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KEY MESSAGES

- Prolonged lake effect rain and at times snow showers will continue through at least Saturday morning, especially in northwest IN.

- Rain will transition to snow in the lake effect band tonight and may result in a few inches of wet snow accumulation that will lead to some localized travel impacts late tonight into Friday morning, including the Friday morning commute.

- Cooler temperatures are expected through the weekend before temperatures warm next week.

UPDATE

Issued at 621 PM CST Thu Nov 12 2026

The going forecast appears to be in good shape.

Over the past few hours, a dominant lake effect rain band has emerged within a focused convergence zone (blowing >30kt on either side of the band) forced by an inverted trough over southern Lake Michigan. Webcams along the southern Lake Michigan shoreline from Michigan City to Gary have shown healthy rain rates and reduced visibility. This is especially true near the western edge of the band where channeled winds are leading to northerly gusts exceeding 50 mph (am noting blowing sand at Wells Street Beach in Gary). With temperatures in the lower atmosphere remaining relatively warm through the evening hours should remain plain rain.

Attention is on the next upper-level shortwave pinwheeling south into northern Wisconsin at press time on a path to move directly over northern Illinois after midnight tonight. Per RAP guidance, the shortwave is anchored by seasonably cold mid-level temperatures (500mb temps near -30C, 700mb temps near -15C). As the wave slides directly overhead, the rapidly cooling/improving low-level thermodynamic profile should support precipitation type within the lake effect band transitioning from rain to snow (as early as 10 pm, but more likely in the 12 am to 3 am time window). Snow will prevail within the band until mid Friday morning and the boundary layer warms, forcing precipitation type back to rain.

Pinpointing exactly where the lake effect band will park overnight into tomorrow morning remains tricky. Generally speaking, model guidance remains too far west in depicting the zone of greatest convergence when compared to actual radar returns. As the upper-level vort max moves overhead, would anticipate at least some westward drift of the convergence zone toward the Illinois shoreline. If embedded mesolows were to develop (which is a distinct possibility given what will be impressive lake induced thermodynamic parameters by daybreak), westward shifts could be even more dramatic and include pivots well into Cook County at times. However, it can sometimes be hard to shift zones of intense convergence owing to a self- feedback loop (strong convergence leads to stronger/deeper convective band, leading to overall stronger vertical circulations on either side of the band). So, cannot discount that the band remains parked in northwestern Indiana all night and tomorrow primarily in Lake County.

As has been mentioned in prior forecast discussions, the relatively warm ground should prove detrimental to snow accumulations overnight into tomorrow morning in most locations where it does snow. (For this reason, explicit model output of total snowfall using any set ratio should not be used). With that said, temperatures and snow rates within the heart of the lake effect band should be sufficient to allow snow to stick within a very narrow zone (literally the center of the band, perhaps only 5 to 10 miles wide). If the band were to remain parked over the exact same 5 to 10 mile wide zone for hours, could easily envision snow totals quickly adding up. This is especially true if a larger mesolow emerges along the southern Lake Michigan shoreline overnight overcharging convergence and snow rates. As a result, do give some credence to model output of snow DEPTH change (which parameterizes ground heat flux, soil temperatures, melting, compaction, and sublimation), supporting a play for a very localized zone of 4 to 8 inches of snow. Again, we'd need the snow band to be parked over the exact same area for multiple hours to support such an outcome, seemingly favoring Lake County IN.

At this point, see no reason to adjust the inherited forecast or messaging. Those in the Winter Weather advisory area should prepare for locally hazardous travel particularly within the heart of the lake effect band late tonight into tomorrow morning (again focused near the IL/IN border).

Updated products will be sent soon.

Borchardt

DISCUSSION

Issued at 213 PM CST Thu Nov 12 2026

A broad upper trough continues to spiral over the Great Lakes this afternoon which has been leading to mostly to completely cloudy skies areawide and some lake effect showers in northwest IN. The trough has also been advecting in cooler air on blustery north- northwest winds that have been gusting around 30-40 mph. While the strongest gusts are expected to diminish after sunset, 20-30 mph gusts will persist through Friday as the area remains sandwiched between a 1030 mb high near Hudson Bay and a 997 mb surface low off the eastern CONUS. Temperatures will also remain seasonably cool through Saturday with daily highs in the lower 40s and overnight lows in the mid to upper 20s inland and lower to mid-30s near the lake.

The main weather concern with this trough will be the aforementioned lake effect showers. While the lake effect rain showers continue to mainly impact portions of Porter and LaPorte Counties in IN, recent radar trends are showing the middle part of the band over central Lake Michigan starting to pivot westward towards IL. This trend will continue through the evening and will allow the lake effect rain showers to pivot into far northeast IL after 4-5 PM CST and persist through most of the night. As the band moves into northeast IL the lake induced instability is expected to increase as warm lake temperatures and the colder air aloft generate equilibrium levels around 10000-12000 ft. These ELs are deep enough to allow the moisture to get into the dendritic growth region and will likely start to transition some of the lake effect rain showers into a rain/snow mix after sunset. Initially, this mixed precipitation wont cause many impacts, but as surface temperatures continue to cool into the lower to mid-30s this wintry mix should transition over to all snow after midnight especially beneath the heart of the band. Given the stout lake induced instability snow rates beneath the band could near 1 inch/hour for a period which may be sufficient for the snow to overwhelm the otherwise marginal surface temperatures.

Now where exactly these highest snow rates setup overnight remains somewhat unclear at this time, however, guidance has been favoring areas near and along the IL-IN state line. Regardless of where the highest snow rates and duration overlap, pockets of localized snow accumulations of 3-4+ inches continue to look possible away from Lake Michigan particularly on grassy and elevated surfaces. For those closer to Lake Michigan and not under the core of the band, snow accumulations look to be limited in the dusting to maybe a couple tenths range, again mainly on grassy/colder surfaces. Couple the potential for these locally higher snow amounts to occur around the Friday morning commute some travel impacts may be seen. Therefore, we have opted to extend the Winter Weather Advisory into southern Cook and eastern Will counties and maintained it for Lake County, IN to highlight this threat.

Lake effect snow will transition back into a rain by Friday afternoon as temperatures warm into the upper 30s and lower 40s which should prevent additional accumulations at least through the daytime hours. While the lake effect band should begin to drift back east Friday afternoon, latest guidance trends show this shift being much slower than originally forecast so there is a chance the band hovers around the IL-IN state line into Friday evening before moving east. Temperatures will once again cool into the lower to mid-30s Friday night beneath the band which will likely lead to another transition from rain to snow. However, with lake induced instability expected to be diminishing by this point (ELs around 7000-8000 ft at best) the snow rates should be more muted and struggle to overcome the surface temperatures. So while some minor (dusting to 0.2 inch) accumulations may occur on grassy surfaces in northwest IN Friday night, little to no impacts to travel are expected.

The lake effect showers will taper from west to east across northwest and northern IN Saturday morning leaving us with dry conditions through at least the middle of next week as a surface high builds into the region. Temperatures will also begin to rebound next week as winds turn southerly resulting in highs in the 50s and nighttime lows in the 30s to around 40 for the early to middle part of the week.

Yack

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 621 PM CST Thu Nov 12 2026

An extended lake effect precipitation event is underway over southern Lake Michigan this evening. The main lake effect rain band has focused into northwest Indiana (including GYY) and should remain stationary or edge slightly west through the evening. A surface trough shifting southward over Lake Michigan is expected to push the band far enough west overnight that SHRA should begin to clip MDW. As colder air filters over the area overnight, SN may also begin to mix in with the rain showers.

The main precip axis may drift farther west into northeast Illinois for most of the morning, with mixed precip over MDW and near ORD. Confidence remains too low to prevail or TEMPO all SN, but a period of IFR visibility in SN with slushy accumulations under 1 inch are possible at MDW.

By early/mid afternoon, the lake effect band will begin drifting east of ORD/MDW, with precip changes greatly diminishing by sunset.

Winds through Friday will remain NNW to briefly N (generally 330 to 360 degrees) with periods of gusts over 20 knots. Outside of any lake effect precipitation, expect periods of SCT to BKN high-end MVFR to low-end MVFR stratocumulus.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Weather Advisory from midnight tonight to 10 AM CST Friday for ILZ105-ILZ108.

IN...Winter Weather Advisory from midnight tonight to 10 AM CST Friday for INZ001.

LM...Gale Warning until 9 PM CST this evening for Winthrop Harbor IL to Burns Harbor IN.

Small Craft Advisory until 4 AM CST Saturday for Winthrop Harbor IL to Burns Harbor IN.

Small Craft Advisory until 4 AM CST Saturday for Burns Harbor to Michigan City IN.