Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/11/19

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
847 PM CST Sun Feb 10 2019 .UPDATE... 829 PM CST Main update this evening was to extend the mention of light snow and freezing drizzle through late this evening across much of the area. Latest radar imagery depicting persistent scattered precip development across northern IL and northwest IN. The stronger forcing has departed to the east, but isentropic ascent remains focused across the area. This should allow for an additional 2-3 hours of precip, before this shifts to the east overnight. The trend over the last couple of hours has been for all light snow north of I-80, and mainly freezing drizzle for locations along and south of I-80. A fine mist has been observed here at the office in Romeoville, with a light glaze noted on the cars. However, at this time, there are some small snow flakes mixing in with the mist. AMDAR soundings from this evening did note some moisture right around -8c, which is right on the lower threshold to support snow development. This has been just enough to support snow in some locations this evening, but also why we are likely seeing light snow and freezing drizzle. So, for the remainder of the evening, think most locations will see this mix before the precip departs. Have issued graphicasts and an SPS to message this potential along with some possible slippery conditions and given the light nature of the precip along with the short window this evening, will hold off on any headlines at this time. Will continue to monitor trends/obs though, especially for locations between I-88 and I-80 where this mix will likely be more prevalent. Rodriguez && .SHORT TERM... 230 PM CST Near term discussion through tonight... Snow came down quickly as we finally saturated the low layers across portions of northeast Illinois this afternoon. The dry slot of this wave is already reaching the Illinois border, thus the window for snow is not very long, only a few hours of the deeper saturation, thus accumulations will not be all that significant, around a half inch up to 1 inch in most locations. The snow should be winding down in the coming hours. There is some weak ascent in the low levels that will continue behind this wave once we lose deeper saturation. Observations in the dry slot across Iowa has indicated a transition from -SN to either unknown precip (likely FZDZ) or freezing drizzle with some snow. Low cloud bases are not widespread upstream which is indicative of the freezing drizzle likely not being long lived, but we will lose the deeper moisture while some lingering forcing with this wave and low level isentropic ascent continues into the evening. Given the aforementioned, just need to monitor the situation to see if any light accums are possible. Late this evening into the early overnight we do appear to be in between waves, so in spite of there being lower clouds, the freezing drizzle potential appears lower after mid evening tonight. The low level frontal zone will setup just to the south across central Illinois. Isentropic ascent into this frontal zone on SW flow ahead of our next wave may increase FZDZ potential late. KMD Short Term Discussion about the Monday into Tuesday event... Quite a messy set-up is unfolding across the CWA Monday afternoon through Tuesday as the system responsible for active weather across the Pacific NW affects the western Great Lakes. Monday: A shallow stratus layer will likely reside under a deep layer of dry air aloft in the morning. Other than possibly some very week isentropic lift, forcing will lack during the morning hours on Monday for all but the far southern CWA. The layer of isentropic ascent will quickly deepen in the afternoon in concert with mid- level saturation, so a shield of a mix of light snow and freezing drizzle may shift north across the north half of the CWA mid to late Monday afternoon. Confidence on occurrence is somewhat low at this time. Monday night: A combo of WAA, the approaching mid-level trough, and the left-exit of an upper jet should allow for a widespread swath of moderate precip to shift NE across the CWA during the night. The increased low-level WAA should push the surface freezing line to along the I-80 corridor, thus ending the threat of freezing rain south. Between the I-80 and I-88 corridors, including Chicago, the dominate precip type looks to be freezing rain, whereas precip should remain snow generally north of I-88. Given thermo profiles only slightly below freezing below 700 hPa to the north, expectation is for any snow to be fairly wet, with potentially <10:1 SLRs. Lastly, east winds may gust as high as 30 mph through the night across the north half of the CWA. Tuesday: The surface low will track NE across the southern CWA into NW Indiana during the morning. CAA will ensure, changing rain to snow from NW to SE across the CWA through the day. Snow accumulations during the day will range from 1-2" north to around a half inch south. Meanwhile, NW winds will quickly increase with gusts to 35 mph by sunset Tuesday across much of the area. Given the wet snow potential, blowing snow does not appear to be a significant issue at this time. Headlines: First, have decided to hold off on any Winter Storm Watches at this time. Had considered one for the northern tier of counties plus Ogle county. Warning criteria of 6"/12hr or 8"/24hr will likely not be met given the low snow ratios. However, impacts from the very wet snow may ultimately justify the need for a warning if guidance continues to indicate higher QPF over 0.5". However, confidence is quite high that solid advisory-level snow will be met across the northern tier of counties Monday evening into Tuesday afternoon, especially given the potential of rather wet snow. Farther south, confidence on ice amounts is low to medium. This results in a conundrum of whether or not to issue an advisory for lower-confidence icing that starts earlier than higher confidence advisory-level snow that starts later to the north. Ultimately decided to issue an advisory across the northern two tiers of counties. While confidence is medium for icing across the Chicago metro and west, a potentially colder solution of more snow than ice would still warrant an advisory for moderate wet snow instead. In other words, while precip type (snow vs. freezing rain) remains unclear, there will likely be impacts either way. Farther south along the I-80 corridor, there are some concerns that slightly higher surface temps may significantly reduce icing amounts, so have opted to not issue an advisory there at this time, but a colder solution require an expansion to the south. Kluber && .LONG TERM... 235 PM CST Tuesday night through Saturday... The main upper trough axis will shift across the area Tuesday night, while the surface low will deepen further and spread to the eastern Great Lakes. This pattern would suggest precip will taper, but increased low level lapse rates and deep enough saturation for dendritic snow suggest snow showers will continue for some areas into the evening hours. Modest pressure rises suggest some wind- whipped snow showers are possible with some light accumulation of a few tenths of an inch. Wednesday may be the quietest day of the week in the weather department. Weak height rises in wake of the trough will allow a surface ridge to slid over the area, making for cool but dry conditions. Our time in the quiet weather zone will be short lived as we key in on the track of another Pacific Northwest upper low which appears like it could take a bit more northern track across the country before reaching us. This trough will induce another decent surface low in lee of the Rockies on Thursday. Southerly flow ahead of the low will push the warm frontal boundary north through a good portion of the area and allow temperatures to get above freezing nearly area wide ahead of this next fairly potent system. There is fair agreement that the low will spread from the central Plains Thursday northeastward toward the eastern Great Lakes into Friday. This could well be another messy system with the baroclinic zone (which includes the 0C isotherm) draped somewhere across the area. This pattern would generally suggest a rain/mix to snow event. While models are in decent agreement on this general pattern, subtle differences in the low track would support anything from a rain/wintry mix turning to snow event, or all snow for some areas, and whether we get significant snow or get dry slotted and miss the better QPF. Needless to say confidence on a messy system is fairly high with several inches of snow a decent possibility, but the details are of low confidence. Weak through transient ridging will make for dry, cold, and breezy conditions on Saturday. KMD && .AVIATION... For the 00Z TAFs... Another tricky forecast ahead with precip type concerns both at the beginning and end of the 30-hr TAFs. Early evening surface analysis reveals a surface trough draped west to east across central Illinois. This trough axis is progged to inch northward through the evening, then shift south again overnight as weak surface wave propagates east along the trough. PRECIPITATION: Overrunning precipitation continues to wane in intensity north of the quasi-stationary front this evening. Anticipate this trend to continue as isentropic ascent weakens over the next 2-4 hours with precip shutting off thereafter for the remainder of the overnight hours. While the terminals continue to report mainly snow early this evening, loss of ice nuclei is expected as moisture becomes more shallow. This may result in a transition to patchy freezing drizzle through mid evening, then anticipate precip to end altogether. Confidence in freezing drizzle is low, especially at ORD and RFD which are farthest from the surface trough and low ceilings. There is a slightly better chance, but still not great, farther south by MDW and GYY which are closer to IFR ceilings. Most of the day Monday should be dry, but attention will turn to the southwest as low pressure lifts from the southern Great Plains towards the local area by late Monday night. Precipitation with this next low is expected to begin overspreading the terminals Monday evening. There remains low confidence in what the prevailing p-type will be given ongoing model differences. Leaning towards NAM thermal profiles which have slightly faster warming aloft as compared to GFS soundings, but at this point couldn`t rule out periods of freezing drizzle, snow, rain, and sleet at various points in the evening and overnight. Accumulating snow is more favored at the northern terminals, including ORD and RFD which could see several inches of snow. While GYY and MDW will also likely see light snow accumulations, the more southern terminals are more favored to see modest ice accumulations as well. WINDS: East winds this evening will back to the northeast late this evening and overnight. Winds will become increasingly gusty through the latter half of the day Monday as the next surface low approaches the region. Anticipate gusts into the low to mid 20 kt range by Monday evening. CEILINGS AND VISIBILITY: IFR/LIFR ceilings are in place near the surface trough axis over central Illinois north to roughly the I-80 corridor. These lower cigs will trend slightly farther north overnight. For the TAFs, do bring IFR cigs into ORD overnight, though there is a small chance they remain MVFR overnight. Anticipate IFR or near IFR ceilings to persist through much of the day Monday. Finally, misty conditions may result in visibilities between 2-5SM overnight. Deubelbeiss && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Winter Weather Advisory...ILZ010-ILZ011-ILZ012-ILZ013-ILZ014...6 PM Monday to 9 AM Tuesday. Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008...9 PM Monday to 4 PM Tuesday. 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