textproduct: Chicago

KEY MESSAGES

- A prolonged lake effect precipitation event will begin today, with lake effect rain showers initially occurring in northwest Indiana and portions of far northeastern Illinois.

- Rain will transition to snow in the lake effect band tonight, and there is growing concern that a few to potentially several inches of wet snow accumulation will lead to localized travel impacts late tonight into Friday morning, including during the Friday morning commute. - Additional periods of lake effect rain and snow showers will then continue through early Saturday.

- Cooler temperatures are expected late this week into this weekend before another warmup occurs next week.

DISCUSSION

Issued at 400 AM CST Thu Nov 12 2026

A much cooler air mass will continue to spill into the Great Lakes region today as longwave troughing over eastern Canada becomes re-centered over the eastern Great Lakes and multiple embedded shortwave troughs along its western periphery nosedive southward from western Ontario towards northern Illinois. The 00Z RAOBs from both GRB and APX last evening sampled an 850 mb temperature of around -1C, which isn't all that remarkable for this time of year at these RAOB sites. However, when paired with lake temperatures that are still in the low-mid 50s (11-13C) and an elongated northerly fetch over Lake Michigan, this initial push of colder air has been cool enough for some relatively disorganized lake effect showers to stream into portions of northwest Indiana this morning.

The lake effect shower activity should steadily increase in coverage, organization, and overall intensity over the course of the day today as 850 mb temperatures gradually fall towards and eventually into the -5 to -7C range over Lake Michigan. By late tonight, water surface-850 mb temperature differentials will be nearing 20C, while water surface-700 mb temperature differentials will be pushing 30C. This will yield a deep convective cloud layer, with an analysis of HRRR and RAP forecast soundings in BUFKIT indicating that lake-induced CAPE values should largely remain at or above 500 J/kg through a large portion of this lake effect event, likely peaking somewhere around 700 J/kg towards midday Friday. With the northerly fetch over Lake Michigan not expected change much anytime soon, a prolonged lake effect precipitation event can be expected for a portion of our forecast area from today through early Saturday, with the lake-induced thermodynamics supportive of efficient precipitation rates at times and also a potential for waterspouts over Lake Michigan.

During the daytime today, air temperatures largely in the 40s will result in rain being the sole precipitation type. The latest hi-res model guidance suggests that the primary lake convergence axis will remain relatively steady with its position and orientation through the daytime hours, which should focus most of today's lake effect showers into our northwest Indiana counties. This is in contrast to earlier model runs, which suggested that a large footprint of lake effect showers could penetrate well westward into Illinois, though areas roughly east of I-355 in the Chicago metro and near/east of I-57 farther downstate may still nevertheless get in on the lake effect action at some point later this morning or afternoon. Still couldn't rule out a brief period of non-lake-induced convective showers or sprinkles farther to the west in interior portions of northern Illinois, though moisture depth/quality will be a factor in limiting the spatial footprint of this activity. In addition, it should also be noted that blustery northerly winds are also expected throughout the day today, which will prevent temperatures from rising much above this morning's lows.

By this evening, a more dominant, singular lake effect precipitation band should emerge. Air temperatures will also fall into the low-mid 30s (and possibly into the mid-upper 20s wherever skies end up clearing out) after sunset as cold air advection continues, which will allow for the rain in the lake effect band to mix with and eventually change over to snow in most locations. The main exception to this may be in the immediate vicinity of the lakeshore, where the mild lake- influenced air advecting inland could potentially prevent an outright changeover to snow. Even in the most bullish/coldest hi-res models, though, near-surface wet bulb globe temperatures aren't progged to fall much below 32F, which -- when paired with relatively mild antecedent ground temperatures -- would make it difficult for snow to accumulate, particularly on warmer paved surfaces.

In these scenarios, snowfall rates need to be fairly high in order to overcome the less-than-ideal thermal environment and lead to meaningful snow accumulation, and in this particular case, the lake effect band should be robust enough for at least lower-end localized slushy, wet snow accumulations to occur. In the worst case (which has been depicted in a handful of hi-res model runs, but not ubiquitously across the hi-res guidance suite), a localized "bullseye" of several inches of wet snow accumulation could occur somewhere if the band were to park itself and remain stationary over the same area. Confidence in such an outcome occurring in our CWA is low at this time, but it's certainly within the realm of possibilities at this point.

Not unsurprising for a lake effect setup like this, uncertainty also remains with where the "average" position of the lake effect band will end up and how far west it will ultimately wobble. It should be noted that hi-res models have been showing increasing support for a more westerly mean positioning of the band Thursday night into Friday morning before gradually shifting back eastward during the daytime on Friday. This scenario verifying would result in more snow occurring on the Illinois side of the Illinois-Indiana state line than previously expected -- though again, this is not a solution that has unanimous support at this time.

While snow will likely continue in the lake effect band past sunrise on Friday, an eventual transition back to rain is expected by the afternoon as air temperatures warm back towards the lower 40s. Then, as temperatures cool once again Friday night into Saturday morning, snow should once again start to become the dominant precipitation type in the band.

All things considered, ultimately opted to err on the side of caution and issue a Winter Weather Advisory for the Thursday night into Friday morning time frame for Lake County, IN, which is the county that currently appears to be the most likely to see travel impacts from accumulating wet snow during this time window. While the borderline thermal profiles, along with the inherent uncertainties that come with forecasting the spatial evolution of a lake effect band, do still leave us with a lot of question marks over how things will play out with this lake effect event, the possibility of the Friday morning commute being affected and the fact that this will be our first accumulating snowfall event of the year tipped the scales in favor of issuing an advisory. Will let the incoming day shift take another look at things and see if the advisory will need to be expanded into any adjacent counties, but for now, will also be issuing a Special Weather Statement for the counties surrounding Lake County, IN to highlight the potential for accumulating lake effect snow where impacts from this snowfall are less likely to occur at this time.

Otherwise, cool temperatures with daytime highs largely in the 40s will persist through the weekend as the region remains entrenched in the cool air mass associated with the aforementioned mean longwave trough. However, ensemble guidance continues to favor this troughing getting replaced by high- amplitude ridging by early-mid next week. This will result in another stretch of precipitation-free conditions and a warming trend that should send temperatures back up to near to above normal readings for mid-November.

Ogorek

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1208 PM CST Thu Nov 12 2026

Key Forecast Messages:

- Lake effect precipitation impacting the main Chicago terminals (ORD and MDW) and GYY at times, especially tonight into Friday.

- Lake effect band of rain will mix with and change to a period of wet snow late tonight into Friday morning, particularly near the IL and IN state line. The snow may impact the main terminals for a couple hour period around and just after daybreak Friday morning.

- Gusty northerly winds, gusting 25-30 kt this afternoon.

Lake effect rain showers are expected to remain focused into northwestern IN over the next few hours this afternoon. Thereafter, the focus is expected to begin to shift back westward towards the northeastern IL shore. , the threat exists for showers making as far west as ORD and MDW later this afternoon into this evening. Precipitation type with this activity through the evening is expected to remain primarily in the form of rain as temperatures hover in the upper 30s.

Cooling temperatures overnight will support a mix with, and possible switch to, a period of wet snow. This looks to particularly be the case for areas across southern Cook southward along and near the IL and IN state line. Accordingly, due to the proximity of ORD and especially MDW and GYY to this lake effect precipitation, we have maintained a mention for a period of snow showers and associated VSBY very late tonight through mid morning on Friday.

Winds will be strong out of the N to NNW this afternoon with gusts to 25 to 30 kt expected. Winds will ease a bit this evening, but will remain breezy through the end of the period with gusts around 20 to 25 kt.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Winter Weather Advisory from midnight tonight to 10 AM CST Friday for INZ001.

LM...Gale Warning until 5 PM CST this afternoon for the IL and IN nearshore waters.

Small Craft Advisory from 5 PM this afternoon to 4 AM CST Saturday for the IL and IN nearshore waters.