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KEY MESSAGES

- Mild conditions are in store today through Tuesday before the pattern turns decidedly more wintry on Wednesday and beyond, with below normal temperatures and occasional periods of snow/snow showers.

- The main concern from a winter weather impacts perspective is Wednesday into Thursday, with wind-whipped snow showers during the day Wednesday likely followed by accumulating lake effect snow Wednesday night into Thursday morning, primarily for parts of northwest Indiana.

DISCUSSION

Issued at 326 AM CST Mon Jan 12 2026

Through Tuesday:

The week will start off on a breezy and milder note, with temperatures in the 40s both today and Tuesday. Some of the warmest spots could even tag 50 briefly Tuesday afternoon. Dry weather is expected into tonight, with the potential for isolated sprinkles on Tuesday.

The upper level pattern features a broad trough across the eastern CONUS early this morning, and ridging along the west coast. A few low-amplitude short waves are evident in the northwest flow from the Pacific Northwest into the Great Lakes/Midwest region, the most notable across the northern Lakes. This wave is currently producing some light snow well to the northeast of the forecast area, and will not be a factor locally. A couple of more subtle waves/speed maxima are noted upstream of the area in GOES vapor imagery, and these will zip southeast from the Northern Plains and across the area through this afternoon. 00Z RAOBs from upstream sites depict fairly dry low-levels across the region however, and the most visible impact of these ripples will be to bring some passing high clouds across the area. Otherwise, we'll enjoy breezy low-level southwest flow across the region (some gusts to around 25 mph especially this morning) which will result in milder temperatures in the low-mid 40s for the forecast area today. Similar to Sunday night, we'll likely lows in the low-mid 30s this evening, with temps then holding fairly steady overnight.

Farther upstream, another short wave topping the western ridge this morning is forecast to gradually amplify as it tracks southeast toward the upper Midwest and northern Lakes region tonight. Guidance indicates an associated sub-995 mb surface low developing near Lake Superior by midday Tuesday, maintaining and further inducing breezy southwest flow into the Midwest in the process. While more significant precipitation is expected to remain north/northeast of the forecast area late tonight into Tuesday, some high-res guidance does generate some spotty light QPF across the cwa mainly during Tuesday afternoon when deeper large scale forcing strengthens. Despite the increasing ascent however, low-level moisture return appears meager due to surface ridging along the Gulf coast. Forecast soundings depict mainly mid-level saturation with substantial dry air below, which suggests mainly spotty light rain or sprinkles reaching the surface here.

While cloud cover is expected to thicken on Thursday, persistent southwest low-level warm advection flow should warm temperatures a bit higher than today. Highs are expected in the upper 40s in many areas, perhaps tagging 50 degrees in a few spots.

Ratzer

Tuesday Night through Sunday:

A prolonged colder and unsettled northwest flow pattern will set up in response to high amplitude ridging near the West Coast. As the ridging further amplifies up to or north of the Gulf of Alaska over the weekend into next week, this will dislodge the coldest air mass we've seen since back in mid December southward into the Midwest. The western ridging- eastern troughing pattern exemplifies the positive phase of the Pacific North American (PNA) teleconnection pattern, while the high latitude blocking setting up near the Gulf of Alaska this weekend and beyond represents the negative phase of the East Pacific Oscillation (EPO) teleconnection pattern. The first prominent short-wave/PV anomaly diving southward on Wednesday will potentially bring an impactful lake effect snow event downwind of southern or southeast Lake Michigan into Thursday.

Most of Tuesday night will be quiet and breezy with temperatures still above normal. This will abruptly change by Wednesday morning as the aforementioned robust PV anomaly dives southward across the area in tandem with a secondary cold front passage packing much stronger cold air advection. As winds shift to north-northwesterly with gusts up to 30-40 mph, strong large scale forcing will result in quickly blossoming snow showers during the morning.

While there's still some uncertainty in the westward extent of sufficient moisture through the DGZ during the morning, run-to- run continuity across the suite of global ensemble systems remains good on the whole and supports the latest NBM initialization's 30-80% PoPs across our forecast area on Wednesday morning (highest PoPs in northeast Illinois and northwest Indiana and lowest PoPs towards northwest and central Illinois). Depending on how this setup evolves from a convergence and inversion height perspective, very steep low- level lapse rates could yield embedded snow squalls. This would be paired with temperatures falling quickly to the mid 20s to lower 30s by the late morning. Initial melting on mild antecedent pavement could conceivably be followed a flash freeze scenario in a more strongly forced snow squall type regime. If there ends up being scattered less intense wind-whipped snow showers, that may serve to limit potential impacts.

The main notable change from the past few model cycles vs. previous cycles is an eastward shift in the trajectory of the PV anomaly. This would focus the highest snow shower coverage for northern and central Illinois into the morning hours and then could result in flurries at most for these areas by the afternoon. Since this was a recent change, to account for a plausible nudge back west some with the forcing, opted to hold onto fairly expansive chance PoPs Wednesday afternoon. Higher 60-80% PoPs were confined to areas primarily near and east of I-65 in northwest Indiana. Aside from the snow shower details, it will be blustery and cold with wind chills falling into the high single digits and lower teens prior to sunset.

As progged 850 mb temperatures plunge to -15 to -18C by early Wednesday evening, increasingly favorable lake effect thermodynamics will come into play. In fact, lake enhancement of the snow showers may occur as early as the late AM and early afternoon into northwest Indiana. However, the main event, so to speak, looks to be towards and after sunset Wednesday. Based on recent guidance trends, can't ignore the eastward shift of the focus of the PV anomaly and the lake induced convergence axis. With this in mind, barring a shift back, the impactful accumulating wind-blown lake effect snow threat will likely be confined into portions of northwest Indiana. Some solutions suggest the higher end threat could aim into LaPorte County, though feel confident that at least some snow will fall into at least northeastern Lake and most of Porter County.

Lake induced equilibrium levels as high as 8-12kft, 300-500 J/kg of lake induced CAPE, convergence down the long axis of the lake, and lingering synoptic lift are factors arguing for heavy snowfall rates (and high ratios) within the heart of the band. Given the recent shifts, opted to cap PoPs at 60-70% into northeast Lake and Porter Counties (likely PoPs extending into northern Jasper) through Thursday morning. Preliminarily will advertise several inches of accumulation, though in reality, the focus area will be much narrower (along with higher amounts in the heart of the band/ie. double digits plausible). Of significant concern as well is the very strong northerly winds gusting to 40-45+ mph modeled in the heart of the band, so blowing and drifting snow will likely exacerbate impacts. If the lake effect snow threat continues to include portions of our northwest Indiana counties as the event draws nearer, we may very well need to hoist a watch within the next few cycles.

Any break following the probable lake effect snow event may be short-lived, with signs pointing toward several additional clipper type systems affecting the region through the weekend. The first system Thursday night could have a period of accumulating snow in the warm advection regime, followed by waves of scattered snow showers Friday-Friday night. Maintained broad-brushed chance PoPs peaking in the 30-60% range Thursday night-Saturday given approximately that much ensemble member support. Due to the deep northwest flow pattern, none of the upcoming systems will have abundant moisture to work with. Nonetheless, with temperatures trending to well below normal to close out the week and through next weekend and beyond, any minor accumulations can be expected to cause some travel impacts. Sub-zero wind chills in the overnight-morning periods are in the forecast Saturday night through Sunday night.

Castro

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 508 AM CST Mon Jan 12 2026

Aviation forecast concerns for the 12Z TAFs:

- Breezy southwest winds with gusts around 20 kt this morning, easing remainder of the day.

- Southwest gusts 20-25 kts develop predawn Tuesday. Could be a LLWS threat early Tuesday morning if surface gusts are slower to develop.

Northern IL and northwest IN terminals will see an extended period of mainly southwest winds through this TAF cycle. Will likely be some gustiness to around 20 kts this morning, though this should ease gradually through this afternoon. Model forecast soundings suggest fairly strong decoupling this evening, which should limit wind gusts during the evening and early overnight hours, after which development of a west- southwesterly 40-45 kt low level jet is expected toward daybreak Tuesday. Expectation is that surface winds will increase gradually with at least some gustiness during this time, mitigating the threat of LLWS. Based on this scenario did not include LLWS in TAFs at this time, but will monitor surface wind trends for early Tuesday for a possible LLWS inclusion if needed (would be toward 12Z and after if needed). Surface wind gusts to around 25 kts from the southwest appear likely later Tuesday morning.

Otherwise, VFR conditions are expected through the period with patchy high clouds today, thickening and lowering to a mid deck later tonight/early Tuesday.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 9 AM CST this morning for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until noon CST today for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.