Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/26/17


Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
759 PM CST Wed Jan 25 2017 .UPDATE... 748 PM CST Current forecast for precipitation type transition is right on cue with the RAP/HRRR suggesting a rain or drizzle for a short time, even some sleet, to transition to snow. This transition to snow is approaching the Chicago metro area, but may initially be slowed in the heart of the city briefly until low level temps cool and weak deep saturation. Road temps and marginal surface temps have limited accums largely to grassy surfaces in our northern and western counties where snow has been the main precip type for a few hours, but expecting some slushy accums on less traveled roads. After a leading band there may be a brief lull, but as cold air filters in from the north, a secondary trough will rotate southward overnight. The precipitation shield, largely of the snow variety, is filled in through central Wisconsin, and this will move through our area later, with lighter snows lingering into the early portion of the day Thursday. KMD && .SHORT TERM... 304 PM CST Through Tonight... As was anticipated, focused ascent across northern Illinois has allowed drizzle to overspread much of northern Illinois, while upper level low lifts overhead and surface low departs to the northeast. Patchy dense fog has also developed over far northern Illinois and while I do think this may linger for a time this evening, don`t think it will become too widespread. With continued large scale ascent in place and more focus per lingering trough axis over northern Illinois, should continue to see precip shield blossom over northern Illinois over the next several hours with the stronger development initially staying focused north of I88. Have once again bumped up timing for transition from liquid to all snow per latest trends. Latest short term forecast soundings, AMDAR soundings, and surface obs all in fair agreement with placement of colder air pushing through the region. Would anticipate as soon as stronger precip shield develops here in the near term, that mixed precip could quickly then be observed before a transition to all snow occurs this afternoon for these areas. While light snow is generally anticipated, there still remains a possibility for periods of light to moderate snow in these locations this evening. Expect precip to further blossom late this afternoon/early evening into the I88 corridor with similar trends of a small window of rain before the transition to rain/snow and then all snow occurs in the 00-02Z time frame. Light snow anticipated in this corridor and although smaller windows for light to moderate snow are possible, think this would be rather brief and isolated. Large scale ascent will be present over much of the CWA tonight, including areas south of I88 in Illinois and northwest Indiana. However, it does appear that the deformation axis will likely glide more east northeast. This should minimize the amount of light snow in these locations to the south. For most locations, a 5-7 hour window of at least light snow will be possible tonight. Have similar snowfall amounts forecast with 1-2 inches over far northern Illinois, half inch to an inch for areas along the I88 corridor, and then under a half inch for areas to the south. With warmer pavement temps, think any accumulation/impacts on the roads will be limited. However, for areas in northern Illinois where the more intense snow could occur, can`t rule out some slushy conditions especially more into this evening. Rodriguez && .LONG TERM... 205 PM CST Thursday night through Wednesday... Seasonable conditions return to the region throughout much of the extended timeframe along with periodic chances for light snow showers or flurries. Area of low pressure lifting across the region today is expected to merge with another low currently off the New England coast then meander over the Canadian Maritimes through the latter part of the work week and through the weekend. This will leave the local area under persistent cyclonic low level flow and northwest flow aloft along with steep low level lapse rates at least through the weekend. This will result in mostly cloudy to cloudy conditions and periodic chances for light snow or flurries as a series of low amplitude waves rotate across the region. Deep upper troughing over the eastern CONUS will begin to shift east and flatten early next week with weak upper ridging developing overhead on Monday. Strong mid level height rises and surface high translating across the area should provide dry weather for Monday and at least a chance to see some sunshine. Forecast details become a little less clear thereafter as upper ridge begins to buckle with another approaching upper level disturbance/Albert Clipper Monday night into Tuesday. 25.12Z run of the GFS shifted the low track farther south and came in faster than previous runs while the ECMWF maintains a more northerly track with less sensible weather for the local area. Could see a brief return of well above normal warmth ahead of the wave on Tuesday. Deubelbeiss && .AVIATION... For the 00Z TAFs... Main concerns: * -DZRA changing to a period of steady snow this evening. * Associated CIG and VSBY impacts. * Additional flurries/snow showers late tonight through Thursday. * CIG/VSBY trends late tonight through Thursday. An upper level disturbance will result in a period of steady snow likely at all terminals this evening, with somewhat lower confidence at GYY. Prior to changing to snow, the eastern terminals will see light rain, drizzle and light fog with occasional IFR VSBY possible. Based off observed temperatures and aircraft soundings, MDW and GYY should take longer to go over to all snow. Radar trends suggest that IFR VSBY is likely in the snow, with temporary LIFR possible, especially at RFD, where 3/4SM was maintained in the TAF. Cannot rule out LIFR VSBY at the eastern terminals, but confidence is not high enough to mention in the TAF. Mild ground temperatures are expected to limit accumulations on runways, with perhaps a wet slushy accumulation possible, especially at RFD and DPA. The lead disturbance will move east overnight, followed by another wave dropping south from Wisconsin, that could bring a period of steadier snow showers in the predawn hours. The overall pattern through Thursday with an upper level trough and steep low level lapse rates supports at least periodic flurries/light snow showers. While VSBY impacts are currently not anticipated, will need to monitor observational trends on Thursday. West-southwest winds this evening will shift to gusty northwest later in the evening and then remain west-northwest for the rest of the period. Castro && .MARINE... 205 PM CST Area of low pressure currently over far southern Lake Michigan will shift to the eastern Great Lakes tonight then merge with another low currently off the New England coast late Thursday into Friday. The low is expected to meander over the Canadian Maritimes through the remainder of the week and through the weekend which will result in a prolonged period of moderate northwest to west flow of 25 to 30 kt that will persist into early next week. A small craft advisory will be in effect for the Illinois nearshore zones from mid evening through Thursday afternoon and for the Indiana nearshore zones from late this evening into Friday night. Weak surface ridge will translate across the region on Monday with winds turning southwest behind the high later Monday into Tuesday. Meanwhile, a clipper low will drop into the Upper Midwest/western Great Lakes or just north of the Great Lakes Tuesday, though there remains considerable uncertainty in track and timing details at this distance. BMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 6 PM Thursday. Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 10 PM Friday. && $$ 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