Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 161735 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1135 AM CST Mon Jan 16 2017 .UPDATE... 947 AM CST Rain continues mainly along and north of I-80. Air temperatures have risen to above freezing mainly south of I-88. No additional ice accumulation is expected south of I-88. As such, removed Cook, DuPage, Kendall, and counties south from the Winter Weather Advisory. Any ice on the ground in those counties should begin to melt as temperatures continue to climb. North of I-88, freezing rain and minor ice accumulation will continue until noon when temperatures rise above freezing. Looking further south, visibilities south of I-80 are mainly below one mile. Reports from various towns and observations suggest that widespread dense fog is not occurring so will continue to hold off on a dense fog advisory. Guidance continues to suggest that dense fog may form late this afternoon into this evening, but only have medium confidence in dense fog occurring. Models may be confusing low cloud heights for dense fog and will continue analyze for the afternoon forecast. JEE && .SHORT TERM... 215 AM CST Through Tuesday... Concerns center on expansion of mixed precipitation this morning, then a transition to some moderate to locally heavy rainfall later today and tonight, along with the potential for at least a period of dense fog. Ascent has and will continue to increase through the morning hours from a lead shortwave ahead of the deep upper low across the southern plains coupled with increased warm advection/isentropic lift in the strong southwest flow. The forcing is not super strong and with a dry layer in place in the lower level generally lighter precipitation is expected initially during the surface subfreezing conditions, but still enough to create concerns. High res guidance does show an uptick in lift and precip as an elevated warm front will move through this morning, thus expect locally heavier elements once saturation occurs which we are seeing on radar and enhancements on IR. Temps and dewpoints have crept upward, and thermally cloud cover has allowed temperatures to remain somewhat elevated this morning, but there is a corridor of subfreezing temperatures (with 20s dewpoints) such that the concern for freezing rain exists, especially north of I-80 (and more so north of I-88) and northeast of I-55 along a line northwest of Morris to ORD to Waukegan (where it is still only 24). Reports along and north of this line suggest untreated surfaces quickly get a light glaze, even under the lighter echoes. This does include the city of Chicago, though at least of the next few hours rain will be light away from north central Illinois. Southeast of here temps are largely at or above freezing and forcing is weaker thus concerns are lower, but still dewpoints are below freezing so a narrow window of concern still. Amdar soundings early this morning show a sufficient warm layer that would suggest FZRA is the prime concern, with possibly some wet bulbing for a period of sleet especially along and north of I-88 before saturation suggests FZRA/RA to dominate. Surface flow will remain ese today as the surface low remains positioned to the southwest of the area and thus a rapid warmup/moisture surge is not anticipated during the daytime hours which initially will limit surface warming , but continued lower level southerly flow will eventually allow all areas to warm above freezing. Current advisories seem reasonable with up to one tenth of an inch of ice or so in the aforementioned corridor and even some light sleet accums, with the much lighter accums are expected farther southeast where more marginal surface temps exist and weaker forcing is expected, but as mentioned a light glaze would be a concern. We do not anticipate any changes, though will have to watch the Will County and Lake/Porter areas for any possible expansion. Heavier precip rates can be expected later today and tonight as the upper low over OK/KS will move northeastward across the lower Great Lakes region. Ascent increases markedly due to the strengthening of the low level jet and approach of the left exit region of the upper jet. Precipitable water values (indicators of available moisture) are on the high side. Scattered showers can be expected in the afternoon, with more widespread coverage this evening as more organized forcing will arrive. With the approach of the upper low and cooler temps aloft, high level lapse rates (500-300 mb) steepen enough to suggest some potent showers with even a few thunderstorms possible with the more significant moisture surge this evening. The upper low will kick east of the area overnight into early Tuesday. The system`s cold front will keep the threat of showers in late tonight into early Tuesday though possibly with less intensity then the evening activity. With a surface warm front lingering in place ahead of and with the surface low passage, forecast soundings depict the low level moisture (already in place) to our south to remain trapped below a strengthening thermal inversion as the just above surface warm (and moist) air surges northward. Expect a quick lowering of clouds this morning and likely decreasing visibilities. This concern will be focused south of I-80 this afternoon where lighter precip will be occurring, and dense fog is suggested by high res guidance. This will shift northward later this evening and overnight. Some of this will be mitigated in periods of heavier rain and thus hints in model guidance of at least some brief improvement during the evening hours, but then potentially redeveloping overnight with the arrival of the low. Tuesday afternoon the northern stream shortwave will pass through. Limited moisture suggests lighter showers with no more than scattered coverage. Thermal profiles get close to maybe a rain snow mix late in the day if there is even enough saturation into the ice nucleation layer which at this point is in question. KMD && .LONG TERM... 215 AM CST Tuesday night through Sunday... Surface high pressure will build in Tuesday night into Wednesday. There could be some lingering low clouds into Wednesday, though increasing subsidence behind the departing northern stream wave would appear to help squash the moisture layer. Dry conditions will be in place for Weds/most of Thurs. Another slow moving cut off low over the southwestern United States (currently off the Baja CA coast). Southwest flow in this pattern will support above normal conditions, but this seems like a moist pattern where at least occasional cloudiness, possibly more often than not, will rule, especially after Thursday. No precipitation type concerns exist through the period, but precip chances do return later Thursday into Friday as this next cutoff low finally will lift northeast with another chance for decent rainfall. The pattern will remain unsettled as additional lows will spread northeastward, though we certainly remain on the mild to warm side, especially with dewpoints forecast to be in the upper 30s to mid 40s late in the week which are quite a bit warmer than actually air temperature normals, thus we are still on tap for well above normal temperatures, though did not stray from a model blend as our warming will be limited given high model RH progs thru the weekend, suggesting cloud cover and occasional rain with additional weak lows headed our way in SW flow. Eventually cooler air will filter in late in the weekend or early next week. KMD && .AVIATION... For the 18Z TAFs... Rain will continue through tonight. The low over Kansas will move over the region late tonight/early Tuesday morning. As such, ceilings will remain very low at LIFR. Visibility is not as straight forward. 1SM vsby is expected through this afternoon, but vsby may drop to less than a mile after midnight. Have medium confidence in how low vsby will go overnight as the models may be confusing the very low ceilings with fog. Isolated embedded thunderstorms are possible late this evening. Have low confidence in how many thunderstorms there will be and exactly where they will form. Rain may also taper off to drizzle after midnight which could also lower vsby. Winds become northwest to southwest behind the low Tuesday morning. The drier west winds will help raise vsby to VFR in the morning. However, guidance suggests cigs will struggle to rise so kept them in the IFR range. Another upper level wave approaches Tuesday afternoon, and it may kick off scattered showers, so kept VCSH in the TAFs through Tuesday afternoon. JEE && .MARINE... 312 AM CST A modest southwesterly gradient is in place across northern Lake Michigan early this morning between a departing ridge over New England and low pressure over Hudson Bay. Winds will peak around 25 kt over northern Lake Michigan this morning but should gradually diminish through the day as the two features shift eastward. Attention shifts to the southwest as low pressure approaches from the Central Plains today. East to southeast flow is expected to strengthen into the 15 to 25 kt range across southern Lake Michigan this afternoon into the evening and gradually spread north across the lake overnight. The low is expected to track over southern Lake Michigan Tuesday morning with winds turning west and eventually southwesterly Tuesday night into Wednesday. BMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008- ILZ010-ILZ011-ILZ012 UNTIL NOON Monday. 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

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