Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/04/20

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
909 PM CST Mon Feb 3 2020 .UPDATE... 900 PM CST Only minor changes needed to the inherited forecast this evening, mainly to trim and slightly reduce precipitation (drizzle) chances. Evening RAOBs indicate that the broad, persistent area of modest isentropic upglide has resulted in some moistening in the low- levels, but recent VAPOR AMDAR soundings out of Milwaukee and Midway do show that things have not fully saturated, with lingering pockets of dry air present in the 900-800 mb layer. This also seems to be corroborated by recent ceiling observations, with only a small sliver of 2000-3000 foot bases creeping into far southeastern Iowa. Additional/strengthening isentropic ascent on the 285 K theta surface through the rest of the evening does look to occur northeast of us across the mid lake which should encourage lowering cloud bases which look to drift into parts of the CWA after midnight. For really solid drizzle events, we`re typically looking for cloud bases down around or under 1000 feet with deeply saturated profiles and cloud depths into the 8-10 kft range and forecast soundings indicate we won`t be getting anywhere near that overnight into Tuesday morning. There does, however, appear to be perhaps a 4-5 hour window where some light drizzle could get squeezed out of the saturating low- level columns, centered most likely straddling I-88 and in the immediate vicinity of the lake. These locales look to remain generally above freezing, limiting our concern for icing potential due to freezing drizzle. Farther to the north and west--where it will be cooler and likely more below freezing--the depth of the dry sub-cloud layer does now appear to be just too deep to allow any small droplets to survive the fall to the surface. As a result, have pared PoPs back north and west of a Mendota to Crystal Lake line. While we can`t discount a brief period of some light freezing drizzle, think the very limited duration combined with still warm road surfaces (36-38 degrees per recent observations across McHenry County) will greatly curtail the potential for travel impacts. Evening guidance is hinting at the potential for a small frontogenetic circulation possibly poking a bit more into our southeastern counties Tuesday afternoon with a potential for some light snow accumulations, but will defer any alterations to this part of the forecast to the midnight shift. Carlaw && .SHORT TERM... 242 PM CST Through Tuesday night... The primary forecast concerns in the short term period center around the potential for a short period of drizzle/freezing drizzle late tonight into early Tuesday, along with the lake shore flooding threat on Tuesday. Temperatures have warmed nicely across much of the area under partly cloudy skies early this afternoon in spite of the switch to northeasterly winds. This is especially the case inland from the lake and outside of the lingering snow field over north central IL. In these areas, temperatures have warmed into the upper 40s, to even the mid 50s far south closer to a surface frontal boundary over central IL. Colder air is on the way, however, for tonight and on Tuesday as a surface high over the eastern Dakotas continues to shift eastward across the Upper Midwest and into the western Great Lakes region. This will result in temperatures on Tuesday remaining in the 30s. There continue to be signs that some lower clouds and possible light precipitation will develop across parts of northern IL overnight in response to a developing band of frontogenesis along the 850 MB frontal boundary. Unfortunately, rather low mid-level moisture is expected, which looks to support rather warm (above -10C) cloud top temperatures, likely not supportive of heterogeneous ice nucleation. For this reason, any light precipitation that does fall late tonight into early Tuesday morning will likely be in the form of drizzle or freezing drizzle, depending on surface temperatures. At the present time, surface temperatures are expected to drop into the lower 30s over northern IL late tonight, especially west and north of Chicago. These areas will hence have the best chance to experience a short period of freezing precipitation through around daybreak Tuesday. Confidence in this becoming a significant problem into the morning commute remains on the low side at this time, so no headlines are planned at this time. Aside from some small chance for some light precipitation over my far southern counties Tuesday into Tuesday night, it appears most of the area will remain on the dry side as we await Wednesdays storm system. The main weather story for Tuesday will be the gusty northeasterly winds likely producing high waves up around 10 feet on southern Lake Michigan. With the continued high lake levels, this is likely to result in at least some minor lakeshore flooding and erosion problems along the southern Lake Michigan shores through Tuesday night. KJB && .LONG TERM... 330 PM CST Wednesday through Monday... The main forecast concern is an increasing likelihood of a plowable accumulating snowfall area wide Wednesday PM into Thursday. Latest thinking would place timing of this snowfall in line with causing travel impacts to both the Wednesday evening and Thursday morning commute. At this time, this looks to be an all snow event for most of or the entire CWA. Prior to today, much of the guidance (especially non ECMWF) had been struggling handling the pattern for this potential snow system, with poor run to run continuity. That changed substantially with the 12z cycle, as global operational and ensemble guidance came into much better agreement in bringing accumulating snowfall to the entire CWA. Should today`s trends continue, we will likely need winter headlines for most of if not the entire CWA. Short-wave energy and a strong mid and upper level jet will round the base of a longwave trough of the western CONUS this evening, with the trough digging across the southwest and then into Texas tomorrow into Wednesday. Latest trends suggest a coupling of the strong upper level jet associated with the longwave trough and another strong jet streak over the northeastern 1/3 of the CONUS. This development entails surface cyclogenesis along the Texas Gulf Coast by Wednesday morning. The low will then track quickly northeast to the middle Ohio Valley by Wednesday night and near Lake Erie by early Thursday while steadily deepening to the lower 990s mb. This non-NAM consensus surface low pressure track is a climatologically favorable one for accumulating snow in much of the local area. In addition to favorable large scale ascent from the neutrally tilted upper trough and upper jet divergence, this system will have a good moisture feed of above normal PWATs due to open Gulf trajectories in the system warm sector. The strengthening synoptic system will interact with and strengthen an already tight thermal gradient/baroclinic zone over the region, yielding good mesoscale forcing from stout lower and mid-level frontogenesis. With all this in mind, expect snow to break out from south/southwest to north/northeast Wednesday afternoon and evening. Assuming no major changes in the favorable aspects of this set-up, the warm conveyor belt Wednesday evening and then deformation area precipitation later Wednesday night into Thursday morning should support steady and occasionally moderate snowfall rates. As to a possible fly in the ointment, the system warm sector will likely have plentiful convection, which in *some* cases (ie. Jan 10-11 2020) can cut off some of the moisture transport and modify mass fields. This system will also be moving rather quickly, which may limit relatively speaking the QPF and snowfall amounts. That said, think that the favorable aspects to this set-up could potentially lead to higher QPF and snowfall than in the official forecast. Finally, depending on the exact track of the system, including 850 mb low, a farther north/northwest track could possibly bring mixing issues into the far southern CWA, and dry slot impingement Wednesday night. At this time, am a bit less concerned with possibility guidance shifts back southeast again. With the poor guidance agreement until today, the explicit QPF and snowfall has been trended a good deal higher than previous forecasts. Do not want to put too much emphasis on the exact amounts yet, as we may not be done trending as alluded to above. However, what is available on the probabilistic snow page gives a 2 to 5" range. The main message is that confidence is increasing in a plowable snowfall for the entire area and the potential for significant impacts to the Wednesday PM and Thursday AM commute. With temperatures in the 20s to around 30 and lift favorably positioned within the DGZ on forecast soundings, snow ratios should be near to a bit above climatological average, in the 12-16:1 range. With a farther north surface low track now favored, it will be breezy along with the snow, which could yield some blowing/drifting issues. Keeping winds in check somewhat though will be the lack of a strong surface high north or northwest of our area. Northeast to north winds will be strongest over southern Lake Michigan (gusts to 25 to perhaps 30 mph), so lakeshore flooding concerns may return quickly on the heels of tonight through Tuesday night event. The system snow will come to an end by Thursday mid day or early afternoon east. Thursday evening and night (and possibly Friday AM), we`ll have to watch for developing lake effect snow showers. It`s too early to say how the lower level convergence will set-up and thus which areas will be most favored, but favorable convergence, thermodynamics, and inversion heights of 9-10kft could add to the system snow totals somewhere near/along IL and IN shore. Expected snow cover over the region will keep temperatures near seasonable through Saturday, with a modest warm-up Sunday as a clipper system passes north, though it too likely kept in check by snow cover. This may be followed by a cold front passage on Sunday night into Monday. Have some low chance PoPs during this time, but confidence is low out in that range. Castro && .AVIATION... For the 00Z TAFs... The concerns and confidence for the Chicago area airports are: - High confidence in winds being between 010-030 degrees for most of the TAF, most frequent 020 degrees, with medium confidence in peak gusts 28-33 kt during the Tuesday morning rush - Low confidence in any IFR and brief freezing drizzle overnight and early Tuesday morning, which if were to occur would most likely be between 10Z- 13Z - Chance for light snow Tuesday evening mainly over the southern TRACON area Saturation has been slow to occur in the low-levels going into this evening, but should happen more readily into overnight resulting in areas of lower MVFR ceilings. The profiles are supportive of patchy drizzle even though IFR ceilings should be somewhat spotty. Without IFR ceilings, it may keep the drizzle notably light. With temperatures inching downward and dew points dropping below freezing early this evening, there is the potential for freezing drizzle. Following a mild day, pavement temperatures are likely to lag milder so with any drizzle expected to be both light and short duration, impacts should be limited. Any low clouds look to clear out between mid-late morning. North-northeast winds will continue through the TAF period, with any wobble in direction forecast to be small. Confidence is high in the peak gust time being late tonight through early afternoon Tuesday, with a gradual diminishing thereafter. A few gusts higher than the 31 kt forecast in the ORD TAF are likely. Gusts look unlikely to top 35 kt. Wind speeds will continue to gradually ease Tuesday night, but being off the lake with slightly colder air, there is potential for lake effect flurries/patchy light snow into far northeast Illinois, but that looks mainly overnight Tuesday night (beyond current TAF). Also Tuesday night should see light snow across central Illinois that may reach up into the southern TRACON area, with medium-high confidence in this staying south of ORD and MDW. MTF && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Lakeshore Flood Advisory...ILZ006...midnight Tuesday to 6 PM Tuesday. Lakeshore Flood Advisory...ILZ014...3 AM Tuesday to 6 AM Wednesday. IN...Lakeshore Flood Advisory...INZ001-INZ002...3 AM Tuesday to 6 AM Wednesday. LM...Small Craft Advisory...Winthrop Harbor to Wilmette Harbor until 3 AM Wednesday. Small Craft Advisory...LMZ741-LMZ742 until 6 AM Wednesday. Small Craft Advisory...LMZ743-LMZ744-LMZ745...3 AM Tuesday to 9 AM Wednesday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: