Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 191142 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 542 AM CST Sun Jan 19 2020 .SHORT TERM... 347 AM CST Through Monday... Today through Monday night... Arctic blast, with blustery winds and bitter cold wind chills are the main forecast highlights in the near term, as well as the potential for some lake effect snow showers later today into Monday. Some of these lake effect snow showers may affect portions of Cook county late this afternoon and tonight. Gusty westerly winds continue to push arctic air into the region early this morning, within a tight surface pressure gradient between departing low pressure to the northeast, and 1040+ mb high pressure building southeast across the Northern Plains. Surface observations as of 3 AM CST indicate air temperatures have fallen to around zero across our western cwa, and the lower single digits across the east. Sustained winds near 20 mph, and gusts of 30-35 mph were producing wind chills in the -20 to -22 range from the Fox Valley westward, where current wind chill advisory remains valid through 10 AM this morning. While some slight moderation of the air mass is expected today, temperatures will struggle to rise from our morning lows into the low-mid teens at best by this afternoon. Winds will slowly diminish, but will remain blustery through the day, keeping wind chills near zero in many areas even into the afternoon. Areas of cold advection stratus will make for partly to mostly cloudy skies, with the relatively shallow but cold cloud layer at ideal temperatures for producing flurries at times. With westerly low level winds this morning, lake effect snow showers have been focused across the lake, east of the forecast area. However, a subtle surface trough across northern Lake Michigan will push southeast today in association with mid-level short wave digging around the trailing periphery of the departing storm to our east. Winds, particularly just above the boundary layer, are progged to veer more northerly with time late this afternoon and tonight, which is expected to allow lake effect snow showers to edge back into our northwest Indiana counties and even far eastern and southeastern Cook county. The dry nature of the arctic air, and relatively shallow inversion heights (maxing out around 6 kft tonight) should preclude significant measurable snow, though amounts of perhaps 1-3" will be possible, especially across Porter County where the longest residence time and strongest convergence appears likely. Minor accumulations of less than an inch may occur into the Cook county shore areas, especially from downtown Chicago southeastward. Backing/decreasing low level winds should push the weakening band back across Lake and Porter counties Monday morning which along with a lowering subsidence inversion should bring lake snow to an end by afternoon. Temperature wise, only very slow moderation of the arctic air mass is expected through Monday. Lows should dip into the single digits in many areas tonight despite some cloud cover, with recovery into the teens and lower 20s Monday. Clearing clouds and diminishing winds Monday night will allow for similar if not slightly colder mins than tonight. Ratzer && .LONG TERM... 347 AM CST Tuesday through Saturday... Upper ridging builds across the Missouri and Mississippi valleys and western Great Lakes into mid-week, while surface high pressure drifts southeast into the Ohio and Tennessee valleys. This will make for dry and slowly moderating weather Tuesday and Wednesday. Potentially active weather looks to return Wednesday night through Saturday however, with medium-range models depicting the development of a closed upper trough across the central CONUS. While differences in the details of various guidance remain, the general consensus is for the development of broad southwesterly warm/moist advection flow to develop Wednesday night ahead of a baroclinic frontal zone which lays out from Ontario back into the Central Plains. While this focus is initially west of the forecast area, there is evidence of a lead short wave lifting northeast along the baroclinic zone Wednesday night, which may spread some very light snow into parts of northern IL overnight. Weak but persistent warm advection flow will present the potential for additional light precip Thursday, though the greater potential for more organized precipitation appears to come late Thursday through Friday night as the upper trough closes off and a deepening surface low develops and lifts from the lower Mississippi Valley toward the Great Lakes. The GFS/CMC/ECMWF and their ensembles vary with the track of the low at this distance (GFS warmer and farthest west, the CMC farthest east), so confidence in details is still low at this distance. It does appear however that the potential for a storm system to produce some sort of measurable wintry precipitation exists during the late week period. Ratzer && .AVIATION... For the 12Z TAFs... 542 AM...Forecast concerns include... Lake effect snow showers late this afternoon into tonight. Mvfr cigs through tonight. Gusty west/northwest winds today. Lake effect snow showers are possible later this afternoon and especially tonight across northwest Indiana and these are expected to reach gyy and may move into far northeast IL. Confidence is low for how far west these snow showers will move and there remains quite a bit of uncertainty regarding duration at any one point. For now have opted to include only prob mention this evening for mdw and for all of tonight at gyy. Scattered flurries will be possible across the entire area today and tonight. There have been periodic holes in the mvfr deck early this morning and this is likely to continue as well as periods with just vfr cigs but for now maintained mvfr cigs for much of the period. Lower cigs would be possible with any lake effect snow showers. West/northwest winds have diminished overnight with gusts now in the lower/mid 20kt range and these gusts will likely continue this morning with a slow diminishing trend this afternoon with speeds under 10kts this evening and wind directions slowly turning more northerly tonight. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Wind Chill Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011- ILZ012-ILZ019-ILZ020-ILZ021-ILZ032 until 10 AM Sunday. IN...Lakeshore Flood Advisory...INZ002 until noon Sunday. LM...Heavy Freezing Spray Warning...LMZ740-LMZ741-LMZ742-LMZ743- Gary to Michigan City IN until noon Sunday. Small Craft Advisory...Winthrop Harbor to Gary IN until 9 AM Monday. Gale Warning...Gary to Michigan City IN until 6 AM Sunday. && $$ Visit us at weather.gov/chicago Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/NWSChicago www.twitter.com/NWSChicago www.youtube.com/NWSChicago

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