300 FXUS63 KLOT 091957 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 257 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... 252 PM CDT Wednesday through Tuesday... A pronounced pattern change will occur during the beginning of this period, with the transition time providing gusty showers/storms on Wednesday and then prevailing wind gusts 30-35 mph Wednesday night into Thursday. This will be followed by the likelihood for frost Thursday and Friday nights if winds can diminish enough. The well-defined closed upper low over southeast Colorado this afternoon will eject across the Upper Midwest tomorrow, with guidance in good agreement on a deepening surface low to just below 1000 mb near La Crosse WI by mid-afternoon. With the negative tilt to the ejecting wave, strong forcing for ascent will be overriding the deep moisture plume and pre-frontal convergence, all a recipe for widespread showers. The instability in the column becomes quite narrow, not that atypical for ahead of an autumn cold front especially one with deeper warm cloud processes such as this. But given the negative tilt to the wave, can see embedded thunderstorms being present. Lightning or not, the unidirectional strong southwest winds in the column provide the potential for downdraft-driven gusty showers. In any more well-defined arcs of convective activity, mainly any that are not far ahead of the front in the afternoon, the potential for isolated severe gusts will exist. Some members of the higher-resolution guidance family try to key in on higher reflectivity/deeper updrafts moving to the east-northeast across the area in the afternoon, but others do not. While there is ample low-level shear, the orientation of the deeper shear vectors to the boundary is not ideal for discrete activity, and this pre-cold front is not an ideal setup for a tornado threat. But surface winds are backed to around due south, so if mode can be more discrete immediately ahead of the main convective line/semi-line, that may not be ruled out. After a mild and humid day with highs in the 70s, the cold front will sweep through Wednesday evening. With the strong isallobaric response and steeping low-level lapse rates in cold advection, westerly winds behind the front will sharply increase. Gusts immediately behind the front may reach 40 mph, with regular gusts of 30-35 mph expected into the day Thursday. Temperatures by daybreak Thursday look to be lower 40s. Despite a mainly sunny sky on Thursday, the northwest winds will keep highs from rebounding much, with highs generally around 50 (around 35 degrees colder than today). Surface high pressure around 1021 mb is forecast to build into the Mississippi River Valley Thursday night. It is likely that some northwest wind will be maintained through the night in our area, at least given current forecasts, though challenging to say if enough to mitigate frost. With the low starting point in temperatures going into the night, readings will likely dip to around freezing in north central Illinois and continue frost mention in the forecast. The Friday forecast became a bit more tricky, as the 12Z EC shifted north with the Pacific-hybrid system moving quickly across the region, and places that more over our area. This would bring light rain potential. Even with that forecast, the quick progression would likely favor a clear sky Friday night and a frost threat and depending on wind speeds again, possibly a freezing threat into north central Illinois. The weekend, especially the latter weekend into Monday, look unsettled in global guidance, but a wide disparity between models and between runs on how the pattern will evolve. This is increased by the fact the flow is split with a phasing potentially over/near the Great Lakes. Chances for rain have been increased within blended model guidance, but apart from that it is presently difficult to give more details in coverage/timing than that. The pattern favors cold advection undercutting the backside of precipitation, but right now the chances for some flakes of snow before ending look to be north in Wisconsin. Highs on Monday are only forecast to be in the mid 40s in the northern CWA and could end up being a little cooler than that in time. 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...Small Craft Advisory...LMZ742-LMZ743-LMZ744-LMZ745...9 PM Wednesday to 10 PM Thursday. Gale Watch...LMZ740-LMZ741...9 PM Wednesday to noon Thursday. && $$ 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