568 FXUS63 KLOT 091911 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 211 PM CDT Tue Oct 9 2018 .SHORT TERM... 210 PM CDT Through tonight... Isolated to scattered showers and storms are expected this afternoon. The only forcing is surface heating so widespread storms are not expected. The latest RAP analysis shows up to 2000 J/kg of CAPE and 25-35 kt of shear. DCAPE values are 800 J/kg or less. While a storm or two may pulse up close to severe limits, the vast majority of storms will not be severe. Main threats will be gusty winds, heavy rain, and lightning. Convection will diminish after the sun goes down and forcing eases. I kept a chance of showers and storms west of a McHenry to LaSalle line this evening as guidance suggests a few showers or storms may percolate there ahead of the main cold front. Speaking of the cold front currently over central Iowa, it will slowly march east tonight. Showers and storms are expected ahead of it, and will mainly impact areas along and west of I-39. I have low confidence in the number of showers and storms tonight because forcing will be mainly tied to the front. A couple transient vorticity streamers may help force precipitation, but the streamers are not forecast to be very strong. Due to cloud cover tonight, low temps should be mild in the upper 60s. JEE && .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