Marine Weather and Tide Forecast for Chicago, IL

DISCUSSION
Issued at 323 AM CDT Wed Apr 30 2025

Through Thursday:

A mid/upper-level trough will eject out of the Desert Southwest this morning, then drive an attendant surface low east- northeastward across the Southern Plains into the Ozarks this afternoon and evening before shifting across our area into Thursday. Ultimately, this system will result in some periods of inclement weather across our area, particularly tonight and on Thursday.

While all is quiet locally early this morning, we are watching an area of showers and thunderstorms currently moving into southwestern MO. This activity is associated with a lead convectively enhanced impulse that is expected to track northeastward into IL later this morning and into this afternoon. Fortunately, it will be moving into a much drier and stable airmass with northeast extent over IL. Accordingly, a weakening trend is expected through the morning.

Nevertheless, some light rain showers are likely (60% chance)
to shift into at least southern sections of the area (south of I-80) later this morning into the midday timeframe. Some of these light showers could even make their way into far northern IL and northwestern IN mid to late this afternoon (after 3 PM).
However, easterly low-level flow across far northern IL is expected to continue a steady feed of drier air in the low levels through most of the afternoon. This may hence result in only some very light showers as far north as the Chicago metro area through early this evening. Otherwise, expect seasonable inland temperatures in the low to mid 60s today, with cooler onshore flow keeping temperatures in the lower 50s along the IL lakeshore.

Rain chances will ramp-up significantly across the area tonight, most notably after midnight as the main slug of moisture advects northward into the region just ahead of the approaching surface low and surface warm front. Steepening mid- level lapse rates overnight will also support some embedded thunderstorms towards morning. This in combination with healthy deep layer moisture (Precipitable Water values 1.4"+) will support the possibility of locally heavy rainfall amounts (possibly as much as 1-2") across parts of northeastern IL and northwestern IN late tonight into early Thursday morning. This is not currently anticipated to cause any significant flooding issues, though if some of these heavy rainfall amounts fall over the urban areas around Chicago, some minor flooding could result.

Very low clouds and possibly some fog may also develop near the surface warm front and low track very late tonight into Thursday morning. While fog is possible, particularly on Lake Michigan, this has been left out of the official forecast for now. Any low visibilities that result will improve into the afternoon as the surface low begins to shift into Lower Michigan.

Additional periods of showers and thunderstorms are expected on Thursday, particularly near and in advance of a surface cold front, that looks to reside near the I-55 corridor into the afternoon. While the threat for strong storms looks low at this point, due to the lack of much insolation, some storms could once again produce some locally heavy rainfall amounts Thursday afternoon.

KJB

Thursday Night through Tuesday:

The center of the surface low should be east of the area on Thursday night, but the cold front will still be trailing behind it. Chances for showers and thunderstorms along the front earlier on Thursday night are expected, but probabilities for rain drop substantially after midnight as the front exits to the east. Drier conditions are expected on Friday morning. As an upper-level trough drops southward from Canada and passes quickly over the region, there is a 30 to 40 percent chance for another quick shower Friday afternoon/evening, but there is low confidence in coverage. And given the lack of moisture (relative to the mid week system), it seems these showers would not pose any significant impacts.

A surface high will grow through the weekend with an upper-level ridge building into early next week over the western Great Lakes. With height rises and a dry air mass, the extended forecast is expected to be precip free. 850 mb temperatures will steady increase as the ridge builds and surface temperatures will slowly return to 10+ degrees above seasonal normal values by early next week.

DK

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/
Issued at 630 AM CDT Wed Apr 30 2025

Key Aviation Messages for the current TAF period:

- Increasing chances for rain through the latter half of the TAF window

- MVFR cigs/vis are expected with the rain, with the potential for IFR cigs after 06Z

A low pressure system will gradually move through the area overnight Wednesday through Thursday. As the warm front associated with it moves north Wednesday evening, the chances for rain increase with it. There are four main windows being monitored:

1.) on Wednesday afternoon, the first push of moisture aloft ahead of the front will increase cloud cover over the area.
Maybe a sprinkle reaches the ground, but drier air near the surface should keep it virga, therefore no formal mention in the TAF.

2.) Wednesday evening will be the first window for the chances for rain as more moisture feeds in. This window was kept as a TEMPO due to the lower confidence on exact timing at any specific terminal. If and when it is raining, MVFR cigs are expected.

3.) Higher confidence in prevailing rain at terminals after 06Z. As the low moves closer to the area, there is a chance for higher rain rates resulting in MVFR cigs/vis with the potential for IFR cigs.

4.) Rain turns more into rain showers after 12Z, with the potential for a few breaks. However, IFR/MVFR cigs are expected to remain.

While the chance for thunder is non-zero through the overnight and Thursday morning, models are keeping the better instability farther south (not arriving until Thursday during the day).
Therefore, thunder has been kept out of the TAFs presently.

Winds are light and east northeasterly. As they increase, they are expected to be out of the southeast between 10 to 15 knots today. An occasional gust under 20 knots is possible, but should be fairly sporadic if it were to occur. Winds will return to the east around 00Z through 12Z as they diminish. After 12Z, there is low confidence in wind trends for Chicago terminals, yet moderate confidence near KRFD. It will be determined by the exact track of the surface low, which models are still differing. Even just a few miles shift in the low track could create a period of southerly winds versus northwest winds at Chicago terminals.

DK

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