Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/04/20

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
858 PM CDT Sat Oct 3 2020 .UPDATE... 857 PM CDT The forecast message remains a wet one through a good part of tonight with the going high PoPs, rainfall amounts, and timing all looking good. Changes to the forecast have all been merely tweaks. There is the chance some lake effect showers could graze the northeast Illinois shore, including downtown Chicago, midday Sunday, while likely some will reach the northwest Indiana shore. The impressive trough for early October continues to dig in on the synoptic scale across the Upper Midwest and Great Lakes, ushered in by a due southward oriented 125 kt upper jet from central Canada into the central United States. The cold air aloft is certainly present, with the 00Z DVN sounding sampling 0C at 850 mb, and that supports the KDVN radar correlation coefficient indicating a mix of rain and snow as low as 2,500 ft in northwest Illinois. The boundary layer will remain sufficiently warm for the rest of the night for all the precip to fall as rain, so no worries there! The primary vorticity maximum center will pass the area during overnight, so will see rain tapering fast in north central Illinois around midnight and in Chicago near 3 a.m. Higher resolution guidance, basically that of convection-allowing scale or close, is depicting a slight higher signal in lake effect showers on Sunday. The steering flow would still support more of far northwest Indiana to see these, but it`s worth noting the HRRR and RAP this evening have trended slightly further west, even grazing the northeast Illinois shore with QPF. RAP soundings do show 200-500 J/kg of lake-induced CAPE with equilibrium levels around 10,000 ft, so certainly sufficient for some showers to develop. However there will be a race against drier air working into the area especially mid-late afternoon. For now no changes to the going forecast PoPs but will note to the overnight shift. MTF && .SHORT TERM... 226 PM CDT Through Sunday night... We`re finally starting to see the influence of a rather sharp shortwave which is slicing through Minnesota and north-central Iowa with a gradual expansion to a region of light rain spreading towards parts of central and north-central Illinois. This is occurring as the remaining region of dry air in the 850-700 mb layer is more fully saturating, as evidenced by recent VAPOR AMDAR soundings. We`ll continue to see this precipitation shield expand and fill in through the afternoon and evening hours as large scale forcing for ascent gradually increases. Model cross sections depict the development of at least a modest f-gen circulation late this evening in response to the deepening larger-scale indirect ageostrophic circulation under a departing 120+ kt upper-level jet. So while most of this event will feature only light rain, periods of locally more moderate activity appear probable late this evening and into the early-overnight hours near locales straddling the I-55/57 corridors where this frontogenetic circulation is maximized. There`s actually a brief window where some non-zero instability to upright convection develops in our southeastern zones as lapse rates steepen up a bit. Probably not enough to worry about electrification, but moreso assisting in the production of briefly more moderate rainfall. Precipitation and cloud cover may clear our far northwestern locales early enough to support at least some threat for fog and/or low cloud development towards sunrise on Sunday. Think that winds just off the deck look too robust for much of a dense fog threat, however with more of just a shallow mist and low stratus potential. Temperatures will also fall into the upper 30s with the arrival of a northwest wind shift, but the low- levels will dry out quickly enough to end any precipitation well before the column cools enough to support any mixing of p-types. Northwest winds remaining up around 5-10 mph will likely curtail the potential for much frost development in our northwest early Sunday morning. Have generally sped up the departure of precipitation chances Sunday morning with this forecast package, confining and straggling chances to northwest Indiana. Lake effect parameters, while marginal, will support carrying chance PoPs through Sunday afternoon into parts of Porter, Lake, and Jasper Counties. Breezy north to northwest winds will then develop through the morning Sunday and afternoon, with some occasional gusts into the 25 to perhaps 30 mph range. The other story will be the period of near-freezing temperatures Sunday night as the spine of an encroaching area of high pressure crests overhead and winds go nearly calm. We`ll likely have to contend with just a bit of high-level cloud cover at times, but the radiational cooling potential looks solid, especially with dewpoints in the lower 30s. We do show a few 31-32 degree readings in our typical cool spots (Woodstock, Aurora, Rochelle areas for example), but the sub-freezing potential looks pretty localized and insufficient in areal extent to warrant any Freeze Headlines. We will, however, likely need a Frost Advisory for much of the region away from the immediate urbanized Chicago corridors, and will let the midnight shift take a look at this orientation. Carlaw && .LONG TERM... 245 PM CDT Monday through Saturday... The primary Storm track is expected to remain north of the area though the period. For this reason, there are no good chances for precipitation through the forecast period. Expect temperatures to be more seasonable next week, with highs generally in the 60s, with low 70s at times. A surface high that move move across the area Sunday night will shift to our southeast during the day Monday. This will allow the winds to turn southerly in its wake for Monday into Tuesday. Moderating temperatures in this warm air advection pattern will push highs back into the lower 60s for Monday, and potentially up around 70 for Tuesday. A weather disturbance passing mainly to our north on Tuesday may drop a weak cold front over the area during the day, but this looks to do little to the temperatures as the flow quickly shift back out of the southwest Tuesday night into Wednesday ahead of a fast moving clipper system dropping southeastward over Ontario. This system will result in a mild and breezy day Wednesday, with a stronger cold front expected to shift southward over the area during the day. It continues to look dry across the area with this frontal passage, so we continue to advertise a dry forecast. We only look to get a glancing blow at some cooler air in the wake of this front, so we can expect temperatures to be in the lower to mid 60s for Thursday. Thereafter, we look to get back into a mild weather pattern, supportive of highs around 70 for next Friday. Even warmer temperatures, potently well into the 70s, are possible for a period next weekend. KJB && .AVIATION... For the 00Z TAFs... 627 PM...Forecast concerns include... Light rain through early Sunday morning. Mvfr cigs tonight. Gusty northerly winds Sunday. Light rain will continue across the entire area this evening and slowly end from northwest to southeast early Sunday morning. The rain may briefly reduce visibilities into mvfr. As the low levels saturate...cigs are expected to lower into mvfr across much of central IL and northern IN. Confidence is decreasing for how low cigs will become across the terminals. Mvfr cigs are still expected but may remain in the 2kft range across most areas. The best chance for 1kft cigs is likely along an arr/jot/gyy line with ifr possible south of that line. Cigs are expect to slowly lift into vfr Sunday morning. If clouds scatter out by Sunday morning across northwest IL there is some potential for either light fog or a low stratus deck but confidence remains low. Light westerly winds will shift light north/northwest this evening with speeds increasing after the rain ends overnight. Speeds into the 12-16kt range are expected Sunday morning with gusts into the lower/mid 20kt range. Speeds and gusts will slowly diminish Sunday afternoon and a lake breeze/wind shift to the north/northeast is expected for ord/mdw. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744- LMZ745...7 AM Sunday to 4 AM Monday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: