686 FXUS63 KLOT 212033 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 233 PM CST Mon Feb 21 2022 .UPDATE... Issued at 1012 AM CST Mon Feb 21 2022 The main update this morning was to issue a dense fog advisory for Chicago and, due to the rapid development of dense fog along a quasi stationary frontal boundary. Recent trends have shown this area of fog undergoing diurnal improvement. This is likely to continue as we head into the afternoon, though some fog may persist near the lake. At the present time, this advisory is in effect through 2pm, but we will likely be able to continue to drop counties from it early as conditions continue to improve. Otherwise, with the boundary expected to remain nearly stationary around the I-80 corridor through the day, expect temperatures to vary widely from north-to- south across the area. The warmest temperatures (in the 50s) are expected to be along and south of I-80, with primary upper 30s to low 40s farther north. As we head into the evening hours low clouds, fog and drizzle are likely to redevelop across northern IL in the vicinity of the surface boundary. While the fog could again become dense at times into Tuesday morning, the other increasing concern for northern IL late tonight into Tuesday morning is the threat of showers and thunderstorms, some of which could produce small hail and locally heavy downpours. These heavy downpours may lead to some hydro concerns, with localized amounts exceeding an inch possible. While normally such amounts would not be of concern, we still have a frozen ground which will significantly limit any soil infiltration, thus leading to significant pounding and higher than normal runoff rates into areas streams and rivers. Also, many area rivers still have large amounts of ice, which will only act to exacerbate potential river flooding issues. While we currently do not plan to issue a flood watch, we will continue to hit the hydro message hard in the ESF and in graphics. Freezing rain and icing potential also continues to be a concern very late tonight into Tuesday, especially for areas near the WI state line. We will continue to assess the need for any type of winter weather advisory for our northern tier counties for the afternoon package. KJB && .SHORT TERM... Issued at 244 AM CST Mon Feb 21 2022 Through Tuesday... Plenty to look at in the short term as spring will encroach upon the forecast area today and Tuesday, while winter tries to hold its ground in the far north. The primary forecast messages are: * Large temperature range from north-to-south maximized by midday Tuesday, with around freezing far north to around 60 south * Areas of drizzle developing late today and this evening, with potential for some fog too * Showers overnight persisting into Tuesday, some likely in the form of thunderstorms, with even small hail potential, as well as the rainfall possibly being enough for minor ponding * Likelihood of freezing rain and/or freezing drizzle near the Wisconsin state line Tuesday during the day; opted to hold off on Winter Weather Advisory for uncertainty on timing and whether the morning commute would be impacted or if it would be after A frontal boundary that will be the source of the temperature gradient across the area is draped from west-to-east near I-80 early this morning. There has not been any fog along this thus far, though low temperature-dew point spreads hint that it is still possible early this morning. Low clouds to the north of the front in Wisconsin are slowly sagging south and will spill into far northern Illinois. isentropic ascent is fairly weak within this and more pronounced further northwest, so not expecting any drizzle through early-mid afternoon. Some higher clouds will expand in as well today, overall making it mostly cloudy for a majority of the area by early afternoon. Today's temperature forecast may be the "easiest" period of the short term as the boundary should remain near stationary. Have mid to upper 30s far north, although if the low clouds are opaque enough that may be a little too mild. In the south have lower 50s going as the remaining snow pack rapidly melts. A strengthening subtropical jet and an ejecting short wave will both move into the Central Plains by this evening with surface cyclogenesis occurring. In response, the baroclinicity along the regional frontal boundary will also strengthen, with temperatures moderating to the south and likely some northward progress of the warm front tonight. How far north is challenging to say, as some high resolution guidance has it near I-90, while some just slightly north of I-80 with little movement (such as the HRRR, although that seems influenced by some convective footprints which is unlikely with any elevated convection). What is in pretty good agreement is that parts of the northern CWA should remain several degrees above freezing through the entire night, with the more robust northeast winds remaining harbored in Wisconsin. With the strengthening warm advection atop the boundary, saturated isentropic ascent quickly increases by early evening, with soundings supportive of drizzle potential. Also with the murky-ness some fog can develop in these setups too. Synoptic scale forcing for ascent increases by early overnight with the subtropical jet moving overhead. Moisture transport at the 850-925 mb low-level jet layer increases within this, and showers should become the more common mode, which convection allowing models (CAMs) agree on. The 2-5 km lapse rates also are fairly steep above this low-level jet layer, with just uncertainty on whether the lifted parcel level (LPL) height will be sufficient to tap into this. If so, MUCAPE values from the RAP and NAM would be supportive of 500-1,000 J/kg and likely multiple arcs of convection moving through between midnight and noon. With freezing levels around 8-9 kft, and the elevated profiles getting well above this, could envision some hail up to half inch occurring with any stronger cores. With precipitable waters at 0.75 to 1 inch (near maximum this time of year), late tonight into Tuesday moring's setup does raise concerns for isolated to scattered rainfall totals over an inch. The 00Z HREF maximum values even indicate spotty 1.5 to 2.5 inch totals in the central CWA. For failure modes to those upper values, there should be some moisture robbing to the south with a large convective footprint forecast Tuesday morning over the Mid- Mississippi Valley to Lower Ohio Valley. Also this is more convective-dependent and showery in nature than our event last week on Wednesday night (i.e. confidence was higher then in QPF > 1 inch occurring over a large area). With the recent hydrology concerns, and still elevated river levels, will go ahead an issue a general ESF (Hydrologic Outlook). As the surface low pressure moves into the CWA Tuesday morning, temperatures will start to inch downward north of its path center. The surface freezing line should move at least into northern Winnebago and Boone Counties during the early to mid morning. This period will still have showers and possibly drizzle as well. So icing remains a concern in the north, including the Rockford metro. National Blend of Models (NBM) membership indicates over 50 percent with some freezing rain in the north, and about 30 percent with a tenth of an inch in Winnebago and Boone Counties. For Tuesday afternoon, the temperatures will start to drop more markedly in the wake of the low and increasing northwest winds. The system dry slot is progged to move directly over the area with a strong signal of low clouds to remain. This can be an area for freezing drizzle too, and that might include a little more area than just the far north. Feel that the potential for a Winter Weather Advisory on Tuesday is over 50 percent for Winnebago and Boone Counties and close to it for McHenry. But just not enough certainty in timing yet, and also knowing that roads will be coming out of basically a 24-hour period above freezing too, which may keep the morning commute fine. So will allow day shift to evaluate again as well. For temperatures on Tuesday elsewhere, the gradient will be impressive, especially as the low is passing around noon. The difference looks on the order of 30 degrees from north to south, with specific values in the middle having large uncertainty and dependent on where the low is. The lakefront also could wildly swing at times too depending on wind direction being off the lake or from the south. As the cold advection wraps in, a few gusts of 30-35 mph may occur mainly west and south of the metro. MTF && .LONG TERM... Issued at 233 PM CST Mon Feb 21 2022 Wednesday through Monday... Primary forecast concern is for accumulating light snow Thursday afternoon into Thursday night. Northwest winds will still be gusting into the 20 mph range Wednesday morning with temperatures in the low/mid teens. This will produce wind chills from 5 above to 5 below zero. Highs will only reach the mid 20s on Wednesday. Winds will slowly turn to the northeast during the afternoon and continue into Wednesday evening when some lake effect flurries or snow showers will be possible, which may allow for a dusting of snow later Wednesday night into early Thursday morning, for areas near the lake. There is also a chance for light snow across central IL into central IN Wednesday night, that appears to mainly affect the far southern cwa as a weak wave lifts north ahead of the main system. This may bring some minor snow accumulations to the far southern cwa but confidence is low. The NAM indicates this light snow may be a bit further north, but there isn't much support from other guidance. The main surface low will move across the Ohio Valley Thursday night with a swath of heavier snow to the northwest of this track. This swath may catch the far southeast portion of the cwa. But for most of the area, it looks like a light accumulating snow starting Thursday afternoon and continuing into the predawn hours on Friday, ahead of the main upper trough which moves through the area. Colder temperatures should allow for a fairly fluffy snow that may accumulate 2 to 3 inches, perhaps higher far south as noted above. While the snow will likely be ending or have ended by the Friday morning rush hour, it will likely still be impacted as untreated roads may still be snow covered/slippery. No significant changes to the blended guidance during this time period, but pops will likely need to be increased as this time period approaches. There is consensus for the overall pattern for this weekend but still quite a bit of uncertainty for the details. A cold front is expected to move across the area later Saturday night or Sunday, with still timing differences. Ahead of this front, on Saturday, there will likely be a brief moderation in temps. Behind the front, the GFS is quite a bit colder than the ECMWF. This will yield below normal temps for the end of February but fairly low confidence for how low temps will be. Some lake effect snow will also be possible Sunday into Sunday night for northwest IN. cms && .AVIATION... For the 18Z TAFs... The forecast concerns through the TAF period are: * Developing and stalled warm front near or just south of the terminals overnight through much of Tuesday leading to lowering cigs and visbys into IFR and possibly LIFR from the overnight period onward. * Showers and a few thunderstorms overnight into Tuesday morning (medium confidence), with potential for a prolonged period of at least VCTS at all terminals. * Trends in winds, with a east to northeasterly direction preferred through the TAF period (low to medium confidence) * Low (30% or lower) chance for freezing rain toward the end of the TAF period at RFD. As a surface low pressure system develops in the Southern Plains this evening, increasing southerly low-level moisture advection will allow the patchy MVFR cloud cover roughly along the Ohio River Valley at press time to fill in and stream northward. While confidence is somewhat low on exact timing, a return to MVFR clouds hence appears probably tonight with a preliminary estimated arrival toward 00Z. At the same time, northeasterly to easterly low-level flow will remain steadfast off Lake Michigan owing to an increasingly packed low-level pressure gradient across the western Great Lakes. Accordingly, a warm frontal zone is poised to develop in the vicinity of the terminals, which presents lower than average confidence on the forecast for winds and cigs overnight. Climatology would favor the warm front developing just south of GYY/MDW and along I-80, more or less close to where a zone of low-level confluence of surface winds already exists. Even as the surface low pressure system approaches toward the end of the TAF period, climatology favors the warm front remaining locked in place and again, south of the terminals. Accordingly, the outgoing TAF package will present the scenario where the terminals remain north of the front through the next 24 to 30 hours. As low-level moisture increases tonight, low clouds and pockets of mist/drizzle/fog appear poised to develop along and north of the warm front. Confidence is high that IFR cigs will develop toward 03Z and medium that LIFR cigs will develop by 7/8Z at all terminals, give or take an hour. Visibilities also appear poised to drop below 3SM at all terminals, with at least some potential for periods below 1 mile particularly at RFD. Low clouds and visbys will likely remain locked in place until the surface low pressure system and associated cold front surge through the area toward 00Z Wednesday. In fact, conditions may lower further to VLIFR (particularly with cigs) toward the end of the TAF period as the surface low approaches from the southwest. Meanwhile, winds north of the warm front appear likely to hold onto an easterly component through the TAF period with a preference for a northeasterly direction especially after midnight. After 18/19Z, winds may become variable if not calm as the surface low pressure system approaches, though confidence is low. Toward 00Z Wednesday, a cold front will sweep across the area leading to gusty westerly winds, though magnitudes will likely be highest south of the terminals. Overall, confidence in the wind forecast is low to medium. In addition, broad and persistent warm-air advection will lead to scattered convective showers from about midnight to mid-Tuesday morning. With steep mid-level lapse rates able to generate MUCAPE >500 J/kg, embedded thunderstorms will be possible terminals through the overnight and Tuesday morning period. While realistically the chance for thunderstorms will be anytime between 7-15Z, opted to target the period of highest confidence in thunder between roughly 11 and 15Z at the terminals after collaboration with CWSU ZAU. Note that the thunderstorms may produce small, non-damaging hail. Finally, at least some potential exists for temperatures to drop to the freezing mark at RFD toward the end of the TAF period while precipitation continues. However, confidence in the air temperatures actually reaching freezing is low (may end up right at 32 or 33 degrees). For now, have offered a PROB30 from 15-18Z for -FZRA with the expectation for refinements in later TAF packages. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011- ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105- ILZ106-ILZ107-ILZ108...midnight Tuesday to noon Tuesday. IN...Flood Watch...INZ001-INZ002...midnight Tuesday to noon Tuesday. LM...Dense Fog Advisory...LMZ741-LMZ742-LMZ743-LMZ744 until 3 PM Monday. && $$ 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