Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 231127 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 627 AM CDT Sat Mar 23 2024 .KEY MESSAGES... - Scattered lake effect snow showers for portions of northeast IL and northwest IN this morning - Seasonably cool conditions for the weekend - Next rain chances arrive Sunday night into Tuesday. Potential for locally heavy rainfall late Monday afternoon and evening. - Breezy to windy conditions at times, particularly on Monday and Tuesday. && .DISCUSSION... Issued at 307 AM CDT Sat Mar 23 2024 Through Sunday: High pressure is beginning to drift into the upper Midwest beneath a broad upper trough that continues to pivot over the Great Lakes this morning. While precipitation associated with this upper trough has largely come to an end across the region, scattered lake effect snow showers have developed across the southern third of Lake Michigan and are starting to wiggle their way onshore near the IL-IN line and points east. Guidance unfortunately continues to struggle with the handling of the showers likely due to fact that most guidance is too dry in the low-levels. Though the RAP is starting to get a better handle and does show some modest lake induced equilibrium levels around 6500 ft persisting through mid-morning which should continue to support scattered lake effect snow showers across portions of northwest IN until instability wanes. However, forecast soundings show very limited moisture in the dendritic growth layer which I suspect will keep snowflake size small and limit any accumulations to under a tenth of an inch if any occurs at all. Otherwise, expect skies to gradually clear through the morning as the aforementioned high takes hold to our north. However, winds will remain out of the north-northeast which will continue to advect in cooler air and cap highs this afternoon in the mid-30s to lower 40s. Though warmer temperatures are expected for Sunday as winds turn southeasterly and warm advection takes hold. While this will allow temperatures to reach into the 40s to lower 50s Sunday afternoon, the warm advection also looks to generate some showers across portions of northeast IA and northwest IL tonight into Sunday morning. Guidance continues to favor that the bulk of this shower activity should remain west of the forecast area but given the modest forcing and instability noted in forecast soundings, I have decided to maintain the 20-25% chances for precipitation along and west of I-39. If this precipitation does pan out, it should fall in the form of snow and could even lead to some minor accumulations (a dusting) primarily on grassy and elevated surfaces. Additional showers may also occur Sunday afternoon across northwest IL as warm advection continues to strengthen ahead of our next weather system but elsewhere should remain dry. Yack Sunday Night through Friday: By Sunday night, attention will turn to a maturing low across Kansas and Nebraska. Persistent modest pressure falls and a robust 40-50 kt LLJ streaming overhead will yield intermittent 25 to 35 mph gusts during the overnight hours. With it looking like the main elevated instability/moisture axis will likely hang west of the Mississippi River through the overnight, precipitation chances may remain fairly low, at least through the late overnight/morning hours on Monday. Thereafter, broad ascent from a combination of increasing DCVA and upper jet divergence will arrive as the surface low barrels towards Iowa. Shower coverage will commensurately increase through Monday morning and afternoon. There may also be just enough elevated instability (MUCAPE) at the eastern fringes of the the main steepened mid-level lapse rate plume to supper a few embedded thunderstorms, although coverage at this time appears a bit too low to justify a formal mention in the gridded database. A trend in guidance over the last 24 hours has been to shift the system`s modest warm sector a bit farther to the west on Monday afternoon with widespread/persistent stratus and intermittent shower activity across our forecast area. Winds will likely also become intermittently pretty gusty through Monday morning and early afternoon, although just how strong these get is a bit unclear as low stratus will likely limit the depth of boundary layer mixing. For now, have capped gusts around 35 to 40 mph. The most intense and cohesive large scale forcing for ascent will arrive across our area Monday afternoon and into the evening as strongly-diffluent flow broadens in advance of a bifurcated upper jet. Precipitable water values look to surge past an inch which is around 200 percent of normal for this time of year. This will yield increasing warm cloud depths and precipitation efficiency. Thankfully, this system will remain somewhat progressive, with the mid-levels drying out readily late Monday night. That said, this looks like a pretty favorable setup for localized corridors of heavier rainfall, and it`s not surprising to see 12-hour precipitation amounts nearing 2 inches in spots from the global guidance. A cold front will sweep across the region on Tuesday, although moisture doesn`t look like it`ll be entirely scoured out until later in the day which will keep at least some shower activity going. Depending on the front`s exact timing, temperatures will probably hold steady or even fall through the day. After two chillier nights Tuesday night and Wednesday night, high temperatures look set to moderate back into the 50s and 60s through the end of next week. Carlaw && .AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
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Issued at 626 AM CDT Sat Mar 23 2024 Aviation weather concerns are: - Lingering lake effect -SHSN near GYY this morning, potentially briefly at MDW. - Potential (30%) for brief period of -SN INVOF RFD late tonight. Flurries not out of the question at ORD, DPA, MDW. Lingering MVFR cigs will continue to lift/scatter to VFR at ORD/MDW shortly, with MVFR cigs hanging on at GYY through this morning. One last batch of lake effect snow showers are expected to push across GYY later this morning, and have included a TEMPO group to catch this. There`s also a potential these snow showers clip MDW in the 14-16z timeframe, but latest radar loops suggest this activity may remain just east of the airfield. North to NNW surface winds will trend NNE with time this morning, and then more easterly this afternoon with a lake breeze. Another disturbance will arrive tonight and will bring a chance for a brief period of snow to the region. The main chances currently are at RFD where a PROB30 for MVFR -SN has been introduced. Some guidance supports a brief period of light snow even at ORD, DPA, and MDW, but chances for this seem to be less than 30 percent, so have left the Chicago-area TAFs dry tonight. Breezy southeasterly winds will develop on Sunday morning. Carlaw
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&& .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 10 PM CDT this evening for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago

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