Marine Weather and Tide Forecast for North Chicago, IL

DISCUSSION
Issued at 308 PM CDT Fri May 16 2025

Through tonight:

A large upper-level low over Minnesota has brought continued SW flow aloft into our region, ultimately resulting a relatively uncommon High-Plains pattern across northern Illinois today.
Earlier today, a very dry airmass upstream per WV 12Z KSGF RAOB and WV imagery over Missouri has advected over the northwest half of Illinois amid deep diurnal mixing. This has pushed RH values as low as the upper teens over the western CWA as S/SSW winds gust 35 to 40 mph. This prompted an earlier issuance of a Red Flag Warning for critical fire wx conditions across portions of the forecast area where little to no rain fell on Thursday, though elevated fire wx conditions are occurring elsewhere (covered by a Special Weather Statement) this afternoon.

Meanwhile, deep mixing into 40kt+ flow aloft should begin to generate sporadic surface gusts to 45 mph or briefly higher mid to late afternoon for areas around and northwest of I-55. Then, as a surface trough crosses the area late this afternoon into this evening, a favorable isallobaric component to an already strong synoptic gradient combined with modest residual mixing with CAA could yield SW gusts to 50 mph for a period this evening. Have therefore maintained a Wind Advisory for the aforementioned locations through midnight. While there is no headline specifically in effect for blowing dust, observations from gusty winds in rural areas on Thursday support maintaining a mention of patchy blowing dust in the forecast grids and Wind Advisory through this evening.

No substantial changes have been made regarding the severe thunderstorm potential late this afternoon into early evening (see the latest Mesoscale discussion update for more detailed information). However, observational trends as of 3pm as well as near-term CAM guidance continue to favor areas south of a Pontiac to DeMotte line for the highest chances of severe convection as the northern edge of the discrete cells over southwest Illinois ride a northward-advacing plume of modest low-level moisture. Farther north, a gradual increase in high-based cumulus coverage will be a focus for at least isolated storm development across the remainder of the forecast area.

Kluber

Saturday through Friday:

The big, vertically stacked, low pressure will be over northern Lake Michigan early Saturday morning with continued gusty westerly winds and extensive stratocumulus deck across our area to the south of the upper low. This system will move eastward away from the area during the day Saturday, which should allow cloudiness to scatter out some during the afternoon. Pressure gradient will begin to ease some Saturday afternoon, though deepening mixing of the boundary layer may offset the decreasing gradient and slow the decrease in winds during the afternoon. By sunset winds will quick subside as boundary layer decouples.

A sprawling ~1030 mb high will build east across Ontario and the Hudson Bay on Sunday with mid-level shortwave ridging forecast to build in across the local forecast area. This high to our north with send a robust afternoon and evening lake breeze/backdoor front through the entire region. This will end up holding lakeside highs in the mid/upper 50s, while inland locales will likely warm into the 70s prior to the lake breeze push. While some guidance hints at the potential for some intermittent lower cloud cover on Sunday, most indications are that any morning cloud cover should scatter into the afternoon.

On Sunday night into Monday, a sharpening warm frontal zone well to our south will slowly advance northward into southern Iowa, northeast Missouri, and southern Illinois. Increasing low-level warm advection in response to strengthening 925-850 mb southwesterlies overtopping this frontal zone will lead to the development of showers and thunderstorms, but given the current location of the aforementioned boundary and MUCAPE gradient, most/all of this activity through Sunday night looks to remain well to our south. Even into the day on Monday any activity locally currently looks like it should remain isolated to scattered, with deeper moisture still relegated to our south.
Forecast soundings off the GFS in particular look like they'd be more supportive of intermittent drizzle/showers as opposed to thunderstorms.

On Monday night into Tuesday, guidance pushes a modest surface low (near 1000 mb) towards northern Missouri/southern Iowa as a robust shortwave pivots across the central plains. During this time, the northward progress of the returning warm front is forecast to slow, with the boundary overall becoming quasistationary across central to downstate Illinois. Isentropic upglide will increase once again, likely leading to a nocturnal expansion of showers and storms which--depending on the precise location of the stalled front--may begin to push into our forecast area, particularly south of about I-80. Could end up being a locally heavy rainfall threat somewhere in the vicinity with this setup.

Thereafter, guidance generally depicts a complex upper pattern developing, with a series of upper lows/gyres pinwheeling around one another into the middle of the week. This general evolution will keep the threat for intermittent periods of rain with embedded thunderstorms going in the vicinity, at least into Wednesday.
There's a signal for things to dry out into the end of the week.

Izzi/Carlaw

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
Issued at 1224 AM CDT Sat May 17 2025

The key aviation weather messages are:

- Gusty W/WSW winds continue tonight into Saturday. A few gusts of 35+ kts at times. Winds easing early in the evening and becoming northwesterly.

- VFR to high-end MVFR cigs into Saturday morning.

Gusty west to west-southwest winds will prevail tonight into Saturday afternoon. Sporadic gusts of 35+ knots will likely occur. Winds will then ease during the evening and will become northwesterly. A few sprinkles can't be ruled out tonight, but chances/impacts appear too low for a mention in the outgoing TAFs.

VFR to high-end MVFR cigs will develop through the night and will then persist through Saturday morning before lifting back to VFR in the afternoon. VFR conditions will prevail thereafter.

Carlaw

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

Small Craft Advisory until 4 PM CDT this afternoon for the IN nearshore waters.