375 FXUS63 KLOT 091807 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 107 PM CDT Tue Oct 9 2018 .SHORT TERM... 348 AM CDT Through Wednesday night... The main forecast concerns during the period will focus on the timing of our next wave of heavy rain producing showers and thunderstorms tonight through Wednesday. Today will be another mild day across the area, though cloud cover will be on the increase throughout the day. Expect high temperatures to once again reach into the low to mid 80s this afternoon as we remain in warm southerly flow ahead of an area of developing low pressure over the Plains. Much of the area is likely to remain precipitation free across the area through at least the early afternoon today as the main axis of showers and thunderstorms remains to the west over Iowa. However, it does appear that some isolated to widely scattered showers and thunderstorms may develop over the area this afternoon as the area looks to have little capping. Storm development could also be aided by what appears to be some rather subtle mid-level disturbances shifting northward across IL and IN from the southeastern CONUS. The main threat from these storms later this afternoon would be locally gusty winds and a brief heavy downpour. However, of some concern is the fact that the southerly surface winds today may back more southeasterly late in the day in response to pressure falls occurring to our west over Iowa. This could set up a small window, by early this evening, for a isolated tornado with any lingering storms. Otherwise, attention turns to a deepening area of surface low pressure, that will shift northeastward from Kansas to the Upper Great Lakes later today through Wednesday evening. This rather dynamic autumn storm system will push a cold front eastward across the area Wednesday afternoon into the early evening. It will be ahead of this approaching cold front that we are likely to see additional periods of showers and thunderstorms over the area. The best chances for these storms will be later tonight through early Wednesday afternoon over north central IL and over eastern IL and northwestern IN during the day Wednesday. The main threat from these storms will be heavy rainfall, and possibly some flash flooding. Abnormally high precipitable water values in excess of 1.6" are expected over the area, and this should support efficient rainfall processes. The main limiting factor for extreme heavy rain rates looks to be rather modest instability. Nonetheless, the potential for training storms (which should be moving south-southwest to north-northeast around 40 mph parallel to the surface front) could result in locally excessive rainfall amounts that could exacerbate ongoing flooding across portions of the area. This is especially true across portions of north central and northeastern IL where recent heavy rainfall has occurred. A flash flood watch may be needed for parts of these areas in later forecast updates. In addition to the heavy rain threat, there is also a small threat for some strong to severe storms over the area on Wednesday. Strong nearly unidirectional flow is expected over the area as these storms move over the area. This could support a strong wind threat with some of the storms. However, as stated above, the main limiting factor to severe storms looks to be rather modest instability. The threat for storms should end from west to east by early Wednesday evening with the arrival of the surface cold front. Following this front expect gusty west-northwest winds to onset and bring a much colder airmass to the area. KJB && .LONG TERM... 403 AM CDT Thursday through Monday... Gusty west-northwesterly winds will continue on Thursday, and this will usher in much colder weather. In fact, temperatures on Thursday are likely to remain in the 50s area-wide in spite of mainly sunny skies. Temperatures Thursday night then look to fall into the lower to mid 30s under this cold airmass. This may set up some frost across portions of the area into early Friday morning. However, it appears that a surface gradient may keep winds slightly elevated overnight into Friday morning, and this could limit the extent of frost development during this period. Therefore, I limited the mention for frost to only patchy at this time, as any frost might be limited to protected areas, where the surface winds could become more likely to decouple. Friday will be another cold day across the area, with upper 40s far north, to mid 50s south likely over the area. It does appear that an upper level disturbance will move over the area during the day. While this feature is expected to increase cloud cover during the day, and possibly result in a few light showers, at this time it appears the area will remain mainly dry. For this reason, I have left the mention of precipitation out of the forecast at this time. Surface high pressure should then quickly build in across the area Friday evening. Assuming that the cloud cover clears out, it appears that we could see a widespread frost across much of the area outside of the immediate city of Chicago. Temperatures may even fall below the freezing mark in some of the favored cold spots of north central IL and the Fox Valley. Late in the period, there is semblance of a stronger wave in the zonal flow pattern sometime during the weekend, with some phasing with a diving northern stream system. A lot of uncertainty obviously with an evolution like that, so for now have gone with the model blend keying in on highest rain likelihood Saturday night into early Sunday. KJB/MTF && .AVIATION... For the 18Z TAFs... Cumulus clouds have developed across the region along with south- southwest winds gusting to around 20 kt. There is no clear source of forcing for convection other than surface heating, but other convective elements are in place for storms. As such, expecting isolated to scattered storms to develop this afternoon. I have low confidence in coverage and exact storm placement, which is why most sites only have a VCTS for this afternoon. Models continue to hint that the best coverage will be near RFD, so went with a TEMPO for TSRA through the late aftn. Storms end around 00Z, and gusts diminish. A cold front will approach from the west tonight bringing more showers and storms with it. Models suggest the line may be slower and weakening as it moves through northern IL so I pushed back rain arrival time slightly. I kept VCTS as I have low confidence in how many storms there will be along the line. Guidance suggests MVFR and maybe IFR cigs will accompany the showers. Gusty south-southwest winds are expected again Wednesday and may be gustier than currently forecast. Rounds of showers with embedded thunder are expected throughout the afternoon. JEE && .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