


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
601 FXUS63 KLOT 181937 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 237 PM CDT Sat Oct 18 2025 .KEY MESSAGES... - Periods of showers and storms are expected this afternoon through Sunday morning as a fall storm system moves through the area. - Much of the area will pick up much-needed soaking rainfall tonight into Sunday. The highest rainfall totals (1-2+ inches) are expected to occur near and east of Interstate 55. - A period of gusty northwest winds (gusts in excess of 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 237 PM CDT Sat Oct 18 2025 Through Sunday: Showers and thunderstorms are starting to redevelop along a pseudo dryline that is currently oriented along a Mendota to Chicago line. Given this orientation and the continued southeast movement of the boundary, still expect the greatest coverage of showers and thunderstorms this afternoon to be along and southeast of I-55 where a plume of better instability resides. Recent aircraft soundings of of MDW do show cape values around 900-1000 J/kg which in combination with the 40-50 kts of shear overhead may support some strong to near severe storms this afternoon and evening. If any strong to severe storms materialize the main threats will be gusty winds in excess of 45 mph and hail up to quarter size. However, there is a non-zero chance that a few of these storms try to rotate especially if they interact with some surface vorticity being noted along the dryline. That said, the environment today is not really favorable for tornadoes due to the somewhat limited instability and weak low-level shear so if rotation is observed in storms suspect it would mainly be a funnel cloud or landspout threat. Heading into tonight, the cluster of showers and thunderstorms ongoing across south-central MO will begin to pivot into north- central IL and northwest IN as a surface low develops. Additionally, the cold front in east-central IA will also be moving through the area tonight. The interaction of these features will allow showers to expand in coverage and fill in across northern IL and northwest IN. Once showers fill in they are expected to persist into Sunday morning as the front and surface low move through the region. From a thunder perspective, the instability should get pushed south of our area this evening and thus limit the thunderstorm coverage but an isolated rumble or two of thunder cannot be ruled out through at least midnight. The showers tonight will bring a nice soaking rainfall to the area with most expected to pick up anywhere from 0.5 to 1.0 inches of rain. Though, a corridor of high rain amounts is expected to develop along the low`s deformation axis which looks to reside across far eastern IL to northwest IN and into Lower MI. Totals in this area could peak in the 1 to 2 inch range with locally higher amounts (possibly near 3.0 inches) possible. Normally rain amounts of this magnitude would be a concern for flooding but with the long duration (12+ hours) and the antecedent dry conditions the threat for flooding continues to look low. Showers will gradually exit the area from west to east Sunday morning leaving us with a dry conclusion to the weekend. However, the strong pressure gradient and deep mixing associated with the cold advection will generate some gusty winds through Sunday afternoon where gusts are expected to peak around 30-40 mph. Aside from the gusty winds, temperatures on Sunday will also be notably cooler with highs only reaching into the upper 50s to around 60. Yack Sunday Night through Saturday: The lingering tight surface pressure gradient will gradually relax Sunday night into early Monday morning. With the upper- level cyclonic shear axis pushing quickly east of the region and lower near-surface dewpoints sloshing in from the west, knocked a few degrees off the low temperatures early Monday morning. With the slackening flow, can`t rule out some patchy frost development west of the Fox Valley in/near sheltered/typical cool spots. The upper-level pattern will remain energetic in the wake of the weekend storm system with a secondary upper-level trough and associated surface low poised to dive southeastward into the Great Lakes region in the Monday to Tuesday timeframe. Even with the usual variance in the evolution of the low depicted among guidance, a signal remains for a period of breezy southwest winds on Monday (WAA ahead of the system) transitioning to breezy west to northwest winds on Tuesday (CAA behind the system). In scenarios where the surface low is strong (sub 1000mb), it is easy to envision peak gusts in either regime tagging 40 mph, as hinted by 40kt of flow at the top of the mixed layer in deterministic GFS and ECMWF forecast soundings. This`ll be something to keep an eye on as we get closer. Unlike the storm system coming through this weekend, chances for meaningful rainfall with the secondary system will be limited by meager moisture return and relegated to a quickily-moving band of showers along the southeastward-surging cold front Monday evening and perhaps a few showers lingering behind as the mid- level cold pool associated with the trough slide overhead. After a relatively warm Monday with highs in the mid 60s, temperatures behind the front on Tuesday will be noticeably cooler and in the 50s. We`ll have to watch for any embedded shortwaves to move over the area along the backside of the upper-level trough to support a diurnal flare in scattered showers, as can happen this time of year. As the system pulls away Tuesday night, temperatures will be poised to fall into the upper 30s to lower 40s away from the lake. At this point, suspect there will be enough cloud cover or lingering winds to limit temperatures from really tanking out, and indeed latest guidance has trended a bit milder with overnight temperatures into Wednesday morning. The colder night could end up being Wednesday night/Thursday morning. Still have some questions related to how quickly the surface flow will diminish overnight, but will need to keep an eye on this period for and increased frost/localized freeze potential. Forecast confidence lowers from the middle of the week owing to spread within ensemble guidance in the treatment of an eastward- moving upper-level low emanating from California. Carlaw/Borchardt && .AVIATION /18Z TAFS THROUGH 00Z MONDAY/... Issued at 1239 PM CDT Sat Oct 18 2025 Key Aviation Messages for the current TAF period: - Showers and storms possible this afternoon, mainly along and south of a KVYS to KGYY line - Showers and storms expand northward this evening, transitioning to just rain after midnight - Gusty northwest winds in excess of 30 knots tomorrow morning MVFR cigs are being observed across much of northern Illinois, though VFR clearing has finally arrived at KRFD. Cigs are expected to gradually break up and lift through the afternoon. Southwest winds around 10 knots are expected through the afternoon. An occasional gust to 20 knots is possible, but it should be infrequent enough to keep it out of the TAF. Winds will slowly turn to the south (potentially a short window to the southeast) late this afternoon as a surface low approaches. While there is a break in the rain at the time of this discussion, attention is turning toward central Illinois where new thunderstorms are forming and moving east-northeast. The threat for showers and storms this afternoon should mainly be along and south of a KVYS to KGYY line. A stray shower north of there is possible which allows a PROB30 to be maintained at KMDW, but the higher confidence is to the south which is why the KGYY TAF now has a TEMPO for -TSRA this afternoon. As a surface low deepens over Illinois this evening, widespread expansion of rain and MVFR conditions across the area is expected. With instability still present, VCTS was kept in the TAF, though chances diminish toward and after midnight. There is a chance for IFR vis during the heaviest rain, but confidence was too low to add it to the TAF currently. Rain will slowly dissipate tomorrow morning from west to east, with dry VFR conditions expected in the afternoon. Winds will turn to the northwest after midnight as the surface low starts to move east. But as the pressure gradient tightens, wind gusts will increase. Wind gusts in excess of 30 knots can be expected toward daybreak, but there is a chance for gusts as high as 40 knots for brief periods of times. Gusty conditions will continue through much of the day tomorrow, though the magnitude will gradually diminish through the afternoon. DK && .MARINE... Issued at 237 PM CDT Sat Oct 18 2025 - A Gale Warning remains in effect from Sunday morning through Sunday evening. A fall storm system will move through the Great Lakes tonight into Sunday, bringing with it periods of showers and thunderstorms and strong winds. The strongest winds are expected to develop overnight tonight into Sunday morning and persist through Sunday evening. Confidence remains high in 35 kt gales being observed across all of southern Lake Michigan, and so the Gale Warning is still in effect. Gales as high as about 40 kts could be observed over Indiana nearshore waters and points farther to the north and northeast. These winds will also lead to very large waves, which will also be highest along the northwest Indiana shore and across southeastern portions of Lake Michigan. Yack/Ogorek && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Gale Warning from 7 AM to 7 PM CDT Sunday for Winthrop Harbor IL to Gary IN. Gale Warning from 7 AM to 10 PM CDT Sunday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN. && $$ Visit us at weather.gov/chicago