Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/30/20

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
803 PM CST Sun Nov 29 2020 .UPDATE... 800 PM CST No significant changes to the forecast message this evening, there is still a chance for some snow showers into NE Illinois late tonight into tomorrow morning, with the better chance for travel impacts still focused into northwest Indiana. Evening AMDAR soundings and the 00z DVN sounding depict dry air in the mid levels, with a shallow lower level cloud deck along and just behind the cold front this evening. The 0z ILX sounding is showing at least a bit more saturation, but still not much precipitation is reaching the ground to our south as well associated with the compact low pressure area across eastern IL into Indiana. With some subtle moisture increase from the SW, we could see some light precipitation begin in NW Indiana late this evening or overnight. Forecast soundings are marginal for snow before midnight. What will be noticeable this evening is the colder and drier air that will advect in from the northwest behind the leading cold front. We are still fairly mixed in the cold advection regime thus expect gusts to 30-35 mph even increase some overnight. The secondary cold front will approach overnight into early Monday. This will be the impetus for lake effect snow showers. No significant changes to the forecast are planned at this point. Surface-925 mb winds do suggest that the main snow shower axis will point into NW Indiana, but some subtle backing to the 850 mb (around 5kft) may steer some of the activity into NE Illinois into early Monday (including the morning commute) also as the moisture player deepens and gets into the better snow growth region. Of note, the NBM (ensemble) produces a ~30-40 chance of a dusting of snow at MDW and points eastward to the lake, and points southward in IL, which could be in play briefly Monday morning, but some high- resolution ensemble guidance has increased the chances in recent runs, but have shown a slight downward trend in IL snow. KMD && .SHORT TERM... 234 PM CST Tonight through Monday night... Focus during this period continues to be on lake effect snow for primarily northwest Indiana and lakeshore flooding from persistent strong north winds. A cold front stretching from Evanston to Aurora to Mendota as of 230pm will continue southeastward through the remainder of the CWA through early evening. Meanwhile, weak phasing of a sharp upper trough centered over Minnesota and an upper low over southeast Oklahoma will begin by early this evening, with a resultant sharper and stronger trough axis crossing the CWA tonight into Monday morning. An axis of post-frontal light precip may develop southeast of I-55 mid-evening into the early overnight hours as the right entrance of an upper jet streak interacts with a marginal increase in moisture ahead of the upper-low. Moisture profiles still appear to be too dry to support any appreciable precip, but a light rain/snow mix transitioning to all snow with little to no accumulation is possible. The arrival of colder 925-850 hPa air concurrent with the passing upper trough late tonight will quickly raise inversion heights and resultant lake-induced equilibrium levels to around 8kft while 850hPa-SST delta Ts increase to around 15C. Adding in an extensive fetch owing to low-level cyclonic curvature following the long axis of Lake Michigan, LES parameters support a potentially impactful LES episode for portions of northwest Indiana. However, there are mixed signals as to whether the LES structure will be in the form of one dominant band or a mix of several bands of varying intensity. Most near-term high-res guidance has been indicating the latter, which is not entirely surprising given the extensive generally uniform field lacking any substantial land-breeze/offshore wind component from WI or MI. This will ultimately result in much more variability in snowfall amounts, with amounts hinging highly on if one of those bands becomes dominant and if individual band motion is transient or stationary. A multi-banded structure and little onshore flow does favor a shoreline convergent band along the Illinois shore, edging a few miles inland across Chicago, Monday morning before winds begin to slightly veer. The overall message of the forecast has changed very little over the past couple days, but the variable nature of LES always lowers confidence in snow amounts. The latest forecast is for totals ranging from around one half inch on the immediate Chicago shoreline to 3-5 inches for portions of Porter County and northeast Jasper County. Locally higher and lower amounts are likely, especially if LES bands remain more stationary. Additionally, marginal low-level temp profiles with surface temps at or above freezing (especially near the lake) and the DGZ residing above the layer of maximum omega in the convective PBL support SLRs as low as 10:1. More pronounced bands may tap into the DGZ, but also have increased riming in the convective column, further complicating SLRs. This may be a limiting factor in snow totals, particularly near the lakeshore. While the snow amounts alone would not be too significant for an LES event, strong north winds will lead to potentially hazardous conditions late tonight through the day Monday. North winds are expected to gust to 35 mph inland and to 45 mph near the lake. The combination of the wind and snow warrants the issuance of a Winter Weather Advisory for Lake, Porter and Jasper counties, especially with rapidly varying conditions affecting the major interstate transportation corridor through northwest Indiana. Regarding the strong north winds, the combination of large waves to 15 feet along the IN shore and surge up to a foot will lead to a potentially significant lakeshore flood event. A Lakeshore Flood Warning remains in effect for Lake and Porter Indiana. Impacts will likely also be noted into the Chicago shoreline, with a Lakeshore Flood Advisory for waves to 10 feet in effect. Kluber && .LONG TERM... 230 PM CST Tuesday through Sunday... On Tuesday, lake effect snow over northwest Indiana should be winding down as the 994 mb low moves northeast into Quebec, allowing surface winds to turn more northwesterly. The pressure gradient also relaxes throughout Tuesday as high pressure ridges up from the southwest. Northwest flow will keep daytime highs just a bit below normal for the first day of December, despite the clearing as clouds exit to the east with the low pressure system. Mostly clear skies and light westerly winds Tuesday night should make this the coldest night of the week, with lows in the lower 20s most locations except for mid 20s toward downtown Chicago. A northwest to southeast oriented ridge over the area Wednesday will support continued clear skies and slightly above normal highs in the lower 40s most locations. Another low pressure system approaches from the southwest Thursday, increasing our cloud cover, but continued west to southwesterly winds will keep temperatures slightly above seasonal norms Thursday and Friday. The low will slip south our of CWA sometime between Thursday afternoon through Friday, bringing a slight chance of rain/snow mix Friday afternoon through Saturday night, mainly impacting the southern half of the CWA. The low moves to our east on Saturday, taking the clouds and weather with it, with northwesterly winds in its wake as a tightening of the gradient brings breezy conditions on Sunday. BKL/Lenning && .AVIATION... For the 00Z TAFs... The primary aviation concerns through the next 24-30 hours are: * Breezy northwest winds gusting 20-30 kts through much of the TAF period * MVFR cloud deck and lake effect snow showers likely to impact GYY but also possibly MDW/ORD from early Monday morning onward Breezy northwest winds will prevail through much of the TAF period in the wake of a strong cold front pushing through the terminals at press time. Generally speaking, VFR cloud cover is expected through the TAF period between various cloud decks through Monday morning and a scattered (diurnal) cumulus deck Monday afternoon. The exception will be at GYY and possibly MDW/ORD as an MVFR cloud deck surges south off Lake Michigan behind a secondary cold front, perhaps as early as 6-7Z. Numerous lake effect snow showers will develop behind the secondary front and stream into northwestern Indiana and perhaps a sliver of northeastern Illinois from roughly 8Z onward. At this point, GYY stands the highest chance of being impacted by numerous showers with associated brief drops in visibility. A lower chance exists for a few showers to sneak toward MDW (and even lesser so ORD) between 11Z and 16Z during which low-level steering winds will have a modest component off Lake Michigan into the Illinois Lake Michigan shore. It`s worth noting the NBM (ensemble) produces a ~30-40 chance of a dusting of snow at MDW Monday morning. At this point, it seems the chances for meaningful snow accumulation or associated drops in visibility are too low (or will occur for too brief of a period of time) to warrant a formal mention in the outgoing TAF package...though again such chances are non-zero. Borchardt && .MARINE...234 PM Current forecast on track with a period of gale force winds late tonight through late Monday evening. A cold front is currently moving across southern Lake Michigan, shifting winds northwest. North gusts will increase to 30kt during the evening. The gradient will tighten overnight into Monday as a ridge of high pressure builds across the central U.S. and a storm system moves northeast across the Appalachians. This storm system will reach the eastern Great Lakes Monday evening and then stall before slowly lifting north midweek. The gradient will slowly weaken Tuesday into Tuesday night with winds turning more northwesterly over southern Lake Michigan. Kluber/cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Lakeshore Flood Advisory...ILZ103-ILZ104...midnight Monday to 6 AM Tuesday. IN...Lakeshore Flood Warning...INZ001-INZ002...midnight Monday to noon Tuesday. Winter Weather Advisory...INZ001...3 AM Monday to 9 PM Monday. Winter Weather Advisory...INZ002-INZ011...3 AM Monday to 6 AM Tuesday. LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744- LMZ745...midnight Monday to midnight Tuesday. Small Craft Advisory...nearshore waters until midnight Monday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: