Marine Weather and Tide Forecast for Winthrop Harbor, IL

DISCUSSION
Issued at 156 PM CDT Thu Apr 24 2025

REST OF TODAY...
This afternoon, an ill-defined warm front and a baggy pressure gradient are in place across southern Iowa into northern Illinois. Weak flow near the frontal trough has allowed a lake breeze to form and push inland. Temperatures well into the 70s and lower 80s will be replaced by temperatures in the 60s and 50s immediately near the lake as the lake cooled air spills inland. Meanwhile, a shortwave trough lifting across the mid Missouri Valley this afternoon is kicking off showers and storms to our west. These showers and storms are expected to move across northern Illinois and northwest Indiana late this evening and overnight, some of which may develop along the remnants of the lake breeze boundary as it settles over portions of east central Illinois and northwest Indiana. Instability will be weak with MLCAPE values less than 200 J/kg while suboptimal deep layer shear of 20-25kt will not support severe storms.

FRIDAY...
Surface low will lift from southern Wisconsin Friday morning to northern lower Michigan Friday afternoon with a trailing cold front sweeping across northern Illinois and northwest Indiana through the day. Favored low level trajectories from an open Gulf will help push dew points up into the 60s for most of the area Friday afternoon and will contribute to modest destabilization Friday afternoon as MLCAPE values increase to near 1000 J/kg. Deep layer shear will remain weak, however, keeping the severe risk quite low. HRRR LPMM indicates a few isolated swaths of 0.75-1.00 inches of rain will be possible with the heaviest showers and storms through HRRR mean values are only around a quarter inch QPF through Friday evening.

THIS WEEKEND...
High pressure will translate east across the Great Lakes over the weekend. North to northeast flow ahead of the ridge axis Saturday will result in a cool down across the region particularly near the lake where onshore winds through the day will result in temps struggling to reach the 50 degree mark.
Further inland temps will warm into the 50s to lower 60s under sunny skies. Winds turn southeasterly behind the ridge axis Sunday which will help temps to recover back into the upper 60s to lower 70s except for the Illinois shores of Lake Michigan where an onshore component to the winds will keep temps in the 50s near the lake.

NEXT WEEK...
Upper level pattern is progged to gradually amplify over the weekend with a deep trough setting up over the Great Plains by early next week. A deepening surface low is progged to lift from the central/northern Great Plains into the Upper Midwest Monday then to northern Quebec by Tuesday with a trailing cold front moving across Illinois and Indiana. Warm air advection will ramp up in response ahead of the low pushing temperatures for much of the area including the lake shore into the 80s. More summer- like dew points well into the 60s will accompany the warmth and set the stage for moderately strong instability for portions of the area both Monday and Tuesday. GFS indicates that around 1500 to possibly 2000 J/kg MLCAPE will be in place Monday across western portions of the state. The instability axis will shift to east central Illinois and northwest Indiana Tuesday. Meanwhile, a jet streak lifting through the lee side of the deep upper trough will contribute to moderate deep layer shear around 40 kt Monday and slightly stronger on Tuesday near 45-50kt. LREF joint probabilities for SBCAPE greater than 500 J/kg and bulk shear greater than 30 kt continue to creep up with 70-80% likelihood Monday (highest in the west) and similar probabilities Tuesday (highest in the southeast). These parameters are more than sufficient to support a severe weather threat with passage of the cold front early next week, and this period will need to be monitored as the forecast details continue to come into better focus over the next few days.

Deubelbeiss / NWS Lincoln

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
Issued at 1254 PM CDT Thu Apr 24 2025

Main Aviation Concerns: -Scattered showers/storms possible (40% chance) near the RFD terminal after Midnight; lower possibility (10% chance) east.

-Scattered showers/storms likely (60% chance) areawide between 16- 22z/11am-5pm Friday.

-Breezy (20-25 kt) north winds and MVFR ceilings will overspread the regional terminals late in the TAF period.

Increasing southwesterly winds in the 2-5km AGL layer may help induce scattered showers/storms (40% chance) across eastern Iowa and northern Illinois in the vicinity of a surface front late tonight.
Confidence and coverage of thunder remain too low to include at the KRFD terminal for now, but trends will be monitored.
Otherwise, a greater (60%) chance for widespread showers arrives around midday Friday along the advancing cold front. Confidence in thunder remains low (20-30% chance) during this time, but we did add thunder mention at KORD and KMDW via a Prob30 group between 17z-21z/Noon- 4pm. Then, immediately behind the front, low-level clouds (MVFR) and breezy (20-25 kt) northerly winds will ensue.

NWS Lincoln

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