textproduct: Chicago

KEY MESSAGES

- Chances for light snow across the region tonight into Sunday which could result in a dusting/light coating in spots.

- Thereafter, accumulating lake effect snow is expected to result in travel impacts across parts of NW Indiana (Porter County in particular) through Monday.

DISCUSSION

Issued at 219 PM CST Sat Feb 21 2026

Through Monday:

Water vapor loops reveal a well-defined vort max pivoting directly overhead early this afternoon. While regional radars are sampling a decent amount of mid/upper-level returns, recent aircraft soundings indicate there's a roughly 6-8 kft thick layer of dry air present between these returns and the widespread low stratus deck.

As the aforementioned vort max begins to pivot across the south end of the lake, a secondary shortwave is forecast to drop out of Wisconsin later this afternoon and evening. Associated ascent will lead to some continued erosion of this dry layer which could eventually support a seeder-feeder process (or even full saturation), allowing some light to develop across parts of northeast Illinois and northwest Indiana. It's still not entirely clear this will be the case, however, with overall modest ascent and a lack a deeper layer of unstable air aloft. Top-down moistening will continue overnight, with light snow development possible, primarily across the northeast two thirds of the forecast area, but uncertainty in whether dry air will erode sufficiently to support light accums remains. Did not make any big changes to the going forecast which seems to handle the uncertainty and potential for light coatings appropriately. Across parts of Porter County, some lake enhancement is expected to develop tonight into Sunday morning.

Lingering light snow will generally come to an end through Sunday morning with intermittent non-accumulating flurries possible at times. Lake enhancement may persist across parts of NW Indiana, and particularly Porter County. Through Sunday afternoon/evening, 850 mb temperatures will cool as another vort max drops southward down the lake. This will lead to modestly increasingly lake-induced ELs, particularly into Sunday evening and overnight. Latest hires guidance suggests that generally multi-banded LES will materialize initially Sunday afternoon and evening, focusing mostly across Porter County but potentially backing into parts of Lake County Indiana at times. Overnight into Monday morning, some consolidation of a primary/dominant band is expected, but given BL winds near 330-340 degrees, it looks like the most intense convergence (and thus snowfall rates) will tend to focus just east of our area. Regardless, steady lake effect/enhancement looks to occur over Porter county into Monday morning, and with temperatures in the 20s, travel impacts appear likely into the AM commute. Inversion heights will lower and the lake boundary layer will dry quickly Monday afternoon, bringing accumulating LES to an end.

The initial period of snow/lake enhancement across Porter county tonight/Sunday morning may not amount to too much, perhaps and inch or two, with the main window of impacts from LES expected to occur later Sunday afternoon/evening and into Monday morning where several additional inches of accumulation are expected, particularly over the northeast quarter of the county. While a brief lull in activity is possible towards midday, elected to hoist a single longer-duration winter weather advisory as opposed to handling tonight's snow with a special weather statement to avoid confusion.

Carlaw

Monday Night through Saturday:

A fairly active synoptic pattern looks to take shape across the northern CONUS through next week as numerous shortwaves dive south and eastward out of Canada and ride along the northern terminus of a fast quasi-zonal upper jet streak.

The first feature of interest is a clipper system which will drop out of Saskatchewan/Manitoba Monday night into Tuesday. Guidance has remained fairly consistent in taking a mid 990s mb surface low across either the UP of Michigan or northern Wisconsin/lower Michigan Tuesday night, with the bulk of the mid-level heights falls displaced just to our north. Upper jet divergence will spread directly overhead, however, and given the presence of slivers of steeper lapse rates aloft, a potential for snow is evident with this feature, particularly north of I-80 where ensemble probabilities for light QPF continue to increase.

Additionally, strong/gusty southwesterly winds will develop Tuesday afternoon in the preceding warm advection regime. Suspect the NBM-delivered wind gusts are a bit too low right now, with some upward adjustments closer to 35 mph a possibility in future forecast updates. Still continue to see at least a modest potential for a heightened grass/brush fire threat Tuesday afternoon (where it doesn't snow) with RHs dropping to around (or even a bit under) 30 percent.

Uncertainty increases markedly during the Wednesday through Thursday period, as guidance varies significantly in the handling of a series of additional shortwaves, and the degree to which they phase (or not). That said, have seen a general clustering of the modeled surface low tracks within the ensemble suite in our general vicinity or just to our south. Today's deterministic medium range suite suggests some increase in the potential for accumulating snow across parts of the region, but given the look of the features upstairs, don't have a lot of confidence in any trends at this juncture.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1208 PM CST Sat Feb 21 2026

Key Messages:

- Periods of flurries and possibly more bona fide snow showers are expected over much of the TAF period, mainly at the Chicago metro terminals.

- MVFR ceilings are expected to remain prevalent through much of the TAF period.

- Blustery northwesterly winds will occur during the daytime tomorrow.

A vorticity maximum that is evident over the Quad Cities on GOES satellite imagery at press time will twirl eastward this afternoon into this evening and pass over the terminals. As it drifts overhead, it may induce deep enough saturation to produce a burst of snow late at the Chicago metro terminals late this afternoon/early this evening. The latest runs of a pair of short-term forecast models have trended towards this scenario, but confidence in this ultimately occurring remains relatively low. Elected to have this possibility highlighted in a PROB30 group for now.

Another period of more bona fide snow showers may be observed later this evening into the early overnight hours. However, confidence in this similarly remains low on the whole, and thus did not make many changes to the inherited PROB30 groups for this period. Regardless of whether these periods of steadier snowfall materialize, periodic flurries continue to look like a good bet through much of the current TAF period at the Chicago metro terminals, with lower chances/confidence in flurries occurring at RFD.

The MVFR stratus that remains overhead early this afternoon is largely expected to persist for the next 24-30 hours. There could be some periods where cloud bases lift to VFR or some breaks in the low cloud deck pass over the terminals, but it remains difficult to pinpoint when that may occur.

Otherwise, expect initially westerly winds to veer northwesterly by this evening while gradually increasing in magnitude through tonight. During the daytime tomorrow, sustained winds are likely to reach or exceed 15 kts with gusts favored to peak at around 25 kts.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Winter Weather Advisory from midnight tonight to noon CST Monday for INZ002.

LM...Small Craft Advisory from 3 AM Sunday to 3 AM CST Tuesday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory from midnight tonight to 6 PM CST Sunday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

Gale Watch from Sunday evening through Monday afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.