Marine Weather and Tide Forecast for Winthrop Harbor, IL

MESOSCALE
Issued at 1240 PM CDT Wed Jun 18 2025

Recent radar scans have depicted intensifying mid-level rotation near Colfax IL (just southwest of Livingston and just west of Ford County). The line of storms entering the south and southwest CWA appears to be in the process of splitting into individual semi-discrete supercells. With strengthening low- level shear/lengthening low-level hodograph on the KILX VWP (vad wind profile), and a very juicy and low LCL air mass (70s dew points with >100 J/kg of 0-3 km MLCAPE), an appreciable tornado threat appears to be unfolding within the Tornado Watch counties in the CWA

We'll certainly keep a close eye on trends with the MCV/surface low path with respect to the favorable parameter space for tornadoes and damaging winds shifting a bit farther north. If this occurs, we'd likely need to do a local expansion northward of the Tornado Watch in effect until 7 PM CDT. For now, however, our thinking on the northern extent of the favorable parameter space for severe weather remains consistent with our earlier messaging (mainly east of I-55 in eastern IL and across NW IN).

Castro

Issued at 1115 AM CDT Wed Jun 18 2025

Compact and rather potent convectively enhanced shortwave (MCV)
over northeastern Missouri will track northeastward across IL today and into lower Michigan this evening. Morning soundings from SGF along with WSR-88Ds are sampling an intense mid level jet streak on the southern flanks of this shortwave with 6km winds of 50-60kt. This jet streak and associated stronger mid- level flow will spread eastward across a gradually destabilizing boundary layer and strong deep layer shear (0-6km 50kt+). Some breaks in the cloud cover is allowing the very moist boundary layer to grow increasingly unstable across eastern IL into western IN. Focus for greatest severe weather threat should be just to the southeast of the track of this mid level vort which looks to roughly bisect our CWA from southwest to northeast.

In addition to the strong deep layer shear, VWPs from LSX and ILX both show strong low level flow (40kt+ 1km AGL). This strong low level flow should continue to overspread the destabilizing boundary layer and result in fairly strong low level shear profiles. Certainly seems plausible there could be a window of time with supercells in a parameter space that is favorable for strong low level mesocyclones and tornadoes. In our CWA, this looks to favor areas near and especially southeast of Valparaiso to Pontiac line.

Confidence is low in how quickly convection could transition to a more linear mode. When/if that transition takes place, the threat for damaging winds would increase, though given the favorable low level shear a QLCS tornado threat would likely persist.

Also very concerning is the flash flood potential along and just to the north of the MCV track where several hours of training convection could result in some pretty extreme rainfall amounts.
Given the near record high PWATs for mid-June, the more intense storms could easily produce 2"+ hourly rainfall rates. This would be especially problematic if it were to occur in the more urbanized areas of the Chicago metro. However, there were also some pockets of very heavy rainfall last night that left other areas, like Grundy County more at risk of flash flooding.

- Izzi

DISCUSSION
Issued at 403 AM CDT Wed Jun 18 2025

Through Thursday:

Main forecast concerns in the near term are thunderstorm trends and severe weather potential across the area today, along with the potential for locally heavy rainfall amounts. SPC's Day 1 outlook has expanded the enhanced risk a little farther north into the southern parts of the Chicago metro area, generally south of where last evenings convection has tracked.

Early morning water vapor imagery depicts a synoptic mid-level short wave trough upstream (roughly through the Missouri River valley), likely convectively-augmented by the MCS/MCV over far southeastern KS. A couple of smaller MCVs were evident in regional radar data as well, one over central NE, another over northeast IA, and one lifting northeast out of southeast WI which brought our evening storms. Expectation is that there will be a relative lull in shower and isolated thunderstorm activity here early this morning in the wake of this departing MCV, before thunderstorm chances increase again mid-late morning ahead of the approaching short wave and embedded KS MCV.

While still depicting more spread than desired, guidance is in general agreement in developing an associated surface low pressure wave across the forecast area this afternoon, backing low-level flow. Meanwhile, mid-level winds are forecast to increase to 40-50 kts along the southern periphery of the MCV, across central IL/IN and the southeast portion of the WFO LOT cwa. Warm and humid low- level air mass (low-mid 80s temps upper 60s dew points) is forecast to support 1500-2000 J/kg of MLCAPE, with 50 kt deep layer bulk shear expected to support organized convection in the form of clusters and line segments, and upscale growth into a more solid squall line appears possible across the southeastern parts of the forecast area this afternoon. A little bit of a warm layer in the 500-400 mb layer per forecast soundings weakens mid-level lapse rates somewhat aloft, with damaging wind gusts the primary expected severe weather threat. Backed low-level winds near/south of the low track will also support a tornado threat as well.Precipitable water values around 2" will also support the potential for torrential rainfall with storms during the midday and afternoon hours, with locally heavy rainfall amounts in excess of 2" possible.

Storm coverage and severe threat should decrease late this afternoon as the low moves east of the area, though the upper trough axis will be slow to move across the region overnight into early Thursday morning. This will likely maintain scattered showers and isolated thunderstorms through the overnight hours and into early Thursday before strong subsidence develops by afternoon. SPS has maintained a marginal (level 1 of 5) risk for Day 2 across our far northern counties, though the potential appears to be higher north of the IL/WI border.

Ratzer

Thursday Night through Tuesday:

Main focus through the extended period continues to be building heat and humidity this weekend into at least early next week.

Broad upper level ridging will be building across the Rockies and high Plains Thursday as upper troughing shifts east across the Great Lakes. This will place the local area under a northwesterly flow aloft ahead of the building ridge to the west. This set up will support the passage of subtle shortwaves through the northwest flow which may result in some isolated shower/thunder chances, especially across far northern Illinois later Thursday or Friday. Better chances look to be to the north so most if not all areas may remain dry.

A closed upper low upstream over the west coast is progged to further amplify the downstream ridge and push it eastward into the weekend. This will bring in much warmer air by Saturday with a breezy southwesterly low level flow setting up across the area. With such an amplified pattern, the ridge axis will extend well north of the area by Sunday, which combined with the fairly northerly position of the base of the western trough would keep the favored track for showers and thunderstorms north of the area through the weekend. Given that we're still a few days away from this pattern shift, we'll have to monitor how the amplitude of the upper features evolves over the next few days to see if the signal for convection to be favored to the north persists. The question going into early next week is how quickly the amplitude decreases and when a favored track for thunderstorms returns to the area as well as how quickly the heat abates.

As far as temperatures, 850 mb temps are expected to warm into the 21-23C range for the weekend. Breezy south to southwest surface winds will support strong warming with highs expected to reach the mid 90s for most areas both Saturday and Sunday, and likely into Monday and perhaps Tuesday as well. Dewpoints are forecast to reach the lower 70s but this will be another element to monitor as drought currently persists across the area. Rain from today's thunderstorms may impact this, but Thursday and Friday will likely allow for drying out of the top layers of the ground. Given this, and the early stages of crop growth (as opposed to mature stages in July and August which allows for crops to add a notable amount of water vapor to the air) there is some concern that current dewpoint forecasts could be a bit too high. A drier airmass would allow for higher temperatures in the upper 90s. Ultimately, a hotter/drier scenario could net a similar heat index as a slightly less hot/more humid scenario with the resultant messaging of taking heat precautions being the same. At this point, the bigger concern is the duration of the heat as opposed to heat reaching an extreme or unusual magnitude.

MDB/Ratzer

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/
Issued at 1256 PM CDT Wed Jun 18 2025

Concerns:

- Thunderstorms with heavy rain this afternoon - Messy winds through the afternoon into the early evening - MVFR ceilings and IFR/LIFR visibility in the heavy rain

Low pressure will continue to strengthen and lift to the northeast this afternoon. An area of showers and thunderstorms will spread north out ahead of the low. The low will move overhead just to the south and east of the Chicago terminals, and likely passing over KGYY. The strongest storms will be out ahead of the low to the east, where gusty southwwest winds will accompany the storms. For the Chicago area, storms may remain elevated. However, the storms will have heavy rain and reduce visibility to LIFR even at KORD and KMDW. The main window of storms will be 19z-22z, before becoming showers with embedded thunder.

The other challenge will be the winds. Winds will remain quite variable given the low track moving close by, but should maintain some easterly component. ORD and MDW also have a lake breeze to contend with, that will also aid winds from transition from a SE to NE, though they will likely bounce around. If any westerly component occurs, they should be on the lighter side or brief, with some easterly component prevailing during the storms.

Winds will shift to north and then northwest as the low pulls away this evening. Some showers and MVFR ceilings will remain in place this evening, with a 40% chance of IFR.

KMD

LOT WATCHES/WARNINGS/ADVISORIES
IL...Flood Watch until 8 PM CDT this evening for ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106- ILZ107-ILZ108.

IN...None.
LM...None.