Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/28/19

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
905 PM CST Fri Dec 27 2019 .UPDATE... 855 PM CST Main updates this evening were to lower temperatures a bit overnight and to add some fog wording to the grids, especially south and east of I-55/57. Skies have remained mainly clear most of the evening which has allowed temperatures to drop off pretty quickly towards their dewpoints across much of the area. That is changing, however, with the approach of a modest mid-level wave which is spreading additional mid-level cloud cover northward. An expansive area of low stratus is percolating at this hour immediately east of the CWA, and this all looks to slosh west and eventually northwestward later tonight as the low-level wind field begins to veer as surface high pressure slinks off to the east. The northern-most portion of this low cloud deck is just that--low clouds, but visibilities have been inching downwards with southward extent where low-level moisture is deepest. Unsure exactly how the incoming mid-deck will impact visibility reductions, but elected to toss in some fog wording into the grids after midnight with the greatest coverage expected mainly along and east of I-55/57 where near-surface winds will remain lightest. With temperatures near and below freezing, any fog/mist would be freezing fog which could cause some slick spots on elevated surfaces, although just how thick fog may get remains in doubt. Will continue to monitor trends this evening. The aforementioned wave to our south has resulted in the development of mid-level radar echos which have, thus far, not resulted in any precipitation due to the dry 850-600 mb airmass in place. ILX`s 00z sounding showed a bit more saturation in thay layer, but recent AMDAR soundings through this layer are still fairly dry. Noting a very modest mid-level f-gen circulation with this feature with some attendant decrease in 500-600 mb lapse rates. Will leave the forecast precip-free given the dry mid- levels, but have tossed in some silent <10% PoPs south of I-80. Finally, another subtle wave is lifting out of Arkansas which could help encourage some precipitation development closer to I-39 across our western counties after daybreak on Saturday. With temperatures already dropping into the mid-upper 20s, could be a potential for a very brief rain/freezing rain scenario but with temperatures pushing north of freezing pretty quickly, don`t envision this causing any notable travel issues. Carlaw && .SHORT TERM... 315 PM CST Through Saturday night... Several forecast concerns through Saturday night, including patchy fog potential later Saturday morning through the afternoon, followed by widespread rain/showers and embedded thunderstorms Saturday night. The pesky stratus deck that kept our temps cooler today finally eroded to along and southeast of I-55 as of this writing. Expecting it continue to slowly erode southeastward, though with low level winds aloft diminishing. However, if it doesn`t, it may tend to expand back north/northwest tonight as cloud steering winds turn to southeasterly. Otherwise, tonight should be quiet with increasing cloud cover overall as surface high pressure transits the area. Temperatures will ease back to the upper 20s to mid 30s this evening and then slowly rise overnight thanks to the increasing clouds, a bit of wind late, and strengthening warm advection aloft. On Saturday morning, attention turns to developing surface low pressure over the southern High Plains in response to deep western troughing and a strong upper level jet streak. In deep southwest flow aloft over our area, a lead mid-level shortwave will clip northwest and north central Illinois with an area of light rain/showers, possibly mixed with intermittent drizzle late the morning as low cloud cover rapidly expands back northward. Think that temperatures will most likely have safely climbed to above 32F by the time any precip ensues, which would preclude risk for any light freezing rain or drizzle. For the rest of Saturday, the local area will be positioned north of a developing warm frontal zone from the Plains surface low that will only gradually lift northward through the day. After the aforementioned morning wave in northwest 1/2 or 1/3 of CWA during the morning, forecast soundings indicate that additional precip if any would mainly be in the form of drizzle. Exception to this could be the far northwest, where additional waves of light rain/showers will be possible. Synoptic set-up tomorrow, with area north of warm front, and dew points gradually ticking upward and into the 40s, is typically favorable for some fog development. Arguing against a significant dense fog risk is steady east- southeast to southeast winds and lack of snow cover over the region. Nonetheless, with temperatures likely staying only a degree or two above the increasing dew points, felt it was reasonable to add patchy fog mention. Of note, the 12z MET MOS guidance was hitting the fog threat hardest in part of the area, which is probably overdone. There will be a strongly non-diurnal component to the temperatures on Saturday, which is also classic to the synoptic set-up. Temperature climb tomorrow will be gradual, with most areas only topping out in the 40s by sunset, except low 50s well south. Far northern areas may not get out of the 30s until early afternoon. Daily high temps will occur in the early evening. The surface low will strengthen into the low 990s mb Saturday night as it tracks toward southern Minnesota. It is during this time that large scale ascent will be at its strongest, along with robust warm advection/isentropic ascent. Expect an area of widespread rain/showers to spread quickly north- northeastward. With PWAT values increasing to or over 1.25", near the December maximum, this will support occasional moderate to heavy rainfall rates. Mid-level lapse rates will remain meager, but the magnitude of the WAA is expected to compensate and still yield a threat for embedded thunderstorms, with a bit higher chance south. Finally, as south-southeast winds remain elevated while warm front lifts north, temperatures will steadily rise through the night, with 50s for all or most areas prior to daybreak Sunday. This will set the stage for another day of impressive warmth on Sunday. Castro && .LONG TERM... 305 PM CST Sunday through Friday... By Sunday morning, the center of the surface low associated with the strong storm system expected to develop over the Central Plains should have lifted to southern MN. An associated surface cold front will quickly push across the plains and should be moving crossing the Middle Mississippi River into wrn IL by the beginning of the period. The latest longer range guidance remains relatively consistent will the general details of the system, particularly in the path and strength of the mid-level circulation center, though there are still some minor timing differences for the frontal passage. These timing differences should have little impact on the general trends and character of the system and impacts on the local area. In advance of the system, strong moist advection will drive temperatures well into the 50s by daybreak, with temperatures likely to rise to, or slightly above 60 F with dewpoints in the 50s before the front pushes through the area. Gusty south winds will set up in advance of the system, with gusts of 25-30 mph. Widespread rainfall should still be ongoing Sunday morning, but should diminish to light rain or drizzle by mid-day as a mid-level dry slot overspreads the region. The upper low is expected to meander around the upper Midwest as surface system begins to occlude. However, a secondary and somewhat strong vort max will rotate around the swrn/srn periphery of the main upper low Sunday evening. Temperatures should begin to drop off quickly as the true cold air begins to filter into the area. This should bring another round of quick hitting rain. Both the GFS/ECMWF have enough column cooling as the pcpn is winding down for the rain to mix with or, briefly, change over to snow before precip ends early Monday morning. Marginal surface temps should limit what minor accumulations may occur, if any, to grassy areas. The cold core of the upper low will spread overhead Monday afternoon and nwly flow aloft with deep layer cold advection should develop as the low lifts to the Upper Great Lakes Monday night. With weak shortwaves rotating around the swrn periphery of the exiting upper low, there will be the potential for some sct light snow showers or flurries overnight into early Tuesday. Conditions should be seasonably cold highs on Tuesday in the lower to middle 30s. Wednesday should be a bit warmer as upper ridging builds across the area. A modest warming trend will set up for Wednesday and through the end of the period as southwest flow returns aloft as the upper level pattern gets more active over the wrn CONUS with a sharp long wave trough setting up. A series of weak shortwaves lifting out of the base of the upper trough and rippling through swly flow aloft, bringing both a modest warming trend with periodic low chances of pcpn. && .AVIATION... For the 00Z TAFs... Main concerns through the TAF period: * Potential for IFR or lower cigs/vis developing overnight into early Saturday morning * Periods of light rain/showers and/or drizzle on Saturday, mainly at RFD in the AM and possibly farther east in the afternoon with IFR/near LIFR cigs. Continue to have a fairly complicated TAF forecast through Saturday. Hi-res guidance is trending toward increased fog/low stratus potential early Saturday morning as the surface high drifts east across the area and light surface winds turn southeasterly. Have updated the TAF to hint at this trend. Things could very well be even lower if fog/stratus does develop toward VLIFR. Temps will also be low enough for some freezing fog potential. As for precip, a first wave of light rain/drizzle should push through in the morning across our northwestern zones that could impact RFD. This still looks to remain northwest of the other terminals. Drizzle will then develop by Saturday afternoon out ahead of the main wave of precip. There is higher confidence in decreasing cigs to IFR/near LIFR Saturday afternoon associated with this precip. Winds will be light and mostly variable through the night, becoming east-southeasterly Saturday morning as the surface high shifts to the east. By Saturday afternoon winds will begin to pick up to 10- 15KT with gusts to 20-25KT possible as the main system approaches from the southwest. Petr && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO