Marine Weather and Tide Forecast for Winthrop Harbor, IL

DISCUSSION
Issued at 245 AM CST Sat Nov 8 2025

Today through Sunday (Rain Changing to Snow Period):

Recent nighttime microphysics imagery depicts a busy scene early this morning with a potent upper-level shortwave pinwheeling into the Dakotas, a second short wave moving across central Nebraska, and blossoming low-level stratus across the Lower Great lakes across a developing baroclinic zone. Recent radar imagery from both KMKX and KLOT depict westward-expanding returns indicative of rain showers streaming off Lake Michigan associated with gradually increasing low-level easterly flow.
This morning is shaping up to be somewhat gloomy across much of our area.

Over the next 24 hours, the Dakotas and Nebraska shortwaves are expected to merge while moving into the middle Mississippi River Valley. As this occurs, a relatively compact precipitation shield is expected to develop along and to the north of the center of low pressure across central Iowa. Deterministic and ensemble model guidance remain in excellent agreement that the system will then shift due eastward this afternoon and tonight, leading to rain spreading from west to east across the area (connecting with the lake effect rain showers closer to Lake Michigan and transitioning to lake enhancement of the broader precipitation shield in northeastern Illinois). As temperatures (and more importantly, wet-bulb temperatures) fall after sunset, a transition in precipitation type from rain to snow is expected from northwest to southeast, taking longest near the immediate Lake Michigan shoreline due to relatively warm onshore flow.

With the precipitation type starting as rain and the ground remaining relatively warm, continue to suspect that snow will struggle to accumulate this evening even as air temperatures fall. With that said, a region of relatively steep mid-level lapse rates intersecting a strong and deep frontogenetical circulation within the compact deformation region does raise concern there could be a narrow/focused corridor where snow rates are high enough to allow for a few inches of snow to stick to the ground and even less traveled-upon roadways (call it a 20-30% chance). It's tricky to pinpoint where such an axis may materialize given the two shortwaves will be merging almost directly overhead. We'll let the day shift watch trends in upstream observations and model guidance to inform any updates to our gridded database.

As the system pulls away from our area early Sunday morning, a trailing band of lake effect snow is expected to migrate southeastward along the shoreline of Lake Michigan. Based on the expected track of the surface low and orientation of the low- level pressure field, do have some concern that the band may last long enough at any given location to drop a quick inch or two of snow, starting near Evanston and Chicago toward daybreak Sunday shifting southeastward into northwestern Indiana by mid- morning. If the band were to stall anywhere for a few hours (particularly in northwestern Indiana), certainly can't rule out the need for a Winter Weather Advisory for a quick 2-4" of snow. Confidence is too low on the evolution of the trailing lake effect snow band to issue a Winter Weather Advisory at this time, so will let the day shift take a closer look at that, too. By Sunday afternoon, the lake effect snow band is expected to shift eastward toward Lower Michigan, giving our area a brief break in precipitation.

Borchardt

Sunday night through Monday (Impactful Lake Effect Snow Period):

Sunday night and into Monday morning, a potent shortwave embedded in the broader polar trough anchored by record-cold mid-level thermal profile (500mb temperatures of -40C) is expected to dive south-southwestward into the western Great Lakes. With increasingly extreme lake instability by Sunday night (marine airmass to 700mb water temperature differentials pushing 35C degrees with effective equilibrium levels of 22-25kft), intense bands of lake effect snow are poised to develop over Lake Michigan and point toward the lakeshore.
Exactly where the polar shortwave tracks will be paramount to guiding where intense bands of lake effect snow push inland.

While there remains spread in the track of the shortwave, am growing quite concerned in the growing consistency among EPS/CMCE/GEFS members showing a track of the 500mb vorticity core down or just west of the spine of Lake Michigan. When paired with the aforementioned extreme lake instability, such a track of the shortwave would support the development of a strong lake-effect mesolow with both intense snow rates and strong/enhanced northerly winds. The EPS/ECMWF suite in particular has been steadfast in showing such an evolution with the mesolow landfalling in northeastern Illinois early Monday morning, accompanied by a 2 to 4 hour burst of intense snowfall rates of >2"/hr and low visibility due to northerly wind gusts in excess of 35 mph (highest lakeside). Thereafter, a lake effect snow band would be poised to shift toward northwestern Indiana. However, if the polar shortwave ends up tracking more southeasterly along the eastern side of Lake Michigan, a north- to-south oriented band of intense lake effect snow may develop instead of a mesolow, and stall over northwestern Indiana through much of Monday. If the latter scenario were to materialize, snow rates and amounts could easily get out of hand and support localized double-digit snow totals somewhere in Lake or Porter counties in northwestern Indiana. Of course, some combination of the above is in the cards, too.

All things considered, feel there is enough of a signal for impactful lake effect snow characterized by intense snow rates >2"/hr and gusty north winds >35 mph to justify the issuance of Winter Storm Watches for Porter and Lake counties in northwestern Indiana and Cook county in northeastern Illinois.
As confidence grows in where the polar trough will go and exactly how lake effect snow bands will evolve, the Winter Storm Watch or eventually upgraded areas may evolve (adding or removing counties). Keep in mind that lake effect snow is a highly localized phenomenon and can lead to very unequal impacts. Much like thunderstorms, lake effect snow can hammer a very localized area while keeping the rest of a county relatively unscathed. So, being included in the Winter Storm Watch is by no means an guarantee warning-level impacts will be felt.

Finally, not to be forgotten, Sunday night will be the coldest so far this season. The combination of overnight lows in the 20s and blustery north winds will drive wind chills down toward the single digits to mid teens by daybreak Monday.

Borchardt

Monday Night through Saturday:

The upper level long wave trough will gradually move eastward Monday night through Tuesday. However, with persistent northwest flow, lingering lake effect snow showers may remain through Tuesday though the focus will be more on northwest Indiana.
Temperatures will slowly warm on Tuesday to allow for the potential for it to be a rain snow mix, but any snow accumulations should be minimal.

There is an upper level short wave that is expected to descend out of Manitoba overnight into Wednesday morning. There is fairly good agreement in ensembles presently that it should head more toward the northern Great Lakes to inhibit any formal PoP mention in the grids. Other than monitoring that that wave remains to the north, the second half of the week looks fairly dry and quiet (compared to how the week will start!). As a surface high takes up residence near Georgia, warm air advection into Illinois will help drive temperatures back up into the 50s and potentially even 60s by Friday (so much for any snow accumulations lingering....). Models are suggesting the potential for another wave to move over the area next Saturday.
Being still over a week away, felt no need to adjust the roughly 25 percent chance PoPs that the NBM provided.

DK

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/
Issued at 532 AM CST Sat Nov 8 2025

Key Aviation Messages for the current TAF period:

- Light rain/drizzle and MVFR conditions linger through the morning for terminals closer to the lake

- Widespread rain this afternoon/evening with lower vis possible during heavier rain bands

- Moderate confidence in the timing of rain transitioning to a rain/snow mix after midnight, and then all snow closer to daybreak

- Northeast winds today becoming north-northwest after midnight.
Wind gusts intensify Sunday morning with gusts in excess of 30 knots around and after daybreak

Winds out of the northeast this morning are focusing some lake effect rain showers over terminals near the lake. Rain rates are fairly light with occasional drizzle at times. Cannot rule out the occasional IFR level FEW/SCT cigs through 15Z, but cigs should remain MVFR through today with occasional reduced vis during steadier drizzle.

There could be a brief break in the light rain/drizzle late this morning into the early afternoon, but the next weather system passes through the region this afternoon evening providing the next chance for widespread rain. Locally IFR vis is possible during the heaviest bands of rain, but kept MVFR vis in the TAF presently.

After midnight, temperatures will drop and wet, slushy snow is expected to mix in with the rain. Eventually, closer to daybreak, precipitation should a.) taper off away from the lake (KRFD) and b.) transition to all snow closer to the lake (Chicago terminals). There is moderate confidence in the timing of the precipitation transition, but can be tweaked in later TAF packages.

Additionally after midnight, winds will slowly turn to the north and eventually northwest. The pressure gradient will tighten and wind gusts will increase. Gusts 20 to 25 knots with occasional to 30 knots is possible before daybreak. Then wind gusts are expected to further increase to 30 knots, with occasional gusts 35 to 40 knots possible, tomorrow morning as snow slowly tapers off in Illinois and focuses over northwest Indiana.

DK

MARINE
Issued at 245 AM CST Sat Nov 8 2025

An area of low pressure will gradually deepen to 29.4 inches as it moves eastward from northern Missouri to central Indiana on over the next 24 hours. Additionally, high pressure over the northern Plains will increase creating a strong pressure gradient oriented across Lake Michigan. As a result, northerly winds will increase quickly this evening along the Illinois nearshore of Lake Michigan and spread to the Indiana nearshore into Sunday morning. A period of gale force winds to 35 kt may occur during the overnight hours into Sunday morning, before diminishing slightly in the afternoon with occasional gusts to 30 knots. Another period of gale force winds to 35 knots may occur Sunday night into Monday morning as a secondary system moves southward down Lake Michigan. For these reasons, a Gale Watch has been issued from midnight tonight through 6 AM Monday for both the Illinois and Indiana nearshore zones of Lake Michigan.

Bands of lake effect rain and snow are expected to develop tonight and last through Monday across central and southern Lake Michigan. Intermittent water spouts may occur in the most intense bands of lake effect rain and snow.

NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES
IL...Winter Storm Watch from late Sunday night through Monday morning for ILZ103-ILZ104-ILZ105.

IN...Winter Storm Watch from late Sunday night through Monday afternoon for INZ001-INZ002.

LM...Gale Watch from late tonight through late Sunday night for the IL nearshore waters.

Gale Watch from Sunday morning through late Sunday night for the IN nearshore waters.