Marine Weather and Tide Forecast for Gurnee, IL

UPDATE
Issued at 1005 AM CDT Mon May 12 2025

The upper level low mentioned in the previous discussion is still seen on water vapor satellite imagery spinning over central Alabama. However, an outer band of showers is moving northwestward from central Indiana over Benton County. The observation in Frankfort, IN did record some light rain, but soundings are still showing a layer of dry air near the surface limiting the widespread impacts. The only change to the forecast was to slightly increase the time of arrival based on trends.
Otherwise, showers are expected to be isolated to widely scattered this afternoon through the evening. Instability looks to be fairly limited, but with daytime heating this afternoon, isolated thunderstorms are possible. Lastly, cloud cover is expected to increase through the day as the system moves northward, but temperatures are taking advantage of the clearer skies currently running 2 to 3 three warmer than originally anticipated. While the max temperatures are still expected in the low to mid 80s, short term temps were bumped up.

DK

DISCUSSION
Issued at 336 AM CDT Mon May 12 2025

Through Wednesday night:

The closed upper low that has been meandering across the Lower Mississippi Valley the past few days has begun to lift north toward the region early this morning. This will advect low-level moisture into the area later today through tonight. Guidance has trended slower with the northward advancement of the higher dewpoints which will likely limit any diurnal pulse showers and storms to areas mainly south of I-80 through the daytime hours.
By the evening these showers and storms will begin to lift across the rest of the area through the overnight hours tied to a 700 mb vort lobe as it drifts northwest across the area. Some of the most recent guidance seems to have a better handle on this feature, tied to convection over far northeast Alabama as of this writing. PoPs only in the slight/20% chance range overnight into Tuesday morning may ultimately need to be bumped up for our next issuance. Given increasing low-level moisture over the cold lake, suspect fog development will commence late tonight through much of the day on Tuesday, some of which could ooze inland along the Illinois shore, particularly into Lake County.

Warm and humid conditions will continue Tuesday and Wednesday as the upper low slowly lifts across the Ohio Valley leading to additional isolated to scattered showers and storms, with the greatest coverage expected on Tuesday across northwest Illinois within the vicinity of the aforementioned vort lobe. Given weak flow aloft, any storms the next few days should be pulse-like capable of lightning and localized downpours. Any of the taller storms would be capable of producing locally gusty winds after they collapse/pulse back down. Low-level instability paired with areas of low-level vorticity suggest a few funnel clouds also can't be fully ruled out. Highs are forecast to warm into the mid 70s to mid 80s inland while still notably cooler near the lake in the 60s to lower 70s.

Petr

Thursday through Sunday:

The summer-like warmth and humidity will then continue into Thursday, with even warmer temperatures expected due to the continued solar insolation and increasing warm air advection. In fact, it's quite possible that we could see our first 90 degree readings of the year in our forecast area on Thursday. If that were to occur, then the daily high temperatures for May 15th in Chicago and Rockford (91 and 90, respectively) would potentially be in jeopardy. For once, these warmer temperatures may reach at least some of our lakeside locales as winds on Thursday should be breezy and from the south.

Late Wednesday into Thursday, the western CONUS trough is expected to eject into the northern Plains and Upper Midwest.
The low pressure system that develops in response will help advect a more pronounced EML into the Midwest and will also cause kinematic fields across the region to strengthen. This will augment the pre-existing warmth and moisture to foster a potential threat for severe weather across the region late Thursday into early Friday. This severe weather set-up remains conditional into our area due to uncertainties regarding impacts of capping, very dry mid-upper levels (dry air entrainment influence), displaced forcing mainly to the north and west of the area, etc., which will impact whether storms will be able to initiate this far south. Thus, while confidence is relatively high at this point in time that the parameter space that will be in place over the region late Thursday will largely be favorable for severe thunderstorms, confidence is still fairly low in whether everything will come together just right for this parameter space to be fully utilized, or whether we'll even see storms at all. Stay tuned!

Our weather for Friday will largely depend on how things play out on Thursday with the low pressure system's evolution. A strong cold frontal passage Thursday night/Friday morning would tend to favor dry and breezy (to potentially windy) conditions across the area on Friday. On the other hand though, the nearby presence of the aforementioned upper-level trough could still support shower and thunderstorms chances here if the cold front arrives too early or too late and/or isn't robust enough to sufficiently scour out the warmth and moisture in the area, particularly south of I-80. While there should be a reprieve from the warmer and at times stormier conditions this coming weekend, the overall weather pattern across the central CONUS is expected to remain fairly active into next week. This could result in renewed chances for showers and thunderstorms in our area during next work week.

Ogorek/Petr

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/
Issued at 624 AM CDT Mon May 12 2025

Main Concerns:

- Potential for low CIGs and reduced VSBY late tonight/early Tuesday morning.

- Chance for SHRA this evening and overnight.

VFR conditions will prevail through at least mid to late this evening. Expect easterly winds to increase to 10+ kt this morning and then ease back below 10 kt this evening.

Showers south of the terminals (and perhaps an isolated TS or two well south) later this afternoon may move into the area on a weakening trend this evening. If showers occur this evening, suspect they'd be non-impactful, so withheld any formal precip mention. A stronger disturbance may support a more coherent area of showers with reduced VSBY overnight into early Tuesday.
Opted to include a PROB30 mention for this at the Chicago metro terminals. Shower chances may increase in the RFD area after 12z Tuesday, beyond their current TAF period.

Some of the guidance brings in low CIGs (low MVFR to IFR) as early as late this evening. This may still be a bit too aggressive, so continued to lean on the slower guidance for now, which suggests lower MVFR CIGs (or lower) developing after 06z Tuesday. If sub-1000 ft CIGs occur, they should be in the 300-500 ft range. Lower CIGs should improve toward midday Tuesday. Finally, the higher humidity air mass and lighter winds early Tuesday should support modestly reduced VSBY. Dense fog may develop over the lake and stream onto the Illinois lakeshore, but likely remain east of MDW and ORD.

Castro

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