Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 232331 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 531 PM CST Fri Feb 23 2018 .SHORT TERM... 243 PM CST Through Saturday Night... For the remainder of this afternoon and into the evening, breezy and cloudy conditions will persist as the the system that brought some additional rain overnight moves off to the east. A zone of height rises over the region in between an exiting low and high pressure building across the northern plains to the middle and upper Mississippi Valley. As the high builds across the Great Lakes overnight tonight, a warm front associated with a southern stream system will set up along the Ohio Valley. A shallow layer of cold air will remain in place at the surface across the region, but aloft, warm, moist advection will set up in advance of the southern stream system, setting up the potential for some freezing/frozen precipitation in the morning. There should be a relatively sharp cutoff to the precipitation on the northern edge, where the cold air will be in place. There is still some uncertainty as to the progression of p-type as the precipitation shield spreads northward, but the general trend should be for an area of all snow on the far northern edge, with a narrow zone of a wintry mix transitioning over to all rain to the south. Through the morning hours, both the nrn edge of the pcpn shield will spread north while the zone of frozen and freezing pcpn also shifts north, to be quickly followed by the transition over to all liquid as sfc temperatures rise into the middle 30s by late morning. However, of greater concern for Saturday into Saturday night, will be the additional rainfall moving across areas already impacted by major flooding. Latest guidance suggests an area of 0.25 to 0.50 inches of additional rainfall for the period from early Saturday morning to early Sunday morning. As of this afternoon, most rivers across the region, including the Illinois, Des Plaines, Du Page and Fox Rivers have either crested or will crest soon. Portions or the Rock River are not expected to crest until Sunday or Monday as the crest wave moves downstream. The Iroquois and Kankakee Rivers and locations near these rivers will remain a concern through the weekend and into next week. The Iroquois River at Chebanse is still rising as water from upstream on the Iroquois river as well as flow from Sugar Creek and smaller tributaries. The Kankakee River in northwestern Indiana will remain above major flood stage well into next week as the river is particularly slow to subside there. The Kankakee River in Kankakee County will also be slow to subside. While any additional rainfall will not be welcome at areas already impacted by significant flooding, since the rainfall will occur over a 24-30 hour period, it should not contribute to significant rises, and perhaps delaying the river levels falling. && .LONG TERM... Sunday through Friday... 153 pm...Primary forecast concerns are the potential for more rain Wednesday/Thursday and the chance of mixed precip/snow Thursday. As low pressure lifts north across the upper lakes region Sunday...strong/gusty westerly winds will diminish on Sunday but there could be a period Sunday morning when wind gusts are still in the 35-40 mph range. High pressure will move from the central Plains Sunday to the Ohio Valley Monday with dry conditions expected through Tuesday evening. With some sunshine expected...temps likely to moderate back near 50 on Monday and into the 50s on Tuesday. With no snow pack...its possible these temps could be a bit low but can trend higher as this time period approaches. Low pressure is expected to develop over the Plains Tuesday night into Wednesday and move near northern IL by Thursday morning. Still considerable differences with the gfs showing a rather strong low at that time vs. a weaker ecmwf low. Ahead of this low...showers will be possible Tuesday night into Wednesday with a period of heavier precip possible Wednesday night into Thursday morning. While the timing looks similar...the large differences in the strength of the low make for an uncertain forecast. In addition...small track changes could make the difference between all rain or a stripe of accumulating snow...which currently would be most favored across far northern IL into southern WI. Current qpf forecasts would suggest the potential for a half inch to an inch of rain with this system. High pressure will then build across the region Friday. cms && .AVIATION...
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For the 00Z TAFs... The main forecast concern will be CIG trends though the period, as well as the potential for more IFR to LIFR conditions Saturday evening with the next batch of rain and fog. Surface high pressure over the eastern Dakotas early this evening will shift eastward towards the Western Great Lakes by daybreak Saturday. As this occurs, the westerly winds will gradually shift northerly later this evening, then east-northeasterly into early Saturday morning. Easterly winds of 10-15 kt are then likely to continue Saturday into Saturday evening. While a short period of a scattering of the low MVFR CIGS is possible this evening, it does appear that most areas will hold on to these MVFR CIGs through Saturday. The chances for precipitation through the daylight hours on Saturday, while not zero, appear to be quite low, with mainly dry conditions expected at the terminals. For this reason we have removed all mention of precipitation from the TAFs through 00z Sunday. Thereafter, the chances for rain and fog will increase across the area in advance of the next area of low pressure approaching the area from the southwest. It appears likely that the area will get a period of very low CIGs (LIFR) and lower visibilities in fog and rain, especially from mid Saturday evening through late Saturday night. KJB
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&& .MARINE... 153 pm...Primary forecast concern remains high end gales and the potential for storm force gusts Saturday night into Sunday. Overall...no significant changes to the forecast or reasoning. Low pressure will develop over the southern Plains Saturday and then begin to rapidly deepen as it moves north across WI Saturday night then crossing Lake Superior Sunday morning. Southeasterly gales are expected to develop over southern Lake Michigan Saturday evening as the low approaches with southerly gales across the entire lake early Sunday morning. As the low continues to deepen over Lake Superior Sunday morning...winds will begin to shift to the southwest and then west as colder air spreads across the region. Its during this time period...Sunday morning...that high end gales are likely and at least some storm force gusts are possible. If any storms occur...they likely would only last a short time...perhaps 3-6 hours. The gales will diminish over the southern part of the lake by Sunday afternoon but gales may persist across north/central portions of the lake well into Sunday evening. The current gale watch has this time period covered well and not planning any headline changes this afternoon. High pressure will move across the Ohio Valley Sunday night and Monday as weak low pressure moves across Ontario...maintaining a west/southwest gradient across the lake. Low pressure may move across the southern Great Lakes region next Wednesday night into Thursday. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Gale Watch...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777- LMZ779...6 PM Saturday to 3 AM Monday. && $$ 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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