Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/24/20

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
905 PM CST Mon Nov 23 2020 .UPDATE... 905 PM CST The overall forecast looks on track this evening. 00Z ILX/DVN RAOBs and recent MDW AMDAR soundings reveal a substantial amount of dry air in the 750-900 hPa layer across northern Illinois. This instills some concern of the behavior of the initial precip band at the surface, with the potential for pockets of heavier precip instead of a solid line shifting northeast across the area during the overnight hours. With that said, latest hi-res guidance still supports moderately to briefly heavy snowfall rates for a 1-2 hour period at onset with the initial band. Have therefore opted to make very little change other than to tighten the PoP gradient on the northeast side of the precip band through daybreak Tuesday. Additionally, have moved up the start time of the advisory for Ogle and Lee counties to midnight as moderate to briefly heavy snow may be ongoing as early as around 1 am. Kluber && .SHORT TERM... 310 PM CST Through Tuesday night... The first Winter Weather Advisory of the season has been issued for parts of the far west and northwest Chicago suburbs, out to north central and northwest Illinois portion of the CWA. Advisory was issued in collaboration with WFO DVN and MKX to emphasize impacts to travel during the morning commute, including slick/slushy spots on some roads and visibility as low as ~1/2 mile at times. Most of the CWA except parts of the far south should have a period of all snow as precipitation type and low visibility from this event. However, with more marginal >32F surface temps and overall lower snow totals forecast, impacts should be mitigated. A potent mid/upper level impulse over the Great Basin region will be the primary driver of the weather system set to impact our local weather through Wednesday. A developing surface low across the front range of the Rockies is occurring in response to the mid/upper level short-wave digging into the Four Corners region of the southwest CONUS. The associated downstream southerly low- level mass response will drive a southerly 50+ kt low-level jet from the Plains east-northeastward into the Upper Midwest tonight into Tuesday, resulting in a rapidly northward shifting plume of deep Gulf Moisture into the region. This will set the stage for rapid precipitation development into the area later tonight, despite antecedent very dry air aloft. The area of precipitation will blossom and/or expand northward across the CWA overnight as warm advection and isentropic ascent become quite robust. As alluded to in intro, precipitation type is expected to be mainly in the form of snow for most of the area for at least a few hours after midnight through the early morning. With baroclinic zone oriented from northwest to southeast and continued strengthening warm advection will gradually transition p-type to rain from southwest to northeast. The warming aloft will gradually offset dynamic cooling processes along with an easing of forcing as a lead positive tilt wave shifts northeast of the area. While it can`t be ruled out that some sleet may briefly mix in during the transition, the increasing southeast surface winds should lead the surface temps to rise safely above freezing as the >0C air aloft rushes in. Thus, think that p-type transition for most locations will be one of snow to rain. While the window for steady snow with this system will generally only be for about 3 to 6 hours for most locations, there is increasing concern that the rates could be moderate, to even briefly heavy into early Tuesday morning, thus accumulations appear likely for many areas of northern IL inland from the lake, and this would impact the Tuesday morning commute. Forecast soundings, especially from the NAM/WRF guidance and RAP have an approximate 2-4 hour window of good co-location of very strong omega through the DGZ, which is a favorable look for moderate to briefly heavy snowfall rates. Looking more in depth, model guidance continues to suggest that a stout, but transient band of 700-500 MB frontogenesis will develop northward into the area very late tonight into early Tuesday morning. The associated ageostrophic response should force good ascent right through the favored dendritic growth zone, as noted above. This in combination with very steep lapse rates through this layer from around 600 mb through 400 mb suggest that the snow may fall at a heavy rate for a couple hour period early Tuesday morning. There is even support for larger snow flakes due to aggregation processes in the lower-levels, resulting from a deep moist isothermal layer just below 0C from the surface to just above 700 mb. This could thus support accumulations on some area roads, in spite of marginal surface temperatures, so impacts to the Tuesday morning commute look probable, especially outside the main urban center of Chicago. Exact response of road temps to heavier snow rates is uncertain, but with signal for strong dynamic cooling evident, think that surface temps will be kept around 32F. This should enable for slushy accums on some roads, especially lesser traveled secondary roads and bridges. Total snow accumulations look to be in the 1-3" range I-80 and north in Illinois and roughly along/west of I-294, with little/no accumulation along the lakefront in Chicago. Can`t rule out some isolated >3" amounts closer to the Wisconsin border within advisory area where change to rain will be the latest. Any accumulation in the city stands to predominantly be on grassy/colder surfaces and main travel impact would be from the lower visibility. Speaking of visibility, favorable factors listed above will also be favorable for periods of visibility below 1 mile during the heaviest snow, possibly as low as 1/2 mile at times. All in all, with the greater impacts to the commute likely focused into interior northern Illinois, opted to issue the Winter Weather Advisory for McHenry, Kane, DeKalb, Winnebago, Boone, Lee, and Ogle Counties, in effect from 2AM to 10AM. After the transition to rain clears the entire area to the north, there will be continued periods of rain of varying intensity through Tuesday night. As the surface low pressure area moves closer after midnight Tuesday night, temperatures in the lower to mid 40s and similar dew points (little/minimal T/Td spread) will mean saturated low levels and a chance for fog development. Especially where rain rates become light coupled with lighter winds, will have to monitor the potential for patchy dense fog in spots. There was a signal for this on some of the MOS guidance in parts of northern Illinois. Castro/KJB && .LONG TERM... 158 PM CST Wednesday through Monday... Extended forecast highlights include mid-week rain, quiet and relatively mild weather for the Thanksgiving holiday into the weekend, then a low-confidence but potentially unsettled period Sunday into Monday with a push of colder air early next week. Guidance remains in good agreement in depicting a closed upper level low near Kansas City early Wednesday, nearly stacked atop its associated occluded surface low. Low level southerly winds ahead of this circulation will continue to advect Gulf moisture into the region during the day, maintaining rain/shower threat as the low tracks northeast across the southeastern part of the WFO LOT cwa by late afternoon. A couple of areas of better focus for precipitation and higher rainfall amounts are noted, one across our southeastern counties where unseasonably mild/moist (temps in the mid-upper 50s and surface dew points around 50) in the vicinity of the triple point of the occlusion will reside, and a second farther north across northwest and far northern IL where better baroclinicity exists along an inverted cold frontal trough north/northeast of the low. Forecast soundings continue to depict some weak MUCAPE across our southern counties Wednesday afternoon near the triple point, worthy of at least some slight chance thunder mention there. Rain should taper off and end from west to east Wednesday evening as the upper trough and surface low move off to the east of the forecast area. Weak surface high pressure ridge moves in behind the departing system late Wednesday night into Thanksgiving day, though given the occluded nature of the departing low there is not much of a push of cooler air, and no deep/strong subsidence to erode lingering low level cloud cover. It will be dry and relatively mild for the holiday however, with high temps around 50 degrees expected. Quiet weather looks to persist through the end of the week and into at least the first half of the weekend, as large scale upper trough becomes positively-tilted across central and western North America. Similarly dry and relatively mild conditions appear in store for the region again Friday and Saturday. Models have shown some significant differences with the evolution of the upper pattern later in the weekend into early next week. Guidance is in generally good agreement in closing/cutting off an upper low across the southwestern CONUS by the weekend, while the northern portion of the aforementioned upper trough progresses eastward with a dry pacific cold frontal passage across our area Friday. Previous runs of the ECMWF and GEM then depicted another upper trough phasing with the southern closed low and developing a strongly amplifying upper trough and deep low surface low which would affect the Midwest Sunday-Monday. 12Z runs of these models have trended toward the GFS solution of a somewhat more progressive southern stream low and no phasing/impressive amplification of a deeper trough across the Midwest. Getting a look at 12Z ECMWF ensemble mean does still spread some lighter precip across especially our south/southeastern cwa Sunday/Sunday night, so am a bit hesitant to pull pops completely during that period so will maintain some slights/low chance focused on Sunday/Sunday evening. Could be some rain/snow mix in colder evening hours, though mainly rain expected with temps well above freezing (40s) during the day. Ratzer && .AVIATION... For the 00Z TAFs... 548 PM...Forecast concerns include... A period of moderate snow predawn hours thru mid morning. Rain showers for the rest of the period. Ifr cigs Tuesday morning...lifting to low mvfr in the afternoon. Gusty southeast winds Tuesday. An area of snow currently over southern IA and northern MO will continue moving northeast and will reach the terminals in the predawn hours. Current timing in the tafs looks on track. Vis/ cigs should quickly drop into ifr as the snow begins. There may be a 1-2 hour period of vis in the 1/2sm range along with lifr cigs. As the precip intensity begins to diminish by mid morning...the precipitation will mix with then change to light rain. Medium confidence on this timing. Ifr cigs are expected to lift to low mvfr for the Chicago terminals...but ifr will be nearby to the northwest. Ifr cigs are possible at ord/ least for a time Tuesday afternoon. Rain showers will continue Tuesday afternoon into Tuesday night. Eventually cigs will lower back to ifr Tuesday evening. Easterly winds will turn southeast later tonight with speeds increasing to 10-12kts. Winds will remain southeast Tuesday with some gusts into the 20kt range. Winds will slowly turn more southerly Tuesday evening. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Winter Weather Advisory...ILZ008-ILZ010...midnight Tuesday to 10 AM Tuesday. Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ011-ILZ012...2 AM Tuesday to 10 AM Tuesday. IN...None. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744- LMZ745...9 AM Tuesday to 6 AM Wednesday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: