769 FXUS63 KLOT 190458 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1058 PM CST Sun Dec 18 2022 .SHORT TERM... Issued at 250 PM CST Sun Dec 18 2022 Through Tuesday morning... Key messages: * Single digit to lower teen temps tonight-early Monday AM. * Brief chance for snow (possibly ending with freezing drizzle?) Monday PM into Tuesday AM with no more than a dusting expected. Sunshine has made a triumphant (albeit an unfortunately brief) return today across northern Illinois and northwest Indiana! Despite the clearing skies, temperatures have remained rather chilly in the upper teens to lower 20s combined with the lingering breezy winds have kept wind chills mainly in the single digits today. Winds begin to ease after sunset as the gradient eases some. Assuming skies remain mostly cleared out overnight, temperatures likely drop into the single digits (with low teens where clouds linger or redevelop). Thus forecast lows may not be low enough in some spots and too low in others but the message of a chilly night and start to the day on Monday remains the same. Warm advection picks back up during the day on Monday in response to an approaching low-amplitude wave emanating out of the southwest with temperatures warming back in the mid to upper 20s. This wave will bring increasing potential for flurries by late Monday afternoon and into the evening. The snow rate may end up high enough to result in a quick dusting in areas generally north of a LaSalle to O'Hare line tonight. Overall not expecting big impacts with this system though. Confidence remains too low to include in the gridded forecast, but will mention that there is still a signal for a loss of saturation in the mid-levels Tuesday morning with lingering low-level moisture and renewed lift which could spit out some light drizzle or poor quality snow flakes (aka "snizzle"). Petr && .LONG TERM... Issued at 221 PM CST Sun Dec 18 2022 Tuesday through Sunday... All eyes in the long term period are on the late week cyclone and intrusion of bitterly cold air along with dangerous wind chills late this week into the weekend. Confidence continues to steadily increase that a potentially crippling blizzard could impact portions of the region later this week, the timing of which couldn't be worse for holiday travel. If, and how severely, our forecast area (CWA) will be impacted depend on the exact track of the cyclone and to some extent the magnitude and timing of the forecast rapid deepening. Medium range guidance has been advertising a powerful cyclone spinning up over the Great Lakes or northeast U.S. for days now, with a pretty sizable spread in where the storm will develop/track. There was a cluster of guidance favoring a Great Lakes target region, which would increase chances for major impacts in our CWA, with a second cluster favoring the New England resulting in disruptive, but far less extreme winter weather locally. Trend in ensemble guidance over the past 48 hours has been decidedly away from a New England cyclone and toward cyclogenesis farther west over the Great Lakes with recent runs of the GFS/GEFS making the significant shift west. While track guidance has shown (pretty typical) variability, the theme of rapid, and potentially explosive, deepening of the cyclone has been a fairly consistent theme in guidance for days now. The expected intensity of the cyclone combined with the very strong arctic high (>1040mb) over the northern and central Plains leads to high confidence in a noteworthy high wind event with this system. The eventual track of the cyclone and timing of rapid/explosive deepening will determine where the very high impact winter weather/potential blizzard will set up. Given recent model and ensemble trends, our CWA lies very solidly within the potential threat region. The incipient wave that is forecast to spawn this system is still located north of Alaska across the Beaufort Sea, so it still has a lot of distance to cover with plenty of time for shifts in guidance. It isn't uncommon for models to make leftward adjustments to the track of powerful cyclones like this, so it is important not to get locked in on a forecast track yet. At this distance, it is also advisable not to focus on individual operational runs of models, as operational runs will likely exhibit variability in the strength and especially track over the next couple or days. This run to run variability in operational runs is often not an actual "trend" but rather "noise" in the range of possibilities at this distance. Rather than focusing on noise level changes in operational runs, focusing on trends in ensemble data the next couple of days should prove more meaningful as we look to hone in on location and magnitude of the threat. In addition to producing blizzard conditions, the high winds will likely cause lakeshore flooding (which shore depends on track of cyclone) and dangerous wind chills well below zero. Currently, most guidance strongly supports advisory level wind chills (-20 to -30F) Friday into at least the first half of the Christmas weekend, with some potential for even a period of warning level (colder than -30F) wind chills as actual air temperatures plummet below zero. All indications are that our high temp Christmas Day will be the coldest since the mid 90s and possibly one of the top five coldest on record. Any power outages resulting from the strong winds greatly increase potential danger of the cold spell. We strongly urge everyone to pay close attention to later forecasts, particularly if you have holiday travel planned. - Izzi && .AVIATION... For the 06Z TAFs... Forecast concerns for the terminals... *Gradually decreasing ceilings Monday afternoon/evening *Chance for light snow showers and/or flurries Monday evening A surface high continues to move across the area this evening which has created light westerly winds around 5 kts. Winds will remain light through the rest of the TAF period but will become southeasterly Monday afternoon as the high begins to depart. VFR cirrus clouds continue to move into Illinois this evening and will continue to do so through the night before gradually lowering as mid-level moisture builds in Monday afternoon and evening with an upper-level disturbance that will be moving through the upper Midwest into the northern Great Lakes. Low end VFR ceilings (around 5000 ft) are anticipated, but some guidance is showing that some spots may briefly encounter an MVFR ceiling late Monday evening. The upper-level disturbance will also bring with it a chance for some light snow showers and/or flurries Monday evening into the early overnight hours. Guidance has continued to show that the bulk of the showers would remain north of the Illinois-Wisconsin state line, but forecast soundings continue to show sufficient moisture present to generate a light shower or flurry. The best chances continue to be near the RFD area where some minor visibility reductions may occur with the more robust showers. As for the Chicago terminals, conditions continue to look more marginal with some high-res guidance showing that showers may just clip the TAF sites. However, given the decent forcing and sufficient moisture I decided to add a TEMPO to ORD, MDW, and DPA for flurries to account for this potential. Snowfall accumulations continue to look minimal with just a dusting to a couple tenths expected near the RFD area. Yack && .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