Marine Weather and Tide Forecast for Highwood, IL

DISCUSSION
Issued at 241 AM CDT Thu May 22 2025

Through Friday:

A sprawling and pesky cutoff upper low will continue to pinwheel across the Upper Great Lakes today (and into Friday). This will keep precipitation chances going today, although the coverage of widespread showers and drizzle will steadily diminish this morning a region of deeper low-level moisture sinks southward out of the forecast area.

Later this morning and into the afternoon, notably cooler temperatures will start to filter into the region aloft, with good agreement that 850 mb temps will fall to around 0 C near the lake. These values are very near/under the 10th percentile based on SPC sounding climatology. Beneath this core of cold air aloft, low-level lapse rates are forecast to steepen with some degree of insolation as the widespread sheet of low cloud cover breaks up a bit. This combination (cold air atop steepening lapse rates) will result in diurnally-building instability below about 10 kft, with latest RAP forecasts suggesting maybe 100 J/kg of MLCAPE materializing during the afternoon, all of which will be in the 0-3 km layer. Several surface boundaries will also be present in the form of a zone of enhanced convergence from northwest Illinois down through the Kankakee River, along with a lake breeze/modest lake-enhanced boundary pushing south across northwest Indiana.

While high res guidance appears pretty split regarding precipitation coverage midday into the afternoon, this seems like a favorable setup for isolated to scattered shower development along and south of the aforementioned surface boundaries. Depending on how these boundaries evolve, the environment could also support a few funnel clouds with steepened lapse rates and low-level instability in the vicinity of enhanced surface convergence.

Lingering showers early this evening will steadily push south of the forecast area and will also diminish with the loss of heating. Lingering vort lobes aloft interacting with pockets of low-level moisture suggest we can't entirely rule out some additional pockets of showers, but chances are too low for a mention in the gridded forecast. Just a bit of flow tonight should inhibit much in the way of a frost threat.

Friday will feature plentiful diurnal cumulus development away from the lake. A few sprinkles/light showers can't be ruled out, but have capped PoPs under 10 percent for the time being, with any coverage expected to be quite low.

Carlaw

Friday Night through Wednesday:

The models and their ensembles have trended drier through Sunday and have shifted back to showing a chance for precipitation in the Monday/Tuesday time period, as they had been show a few days ago. The blended pops have generally followed these trends with only slight chance pops across the far southern cwa Sunday/Sunday night and then chance pops for Monday/Tuesday areawide. ECMWF remains the driest and would suggest that even Monday could be mainly dry for the area.

Cool temps will continue with lows mainly in the 40s through next Monday night, then perhaps lows in the 50s by the middle of next week. High temps will generally be in the 60s through next Tuesday with 70s expected later next week. Still possible for some lower 70s well inland on days with mainly sunny skies.
Temps will remain cooler near Lake Michigan with persistent east/northeast winds. cms

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
Issued at 1229 PM CDT Thu May 22 2025

Aviation concerns for the 18Z:

- Brief MVFR ceilings for ORD/MDW early this afternoon.

- Generally northerly winds this afternoon and Friday, though lake breeze has already shifted ORD/MDW northeast and expect another early afternoon northeast shift for those sites Friday.

Forecast area remains within a region of cyclonic northerly low-level flow, to the west of a slowly-departing low pressure system over the eastern Great Lakes. Lake-enhanced surface trough has just moved through both ORD and MDW shifting winds to the northeast, and these will persist this afternoon before diminishing this evening and becoming light/variable overnight.
Winds should become light north-northwest after sunrise Friday (though model guidance shows some spread with exact directions), before another relatively early lake breeze shifts winds northeast for ORD/MDW/GYY midday/early afternoon.

Extensive area of diurnally-enhanced cloud cover has resulted in redevelopment of MVFR cloud bases this morning, but these should lift quickly to VFR early this afternoon as drier low- level air spreads in from the north. VFR conditions are then expected for the remainder of the forecast period. A few spotty showers are possible this afternoon mainly south/southwest of the terminals.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES
IL...Beach Hazards Statement until 10 PM CDT this evening for ILZ103-ILZ104.

IN...Beach Hazards Statement through late tonight for INZ001-INZ002.

LM...Small Craft Advisory until 10 PM CDT this evening for Wilmette Harbor to Calumet Harbor IL.

Small Craft Advisory until 1 AM CDT Friday for the IN nearshore waters.