949 FXUS63 KLOT 220007 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 607 PM CST Thu Dec 21 2023 .SHORT TERM... Issued at 256 PM CST Thu Dec 21 2023 Through Saturday morning... Key Messages: * Drizzle and a few showers develop by this evening with a 20% chance that patchy drizzle lingers into Friday AM. * Widespread rain arrives late Friday AM/early PM and continues through Friday night, ending with drizzle Saturday AM * Reduced visibility in fog and rain/drizzle Friday PM-Saturday AM A pair of shortwaves currently moving across the central CONUS will be responsible for our rain and drizzle chances over the next 48 hours or so -- the first being a subtle lower amplitude wave moving along the IL/WI/IA border and a second and more defined wave moving across western KS. The lead wave has resulted in a gradual increase in low stratus coverage across portions of northern Illinois. More recently, weak radar echoes have developed west of the Chicago metro between the I-80 and I-88 corridors, and wouldn't be surprised if there are spits of drizzle already reaching the ground in those areas. This will continue to blossom across the area through the evening. As the wave departs to the east, cloud depths begin to decrease overnight into Friday morning, suggesting any light showers would transition over to mainly drizzle to potentially even a period of dry conditions. A broader region of rain has developed across central Kansas this afternoon associated with the western and stronger of the two waves. This will continue to lift east-northeast toward the region overnight with the expectation that rain will overspread the local area by late morning into the early afternoon hours. Rain will then persist through the evening and into the overnight hours before ending from west to east, though cloud depths remain deep enough to support patchy drizzle into Saturday morning. Given the moist atmosphere in place, reductions in visibility due to fog are expected by late Friday into Saturday morning. Be prepared for slowed travel during the afternoon/evening commute and those that are traveling for the Christmas holiday. In spite of widespread cloud cover, temperatures today have managed to climb into the mid-upper 40s, with a couple spots even reaching 50! The expectation is that diurnal temperature differences will be fairly narrow thanks to a continuation of cloudy skies through the overnight hours with lows in the upper 30s to lower 40s. Highs tomorrow warm back into the mid-upper 40s. Petr && .LONG TERM... Issued at 256 PM CST Thu Dec 21 2023 Saturday afternoon through Thursday... Key Messages: * Well above normal temps through the period, mildest Christmas Eve * Drizzle may persist at times through early Sunday (20-30% chance of measurable precip), along with potential for patchy fog late Saturday evening into Sunday * Rainy, mild, and breezy Christmas Day into Monday night * Showery weather (30-60% precip. chances) lingering into or Wednesday Low stratus should hang tough Saturday afternoon through Sunday morning. Contingent upon sufficiently deep lower level saturation (usually 1500 ft+ is more favorable), modest isentropic lift could continue to wring out drizzle at times. Maintained patchy drizzle mention Saturday afternoon and then slight to low chance (20-30%) of drizzle Saturday evening into early Sunday given uncertainty in saturation depths. Given the light southeast winds, moist boundary layer and likely already low cloud bases, the stratus may build down to patchy fog late Saturday evening and persist into Sunday morning, eroding mid-late AM when southerly winds increase. Patchy fog mention in the grids appears reasonable given some uncertainty, but also support from some of the visibility guidance. High temperatures Saturday should top out in the upper 40s to lower 50s and then drop very little Saturday night. Sunday is a bit of a wild-card temperature wise. Any way you slice it, it will be unseasonably mild, with high confidence in at least 50s. Our current forecast leans on the cloudy side with highs in the mid 50s to locally upper 50s I-80 and south (except rare modest December lake cooling IL shore). However, if the low clouds can scour out and yield filtered sunshine, in concert with increased southeast flow and dew points around 50F (!), ~60F readings are not out of the question. The overall best chance for some sun prior to sunset looks to be I-80 and south where relatively warmer temps are featured. Some spotty light showers may occur Sunday afternoon, though most recent trend has been a drier one vs. previous cycles. Sunday night into Monday, troughing emanating from the Rocky Mountain region will consolidate into a closed mid-upper low over Missouri. The surface low will deepen some as it lifts north through Monday night and then occlude and then meander eastward through mid next week. As is common with closed mid-upper level circulations, the guidance diverges on the exact handling of this feature, which will determine how long into the week showery conditions hang on. Confidence is high in rainy and breezy conditions much of Christmas Day, with highest (~80-85%) PoPs in the afternoon and evening in line with highest ensemble PoPs. Temperatures will be in the lower 50s, with east-southeast winds gusting to around 25 mph Monday-Monday evening, so mild but unfortunately inclement conditions. Regardless of differences in handling the evolution and path of the upper low, a good multi-model and ensemble signal suggests waves of showers persisting late Monday night through Tuesday evening, supporting likely (~60%) PoPs for most of the CWA. PoPs drop into the chance range late Tuesday night through Wednesday evening (~30-40%) and then slight chance (~20%) late Wednesday night-Thursday, reflecting increased uncertainty. Temperatures will step down to merely solidly above normals Tuesday onward, especially at night. Our forecast highs are in the upper 40s-low 50s Tuesday and in the 40s Wednesday-Thursday, with some guidance a bit cooler than this. Nonetheless, no truly late December-like temperatures in store over the next week. Castro && .AVIATION... For the 00Z TAFs... 607 PM...Forecast concerns include... Mvfr cigs through early Friday afternoon. Ifr/lifr cigs late Friday afternoon into Friday night. Scattered showers through Friday morning. Periods of rain, ifr vis Friday afternoon/evening. Scattered showers will continue moving across the area through mid/late evening. Expected coverage and duration will likely not reach prevailing conditions and have opted for tempo with vicinity mention through the night. There may be isolated showers overnight into Friday morning then periods of rain are expected Friday afternoon into Friday evening. During this time period, as the rain becomes prevailing, mvfr cigs are expected to lower to ifr and possibly lifr Friday evening. Visibilities may also lower into ifr Friday evening and there is some potential fog may develop early Saturday morning and that will need to be monitored with later forecasts. East/southeast winds are expected to turn more southerly Friday for the Chicago terminals and then slowly drift back to the southeast Friday evening. Wind speeds are expected to remain under 10kts through the period. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ 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