Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 252014 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 314 PM CDT Tue Jul 25 2017 .SHORT TERM... 308 PM CDT Through Wednesday... Most of the forecast concerns will be after the short-term period by Wednesday evening, with the main focus on very warm and humid conditions on Wednesday. In addition, can`t rule out a few thunderstorms making it into portions of northern IL (mainly far NW and north central IL portions of the CWA during the late afternoon). A persistent cluster of showers and thunderstorms to our northwest finally is dissipating, with only high cloud cover spreading over the area. The rest of the night will be quiet with increasing high clouds from convection to our north and over the Plains. Overnight and early morning temperatures will be milder than today`s lows and mainly in the 60s. A warm front will lift north on Wednesday, bringing a return to uncomfortable humidity levels, with dew points rising to the upper 60s to lower-mid 70s. There should be plenty of convective mid and high cloud debris overspreading the area, which will limit temperatures somewhat. Progged 850 mb/925 mb temperatures would typically support around 90, but with the cloud cover limitations, expecting mid to upper 80s (peak heat index values in the mid to high 90s over IL corn belt areas). Forecast soundings indicate fairly stout capping over the local area into the afternoon, which should effectively prevent thunderstorms until the mid to late afternoon. With the arrival of main short-wave from the west toward evening, have indicated slight to low chance category PoPs for the potential of lead convection spreading into portions of northern IL as mentioned above. Will need to watch for a slightly quicker onset of higher coverage showers and thunderstorms just prior to 00z. The Long Term discussion below contains full details on the risk for heavy rain/flooding and strong to severe storms Wednesday evening and night. Castro && .LONG TERM... 311 PM CDT Wednesday night through Tuesday... Main forecast concerns/challenges are at the start of the period with chances for thunderstorms, and potential for strong to severe storms with heavy rainfall Wednesday evening through the overnight hours Wednesday night. Details of the forecast to fall out with later updates, but with the general idea still on track with this morning`s guidance. Period of interest during this time frame is Wednesday evening, when guidance fairly consistent in ushering energetic shortwave and associated surface low through the Midwest and Great Lakes region. Still come uncertainty with exactly how the mid and lower levels of the atmosphere will respond that include strength and placement of surface low, as well as the strength of the LLJ. This will play a part to the extent/magnitude of the strong/severe storms and heavy rainfall. Nonetheless, instability axis shifts back overhead during this time, along with PWAT axis of around 2 inches. All of these ingredients coming together will support a continued chance of thunderstorms for much of the CWA Wednesday evening/overnight. Will likely see showers and thunderstorms developing across the central CONUS early in the day Wednesday, and then continuing to move east into the afternoon. Some diminishing may occur early on, but as this precip/forcing encounters increasing instability, will likely see expansion and strengthening of these thunderstorms by mid to late afternoon just west of the CWA. Some slight variability with timing, but most guidance in fair agreement with these thunderstorms not reaching the western CWA (West of I-39) until after 00z. Given the time any storm would reach the CWA, wonder how widespread the severe weather threat will be and also window that this will occur Wednesday evening. At this time, the building instability will support the highest threat of severe weather (damaging wind and hail) across the west and southwest portions of the CWA. This would be along/west of I-39 and south of I-80. However, don`t want to exclude any remaining areas in northeast IL and northwest IN at this time, as there is an isolated severe threat in these locations with possible additional thunderstorm development to the north/northwest along the approaching frontal boundary. Also, strengthening LLJ could offset any diminishing trend with instability and support an additional severe threat into the overnight hours area wide, with thunderstorm coverage also possibly increasing. At this time, it appears setup will favor continued thunderstorm development across much of the CWA into early Thursday morning, though a favored location would be south of I-80 Heavy rainfall and possible flooding are concerns Wednesday evening through early Thursday morning. Still too early to finalize an exact location for highest rainfall amounts and although south of I-80 appears to be favored location, its still possible for higher amounts to the north. With the high PWATS, high rainfall rates are a concern area wide. Have continued to highlight these threats with an ESF, but as was noted with the previous forecast, could see the need for a flash flood watch for a portion of the area Wednesday night. Rodriguez && .AVIATION... For the 18Z TAFs... Primary concern through this afternoon is wind direction at ORD/MDW. Winds at many sites have been southeast to south- southeast. However, over the Chicago metro, particularly at ORD, there has been variability in the southerly component between south-southwest and south-southeast. Have held with a due southerly direction in the ORD TAF to account for this, with a potential to become more prevailing southeast this afternoon even without a lake breeze shift. Regarding the lake breeze, the primary boundary is visible on radar from near Calumet Harbor to just northeast of PWK, and it`s only slowly pushing inland. Thus confidence is low in a prevailing easterly wind shift at ORD/MDW. MDW has had a more pronounced southeast direction thus far but expect variability from south to southeast there as well at least for the next few hours. Otherwise, a slowly decaying cluster of thunderstorms is expected to dissipate, with only air space impacts and high cloud cover overspreading the region, and few-sct CU below. Light south- southeast winds can be expected overnight and south-southwest near 10 kt on Thursday. VFR conditions are likely for the eastern terminals through the afternoon, with some potential for a period of MVFR CIGs at RFD in the morning. SHRA/TSRA potential will arrive after the current TAF cycle. Castro && .MARINE... 311 PM CDT Despite high pressure just to the east of the lake this afternoon, seeing surface gradient and winds beginning to increase across the lake. This is occurring as surface low and trough track through the upper Midwest and southern Canada. Expect 15 to 25 kt over the northern two thirds of the lake this afternoon and evening, with winds continuing to increase over the northern half through the night. As gradient further tightens, winds to 30 kt will are likely over the north half later tonight. These stronger winds will persist Wednesday morning but will see a diminishing trend through midday and the afternoon as the surface trough moves overhead. Expect these lighter winds to persist for a time Wednesday night but as the surface trough/low depart to the southeast Thursday, will see increasing northerly winds across the entire lake. Winds to 30 kt return Thursday afternoon and evening. Northerly winds will then likely continue into the start of the weekend, with speeds periodically bouncing from 15 to 25 kt, to 30 kt during this time. Rodriguez && .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 is the U.S. government's official web portal to all federal, state and local government web resources and services.