FXUS63 KLOT 190845 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 345 AM CDT Tue May 19 2026 .KEY MESSAGES... - Scattered thunderstorms diminish early this morning across northern IL. Thunderstorm chances then increase again this afternoon, mainly southeast of a Peru IL to Valparaiso IN line. A few strong storms are possible. - Summer-like warmth and breezy winds today will give way to cooler and dry conditions for the middle of the week. - Intermittent shower/thunderstorm chances return late in the week and into the weekend, though with a return of warmer weather by Memorial Day. && .DISCUSSION... Issued at 345 AM CDT Tue May 19 2026 Early morning surface analysis places 1005 mb low over northwest Wisconsin, with a cold front trailing through central Iowa into southeast Kansas. Strong/severe thunderstorms that had developed Monday afternoon ahead of the front across the mid- Missouri Valley and elsewhere ahead of the boundary, have since evolved into another linear MCS, which currently stretches all the way from the central upper peninsula of Michigan to northwest/western Illinois, then southwest to Oklahoma. Aloft, GOES vapor imagery indicates a deepening mid-level short wave trough propagating northeastward from the northern/central Plains toward the northern Great Lakes. Strong large-scale ascent, depicted by mid-level height falls of 60-80 meters across the upper Mississippi Valley, is aiding in maintaining a strong southwesterly low-level jet across our region, which latest RAP soundings/mesoanalysis depict at 60 kts across western IL. While low-levels have diurnally stabilized, the strong low level jet and associated warm/moist advection has maintained an elevated MUCAPE axis of 1500-2000 J/kg per RAP soundings. This, combined with low-level forcing provided by the eastward- propagating outflow boundary along the leading edge of the aforementioned MCS has allowed scattered convective cells to continue to initiate atop the outflow boundary across western and northwest Illinois. Forecast soundings do indicate a weakening and veering of the low-level jet over the next few hours however, along with a decrease in elevated instability. Combined with the stable boundary layer conditions, this should continue to result in weakening trend to the convection as the outflow continues to shift east across northern IL this morning, which is already evident in recent radar and lightning data. Stable outflow and lingering scattered morning showers, combined with a stream of high-level cloud cover off of stronger convection well to our southwest should limit local convective redevelopment for the balance of the morning. The surface cold front is progged move into the forecast area early this afternoon, pushing southeast across the area into this evening. Forecast soundings are generally capped across the area initially, though become conditionally unstable with diurnal warming later this afternoon ahead of the front mainly across the southeast half of the forecast area (and especially southeast of a Pontiac-Valparaiso line). Guidance has generally backed-off with coverage of afternoon convection across our area, though frontal forcing and favorable diurnal timing especially in our far southeast should support an increasing thunderstorm threat mid-late afternoon. Forecast 1000-1500 J/kg MLCAPEs and up to 35 kts of deep-layer shear (generally parallel to the frontal zone) would support some storm organization with some wind/hail potential. Note SPC has moved earlier slight risk out of the WFO Chicago area with the new Day 1 outlook, leaving a marginal (level 1 of 5) risk in our far southeast. A few showers may linger across our southeastern tier of counties this evening until the front exits the region, with a period of dry and cooler weather then expected Wednesday through Thursday. After several recent days of highs in the 70s and 80s, daytime temperatures in the 60s are expected mid-late week with 50s closer to Lake Michigan where breezy northeasterly winds will persist around an area of high pressure building across the Great Lakes region. Nighttime temps both Wednesday and Thursday night will dip into the 40s in most areas. Rain chances then return Thursday night into Friday, as guidance depicts an area of low pressure tracking from Missouri into Illinois and Indiana in association with an upper level trough transiting the region into the weekend. Some detail and timing differences remain within the global guidance suite, though rain chances appear to be highest on Friday, then more spotty from Saturday into Monday. It does appear that warmer temperatures will return later in the weekend, but likely with cooler conditions near Lake Michigan. Ratzer && .AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 103 AM CDT Tue May 19 2026 Forecast concerns for the terminals... - A line of showers and embedded thunderstorms will move across the terminals late tonight into early Tuesday morning. - Breezy southwest winds to develop Tuesday morning behind the rain and persist through the afternoon. - Period of MVFR ceilings is expected Tuesday afternoon behind a cold front, but skies should scatter back to VFR around sunset. A line of showers and embedded thunderstorms is progressing eastward across the Mississippi River and will be encroaching upon the terminals over the next 1-3 hours (earliest at RFD, latest Chicago area sites). While the line is expected to be gradually weakening as it moves through, recent AMDAR soundings out of MDW do show some developing mid-level instability which may allow for a couple of lightning strikes to occur. Despite confidence in thunder actually reaching the Chicago terminals being somewhat low (~20% chance) have opted to introduce a PROB30 for TSRA at DPA, ORD, and MDW as a precaution give the aforementioned instability and lack of broad weakening in the line so far. That said, still think that the most likely scenario at the Chicago sites (especially GYY) is for some isolated to widely scattered showers to occur. The line of showers/storms should diminish by daybreak and result in a mostly dry morning. Winds will be increasing with southwesterly gusts peaking around 25-30 kts through early Tuesday afternoon, but locally higher gusts could materialize if deeper mixing is achieved. As the main cold front approaches Tuesday afternoon it will bring with it a period of MVFR ceilings and perhaps an isolated shower or two though the greatest shower/storm coverage should establish more into northern and central IN. With the increasing cloud cover, gusts should ease a bit into the lower 20 kt range through the rest of the afternoon. Directions during this time will also transition from southwest to west-northwest before turning northwesterly behind the cold front early Tuesday evening. While a few upper teen to lower 20 kt gusts cannot be ruled out Tuesday night, it seems they frequency of gusts should be more sporadic so have opted to remove gusts from the TAFs from this point onward. In the wake of the cold front, ceilings should rise back to VFR Tuesday evening and remain as such for the rest of the forecast period. Lastly winds will finally settle into a northeast direction early Wednesday morning. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 4 PM CDT this afternoon for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago