound nicely with highs mostly in the 70s, though a south southeast synoptic gradient should allow a lake breeze to penetrate into northeast IL Tuesday afternoon resulting in cooler conditions there.

A transition to a zonal mid-upper level flow is expected Tuesday with very low amplitude perturbations in the flow and low level theta-e advection leading to some potential for showers and thunderstorms. Unfortunately, it isn't uncommon for guidance to struggle with low amplitude shortwaves like this, which is resulting a wide range of solutions with regards to placement and timing of potential convection Tuesday/Tuesday night. Cannot rule out some dissipating elevated convection making it into our northern CWA Tuesday morning, so have some slight chance pops for that. Developing surface front could also become a possible focus for afternoon convection over southern WI or far northern IL, but confidence remains low, so stuck with just low-mid range chance pops in our northern tier or two of counties.

- Izzi

Tuesday Night through Sunday:

The previously advertised zonal (west-east oriented) upper pattern is expected to be in place by Tuesday night and will persist through the end of the week. Therefore, mostly dry conditions and above normal temperatures can be expected each day with highs in the 70s (possibly near 80 in spots Wednesday and Thursday) inland of Lake Michigan. However, there will be a couple periods where chances for showers and thunderstorms can be expected.

The first period of possible rain is expect to occur Tuesday night into Wednesday morning as a shortwave trough pivots across the northern Plains. At the surface a stalled frontal boundary is forecast to be in place from northeast KS to southern WI which will be the focus for showers and thunderstorms during this time. Since the rain will be tied to the boundary, it appears that only the northern third to half of our area will actually see precipitation while areas south of generally I-80 remain dry. That said, the nearly stationary rain would normally pose a concern for flooding, but the generally dry soils should be able to soak up the 0.25-0.5 inch amounts and limit the flooding threat. While showers should taper by mid-morning on Wednesday, some guidance tries to redevelop showers and storms Wednesday afternoon along the lingering boundary. Though, the dry lower atmosphere and lack of broad forcing should limit the coverage of showers/storms and keep the bulk of this activity more into northwest IL and near the IL-WI line.

The second and arguably better chance for rain is forecast to occur during the Thursday night through Friday night timeframe as a broader trough swings through the region. While guidance remains in generally good agreement on rain occurring, there remains a fair amount of uncertainty in how the system will evolve, particularly in regards to a southern stream shortwave and its associated surface low. In short, if the southern wave and low are stronger, then rain coverage will likely be more widespread with higher amounts as opposed to the weaker solutions. For now saw no reason to deviate from the 50-70% POPs offered by the NBM but suspect further refinement will be needed as we get closer. Regardless of the evolution, rain should taper on Saturday leaving us with a mostly dry weekend.
Though, temperatures behind this system are forecast to be notably cooler with highs back into the 50s and 60s for the weekend.

Yack

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/
Issued at 1124 PM CDT Sun Apr 20 2025

Aviation forecast concerns for the 06Z TAFs:

- Period of SHRA (~10% chance of brief TS) ends by 07Z-08Z.

- MVFR ceilings develop predawn KRFD, by sunrise CHI. Brief IFR possible at KRFD.

- Breezy SSE winds becoming WSW predawn. Gusts 30-35 kt possible through early afternoon, then easing.

Surface low pressure was over northeast IA late this evening, with an occluding warm front extending southeast across northern IL. This front will move east of the terminals during the predawn hours, allowing breezy SSE winds to shift SW and eventually WSW later in the morning with gusts increasing into the 30-35 kt range. Gusts of this magnitude will persist into early afternoon before gradually easing, more quickly with the approach of sunset. Weak surface high pressure then drifts across the area Monday night, with light/variable winds.

A band of SHRA and a few isolated TS will shift east of the terminals within 1-2 hours of TAF issuance. While a few spotty SHRA can't be ruled out through sunrise, it should be mainly dry. MVFR ceiling development is expected however, with a period of IFR possible at KRFD. MVFR ceilings will persist through much of the day, though should gradually rise this afternoon and eventually scatter out by early Monday evening.

Ratzer

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