FXUS63 KLOT 021753 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1253 PM CDT Fri May 2 2025 .KEY MESSAGES... - Additional rounds of showers are possible at times later today through early next week, with the highest forecast confidence on Sunday. - Expect cooler-than-normal conditions this weekend followed by a mild stretch during the middle of next week. Cooler conditions will prevail along the lakeshore next week with persistent onshore flow. && .DISCUSSION... Issued at 324 AM CDT Fri May 2 2025 Through Saturday: Early morning surface analysis depicts a weak low pressure wave roughly between KMLI and KSQI in northwest IL, with a west-east nearly stationary front extending eastward along the I-80 corridor into northern IN. This wave appears to be a reflection of a mid- level short wave trough axis evident near the Mississippi River in GOES vapor imagery. Relatively weak forcing associated with this wave has generated isolated to widely- scattered showers across eastern IA and parts of northern IL since last evening, as well as a few isolated thunderstorms within a region of weak elevated MUCAPE mainly south of the cwa. Overall, coverage of showers has been significantly lower than much of the guidance has indicated, and have maintained only slight chance (~20%) eastward-moving pops across the area through about 14Z/9 AM CDT until the mid-level short wave axis moves east of the area. Weak subsidence develops in the wake of this disturbance from mid-morning through mid-afternoon, which should allow for dry conditions for most of the day. Farther to our west, another mid-level short wave was noted digging south-southeast across the Plains. This wave is progged to help deepen and amplify a long-wave upper trough from the northern Lakes to the lower Missouri Valley region by this evening, which will again result in height falls developing across the forecast area later this afternoon and tonight, and a return of low (20-30%) chance pops especially across our western cwa by late this afternoon. Scattered showers will likely persist across a good portion of the area tonight into Saturday as the upper trough continues slowly moving across the area and deepen, eventually forming a closed low over downstate IL by Saturday afternoon. Greatest shower coverage (40-50%) is expected to be across western and southern parts of the forecast area, in the vicinity of the trough axis and developing mid- level low. Drier northeast low level flow looks to result in decreasing shower coverage north by Saturday afternoon. Northeast winds and onshore flow was already developing north of the aforementioned stationary front, and will increase across the area as the surface wave moves off to the east this morning. Combined with mostly cloudy conditions especially across the northern cwa, this will result in cooler temperatures today ranging from the 50s near the lake to the low-mid 60s farther inland. South of I-80, temps may approach 70 degrees. Most areas are expected to only be in the 50s to low 60s Saturday, with breezy northeast winds, cloud cover and scattered showers. Ratzer Saturday Night through Thursday: This weekend, the base of an upper trough will swing into the Tennessee Valley before breaking off from the Polar Jet late Saturday and becoming a cutoff upper low with core focused directly over the Midwest. The system will meander over the region during the day on Sunday before getting shunted northward into the Great Lakes Monday into Tuesday by a deep, encroaching upper low swinging into the Desert Southwest. This system is expected to bring us intermittent bouts of rain during the early part of next week. The forecast in the long term picks up Saturday evening when we could maybe see a few showers in our south, although better chances will be well south of the CWA. But, as moisture and forcing gets wrapped around to the northern flank of the upper low, scattered showers are anticipated to fall on our area Saturday night and Sunday. Additional periods of showers are likely Sunday night and Monday as the low begins to work northward across the region. Despite an unimpressive kinematic field around the low, guidance is resolving decent lift on the northern periphery of the low via DPVA. However, profiles are expected to dry out from the top-down as the center of the low nears overhead, which will likely keep coverage rather scattered and intensity largely on the lighter side, especially Sunday night and Monday, although some pockets of more moderate to heavy rain will be possible amid the deep forcing. No thunderstorms are expected with this low as all of the instability looks to remain east of the system out into the Ohio Valley. The generally better rain chances and higher coverage is anticipated across the eastern half of our CWA nearer to the track of the low level circulation. By the time the core of the low is positioned over the CWA Monday evening, precip chances should be off to our northeast and Tuesday looks to be rain- free. Temperatures on Sunday will be similar to Saturday's, in the 50s and lower 60s. Monday looks to be a few degrees warmer followed by a pleasantly mild middle of the week. Highs Tuesday through Friday are forecast in the upper 60s and lower 70s, likely stuck in the 50s along the lake though with persistent onshore flow through the week. That other aforementioned upper low moving into the Desert Southwest this weekend will get sheared off across the central CONUS next week. Its remnants may push a disorganized system of scattered showers across the region during the day on Wednesday. A second wave of showers also associated with this weakening upper low may impact the area late in the week. Thunder chances next week are generally low for now as models have just about all instability staying away from the area. Doom && .AVIATION /18Z TAFS THROUGH 00Z SUNDAY/... Issued at 1253 PM CDT Fri May 2 2025 Main Concerns: - A few opportunities for -RA/-SHRA and the timing of these. - Lower CIG trends through the period. The terminals are north of a frontal boundary that sagged south earlier today and as such remain blanketed with lower CIGs (currently lower MVFR). Expectation is for cloud bases to lift and/or scattering to lower VFR by the mid to late afternoon. Lower MVFR CIGs with some potential for IFR, especially in RFD area, will return late this evening and overnight. A similar improvement to lower VFR should occur by mid Saturday afternoon. Regarding rain chances, near term guidance has been hinting at SHRA development INVOF RFD in the mid afternoon, with this activity then drifting east, likely in a diminishing trend. Maintained VCSH mention at RFD, but will monitor radar and satellite for renewed mention of precip farther east if needed. A better chance for rain, but still lower confidence at the terminals, is associated with the SHRA over southeast IA early this afternoon gradually pinwheeling eastward overnight into early Saturday. With a chance this activity stays south of the terminals, maintained PROB30 mention for -RA and MVFR VSBY in BR. However this plays out, expecting dry conditions the rest of Saturday beyond the mid AM. East-northeast winds will strengthen some near the lake this afternoon and then the wind direction will trend to north- northeast overnight with speeds in the 10-15 kt range. Gusts to around 20 kt appear likely Saturday afternoon from an 020-040 direction. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago