


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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998 FXUS63 KLOT 180546 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1246 AM CDT Sat Oct 18 2025 .KEY MESSAGES... - Periods of showers and storms are expected late tonight through Sunday morning as a fall storm system moves through the area. - Parts of the area may pick up much-needed soaking rainfall over the weekend. The highest rainfall totals (1-2+ inches) are currently favored to occur near and east of Interstate 55. - A period of gusty northwest winds (gusts of 30 to 35 mph) are expected on Sunday. Additional periods of breezy winds are possible on Monday and Tuesday as another fall storm system moves through the region. - Temperatures will be on a see-saw through the middle of next week. && .DISCUSSION... Issued at 314 PM CDT Fri Oct 17 2025 Through Sunday: The mid-level ridge that has brought us the warm and dry conditions the past few days continues to move eastward as deepening trough moves into the Upper Midwest and Plains. That said, expect the rest of our Friday afternoon and evening to see increasing cirrus clouds with temperatures generally holding steady in the mid to upper 70s before cooling into the low to mid-60s overnight. The ongoing breezy south-southwest winds will also persist through sunset before subsiding to around 10 mph tonight. Though, some occasional 20 mph gusts may be seen tonight as a low-level jet develops overhead especially in urban areas where the atmosphere may remain more mixed. Our attention turns towards the aforementioned trough entering the Plains which is part of a developing storm system that will be in control of our weather this weekend. The trough will continue to advance towards the Midwest tonight and Saturday and then begin to phase with the shortwave diving into the Mountain West as it pivots into the Great Lakes late Saturday into Sunday morning. At the same time, the cold front stretching from eastern MN to OK will begin to move towards northern IL. A line of showers and embedded thunderstorms have already developed along this front in IA this afternoon and those showers are expected to move into northwest IL towards midnight and then swing through the rest of the area Saturday morning. Given that these showers and storms will be outrunning the cold front and entering an area of more limited instability, suspect that their coverage may be somewhat scattered in nature as they move across northern IL and northwest IN. While the initial wave of showers/storms may be more hit and miss, a second round of showers and thunderstorms is expected to develop Saturday afternoon as the phasing trough pivots overhead and a surface low begins to develop. Guidance continues to vary on where exactly the low will take shape but the consensus appears to be converging towards it forming over IL and IN and then lifting into eastern Lower MI Saturday night. Nevertheless, the strong forcing will generate a broad area of showers across a good portion of the area Saturday afternoon through Saturday night. Forecast soundings continue to show the instability Saturday afternoon to be rather limited (around 200-400 J/kg) which should keep the severe threat low, but the 35-45 kts of shear and strong synoptic ascent may still allow a isolated stronger storm to develop. If a stronger storm does materialize the main hazard with them will be locally gusty winds but a brief spin up cannot be completely ruled out particularly south and east of I-57 closer to the better instability. In terms of rainfall, a swath of 1-2 inch rainfall amounts continues to look likely along the northwest periphery of the surface low. With the current forecast consensus it appears the axis of highest rain amounts may stay more into northern IN and Lower MI, but rainfall totals generally in the 0.5 to 1 inch range are expected area wide. That said, the ongoing drought conditions and long duration of the rain (36-48 hours) suspect the threat of flooding is still very low. However, if some of the stronger storms train over the same spots localized ponding cannot be ruled out. The surface low and the associated trough will begin to shift east allowing drier air to filter back into the area. Therefore, rain should taper from west to east Sunday morning leaving us with a dry afternoon. Though, temperatures will be notably cooler behind the system with highs forecast to be in the mid to upper 50s Sunday afternoon. Winds will also be quite gusty on Sunday as the lingering pressure gradient and cold advection allow gusts to peak in the 30- 40 mph range Sunday afternoon. Yack Sunday Night through Friday: An active upper-level pattern will continue next week with several disturbances slated to quickly shift across the northern CONUS. The next such wave is forecast to dive southeastward across the Upper Midwest Monday through Tuesday. The ongoing trend with this system has been towards both a deeper parent upper-level wave and associated surface low developing somewhere across northern Great Lakes region Monday night into Tuesday morning. Despite this deeper trend, moisture return remains a wildcard, as this system will be following quickly on the heels of this weekend`s disturbance, and it remains a bit unclear how much boundary layer moisture/dewpoints will manage to recover prior to the arrival of the trailing cold front. The GFS, which may be over mixing the BL a bit, shows dewpoints falling into the upper 30s/near 40 degrees ahead of the front, while most other guidance suggests surface Tds remaining quite a bit higher. This will have a considerable impact on the degree of precipitation coverage with the front. At this point, suspect the GFS remains a bit too dry, with the NBM- delivered chance PoPs looking appropriate at this range. The main item of note with this system looks to be a potential for strong wind gusts, both in the warm sector and subsequent cold advection regime. Warm sector southerly wind gusts could push 40+ mph Monday afternoon, but will be dependent on mixing depths which remain uncertain at this time. Then late Monday night into Tuesday, as low-level lapse rates steepen behind FROPA, an additional surge of west to northwesterly gusts will also be possible. Cold advection will send temperatures falling into the 30s and 40s Tuesday night/Wednesday morning. At this time, a widespread freeze appears unlikely, but lows could push into the low/mid 30s with slackening winds late Tuesday night. Forecast confidence lowers from the middle of the week onward. With that said, ensemble model guidance is starting to exhibit a signal for yet another system approaching the general area toward the end of the workweek. Carlaw && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1246 AM CDT Sat Oct 18 2025 The primary aviation concerns are as follows: - Threat for thunderstorms this morning, with highest coverage favored in the 10-14Z window across the C90 airspace. - Development of SCT to BKN MVFR cigs this morning and lasting through the remainder of the TAF period. - Threat for isolated thunderstorms between 19-22Z across the C-90 airspace. - Expectation for rapid development of stratiform rain with embedded thunderstorms this evening, with relative highest threat for thunder near RFD between 03-06Z. - Backing winds toward south to southeast this evening, followed by a northwesterly wind shift overnight. Northwesterly wind gusts may exceed 30kt by daybreak Sunday. Discussion: Recent radar and satellite imagery depicts an axis of scattered showers and thunderstorms extending from MSN to TOP. The showers and storms are tied to isentropic ascent being forced by a 35-40kt LLJ ahead of a mid-level shear axis slicing through the Upper Midwest. While the axis will shift eastward through the night, current thinking is that coverage at the terminals will be highest at around daybreak as subtle wave currently passing over MCI swings over the central ZAU airspace. Will lean on the HRRR to refine the timing windows of the inherited TEMPO groups to favor 10-14Z at any given terminal given it appears to be resolving the subtle wave well. The main threat with thunderstorms this morning will be lightning and brief drops in visibility to 2-3SM. Showers and storms should taper by mid-morning as the low-level jet decays. Winds should remain southwesterly through the morning, though developing stratus may tend to limit mixing heights and thus the potential for frequent gusts. For this reason, will cautiously remove gusts through the daylight hours in the outgoing TAF package. Attention then turns toward an upper-level shortwave currently lifting from New Mexico into Texas and due to interact with a broader upper-level trough digging southeastward toward the Midwest. As the shortwave approaches, additional showers and storms should develop mainly across the southern ZAU airspace. However, with forecast soundings depicting minimal capping by early afternoon, do have some concern there could be at least isolated thunderstorm development as far north as DPA/ORD/MDW/GYY. So, will maintain the inherited PROB30 groups for thunder in the 19-22Z timeframe. As the southern-stream upper-level shortwave and northern-stream trough merge directly overhead this evening, rapid cyclogenesis is expected to take place over the central or eastern ZAU airspace. As a result of area-wide pressure falls, suspect winds will back in direction toward the south or even southeasterly for a time this evening. As this occurs, broad upper- level diffluence forced by couplet jet streaks will support the rapid development and expansion of stratiform rain and MVFR cigs this evening atop the terminals. In fact, would not be surprised to see a few embedded thunderstorms in the 03-06Z timeframe given lingering instability as well as the threat for mesoscale gravity waves (very unbalanced upper-level flow). At this point, the signal for yet another period of thunder appears relatively highest near RFD near the crashing cold front and where the remnant instability should be maximized, so will go ahead and introduce a TEMPO group for thunder the tailing hours of the TAF. Finally, a northwesterly wind shift is expected toward the end of the 24-30 hour TAF window as the cold front sharpens and moves through the area. While wind speeds should be somewhat muted (10-13kt) initially behind the front, sustained and gust magnitudes will increase markedly by daybreak Sunday with frequent quests of 30-35kt, if not even higher. Borchardt && .MARINE... Issued at 314 PM CDT Fri Oct 17 2025 - Gale Watch is in effect Sunday morning through the evening. A fall storm system will move through the Great Lakes this weekend bringing with it periods of showers and thunderstorms and gusty winds. The strongest winds are expected to develop late Saturday night and persist through Sunday evening with northwest gales to 35 kts expected. These winds will also lead to large waves especially in northwest IN and the eastern portions of Lake Michigan. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Gale Watch from Sunday morning through Sunday evening for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago