Chicago (LOT) — the National Weather Service forecast in plain English · Plaincast

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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 through early this morning, resulting in a few inches of wet, slushy accumulations that will lead to some localized travel impacts. At or above- freezing temperatures are expected to limit the coverage of impacts, however.

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

Discussion

A single, dominant lake effect band stretches nearly down the entire fetch of Lake Michigan early this morning with upstream radars largely overshooting the shallow convective cloud depths across the northern reaches of the lake. A quick-moving shortwave continues to drop southward out of Wisconsin, and is delivering a surge of colder mid/upper-level temperatures (500 mb temperatures analyzed near -30 C across southern Wisconsin). This in turn is resulting in increasing lake-induced thermodynamics with equilibrium heights progged nearing 12-14 kft which generally matches radar returns showing cloud tops nearing similar heights over the mid lake. The lake thermodynamics will peak from late tonight through early this afternoon before gradually diminishing through Friday night and Saturday morning.

Near surface convergence is quite intense at this time, with slow veering noted in observation trends across western lower Michigan and persistent gusty northwesterly winds on the western side of the lake. While earlier guidance was having some difficulty handling the location of the primary convergence axis, latest high-res guidance is generally doing a better job with near-term trends. With the passage of the aforementioned shortwave aloft veered low-level flow across the southern end of the lake, expecting the lake band to make a gradual westward push towards the Illinois shore through late tonight and into the early morning hours.

Have recently noted a few mPing reports of a rain/snow mix developing across interior portions of Lake County, Indiana where surface wetbulb temperatures have started to cool towards 36 degrees, although will note the transition has been slower than earlier guidance would have suggested. Expectation is for the rain/snow mix area to gradually expand over the next few hours (away from the immediate lake front) and eventually transition to all snow, particularly on the western portions of the band where surface temperatures are coldest.

It's always difficult pinpointing exactly where these lake effect bands will move, particularly with the potential for embedded meso-lows. However, the latest high-res guidance suite remains steadfast in suggesting the longest duration of precipitation setting up in the vicinity of the IL/IN state line around the eastern Will, western Lake (IN), and far SE Cook county border regions or thereabouts. Marginal surface temperature profiles (wetbulb temperatures generally in the mid 30s) and a warm antecedent ground will tend to limit snow accumulations in an otherwise highly favorable environment for intense snowfall rates through this morning. Indeed, it will take the presence of intense snowfall rates to result in any meaningful accumulations as snow will slowly melt when/as rates ease. As such, long-duration snow accumulation output is likely not a representative solution for most of the area, as additional melting will take place this afternoon as temperatures warm into the upper 30s.

In the heart of the heaviest part of the lake effect snow band, the potential exists for a hyper-localized corridor of potentially more significant accumulations north of 5 inches, but this if only IF high snowfall rates remain persistent for more than a few hours this morning. At this time, the areal extent of this higher snowfall threat looks to be much too small to meaningfully capture in the gridded forecast, and it's still not totally clear this will materialize given the already- mentioned marginal thermal profiles. Latest forecast remains largely similar to the late-evening update, with a general 2-4 inch swath across SE Cook, eastern Will, and western Lake Counties, and even so, most accumulations may end up relegated to elevated or grassy surfaces. Did also gently expand some light accumulations farther south into eastern Kankakee County and also farther inland towards Kendall/Grundy/eastern Livingston counties as there will be a potential for more inland movement of the band this morning as low-level lapse rates steepen.

While lake effect precipitation will continue this afternoon, any additional accumulations look to be minor and relegated to grassy surfaces. This will result in a transition back to a rain/snow mix or all rain in most locations. Lake thermodynamics will still support periods of heavier precipitation into this evening and early overnight, but the potential for persistent heavy snowfall rates will diminish. As such, while additional slushy accumulations are possible this evening, potential for hazardous travel conditions appears too low to justify extending the Winter Weather Advisory through the night, particularly given the potential for melting this afternoon.