Scientific Forecaster Discussion

000 fxus63 klot 231108 afdlot Area forecast discussion National Weather Service Chicago/Romeoville, Illinois 608 am CDT Tue Apr 23 2019 Short term... 330 am CDT Through Wednesday... Low pressure over northern lower Michigan has a pair of cold fronts, one extending south-southwest from the low which has largely cleared our County Warning Area. The secondary cold front extending west- southwest from the low has just pushed across the Illinois/WI border as of 0800 UTC and will continue to move south across the County Warning Area. The real push of cooler air resides behind this secondary front with temps largely in the 40s upstream across Wisconsin to the north of this boundary. Post-frontal stratus deck continues to engulf the County Warning Area and should blanket the area this morning, before the clearing from north to south this afternoon. Temps will be a bit tricky today. Southern County Warning Area looks to be last to clear out this afternoon and will probably experience the coolest highs in the County Warning Area. Closer to the WI/Illinois border, skies will clear earlier allowing for a longer period of time for the late April sunshine to do its magic in warming temps up. Further complicating temps today is Lake Michigan, which should initially provide a lake enhancement to the cooling over mainly northwest in, most pronounced closest to the lake. However, by this afternoon, lake breeze will begin advancing westward across northeast Illinois resulting in temps leveling off or even falling this afternoon, again most pronounced near the lake. With all this competing factors at work, certainly plausible that there will be some fine tuning needed to today's highs by the day shift as trends more fully reveal themself. Sharp southern stream trough that guidance has largely kept safely south of the County Warning Area mid-week, still looks to stay south, but now guidance has a weak northern stream shortwave on the northern flanks of this southern stream wave. Starting to see more guidance trend toward higher chances of at least some showers Wednesday, especially southern County Warning Area. For now, just bumped up pops from near 0 closer to 14 percent, but kept chances below mentionable precip thresholds for now. If this trend persists with the 12z suite of guidance, day shift will need to entertain the idea of adding some chances for light showers to the forecast for Wednesday. - Izzi && Long term... 330 am CDT Wednesday night through Monday... Northwest flow shortwave trough will ripple southeast through the Great Lakes region Thursday with associated surface cold front progged to move across the area Thursday afternoon/evening. While this is favorable diurnal timing for thunder chances, this system should be relatively moisture starved and not particularly dynamic. Have maintain slight chances of thunder, though confidence is low. Shot of seasonably chilly air will arrive in the wake of thursday's front to close out the week. Heading into the weekend and early next week, medium range guidance suggests we could end up getting into a more active pattern with parade of systems moving quickly across the region, each bringing chances of showers and some thunderstorms. Depending on where exactly the surface baroclinic zone sets up will dictate temps trends, which is difficult to pin down at this distance. Given this, so no compelling evidence to make any changes to blended model guidance. - Izzi && Aviation... for the 12z tafs... The main forecast concern today will be the timing of the northeast wind shift. Surface low pressure is shifting over lower Michigan early this morning, and this has allowed a cold front to push across the terminals. North-northwest winds in the wake of this boundary will continue this morning, but at some point by early to mid afternoon, expect the winds to shift northeastward off the lake. Confidence on the exact timing of this shift is a bit low at this time, but it could occur as early as midday. For this reason we pushed up the timing of the wind shift to 19z at the main Chicago terminals. Otherwise, lower MVFR cigs will stick around through the morning, before scattering out this afternoon. Expect VFR conditions to prevail thereafter. Kjb && Lot watches/warnings/advisories... Illinois...none. In...none. Lm...Small Craft Advisory...nearshore waters until 10 am Tuesday. && $$ Visit US at http://weather.Gov/Chicago (all lowercase)

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