


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
50
118 FXUS63 KLOT 090532 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1232 AM CDT Wed Jul 9 2025 .KEY MESSAGES... - Scattered showers and isolated thunderstorms, mainly near and south of I-80, continue to diminish late this evening/overnight. - Dangerous swimming conditions possible at Lake Michigan beaches Wednesday afternoon into early Thursday. - There is a low chance (20 to 30%) for isolated thunderstorms on Wednesday, primarily near the lake breeze. - Shower and thunderstorm chances increase Friday afternoon and overnight. Timing differences remain, and uncertainty regarding the magnitude of any strong-severe threats exist. && .UPDATE... Issued at 859 PM CDT Tue Jul 8 2025 Showers and a few isolated thunderstorms continue at mid- evening, mainly near and south of the I-80 corridor. A remnant MCV, embedded within a weak synoptic scale short wave trough, appears to be the primary forcing, along with a myriad of lingering surface outflow boundaries. Convection intensity has decreased fairly quickly over the last hour or so however, with the loss of diurnal instability with sunset. While light rain and showers will continue to track slowly east across the south/southeast parts of the forecast area with the MCV overnight, thunder and associated gusty wind/flooding potential should continue to quickly diminish. Farther to the northwest, isolated thunderstorms over southwest and central WI are also on a quick diminishing trend. A few showers may reach the Rockford are in the next couple of hours, but again thunder potential appears low. Otherwise, with Light winds, clearing skies and surface dew points holding near 70F, we`ll have to keep an eye on the potential for some patchy fog development prior to sunrise, Areas of northern IL west of the Chicago area appear to be most favored, where decreasing cloud cover is most favored. Ratzer && .DISCUSSION... Issued at 304 PM CDT Tue Jul 8 2025 Through Wednesday Night: The slow moving MCV from our earlier update is located roughly west of Lacon, IL as of this writing. Widely scattered thunderstorms have developed on the eastern flank of the MCV near and south of I-80 in Illinois to near the lake breeze in Indiana. To the west of the MCV, nearly stationary pulse-type convection resides near the MS River. Rain and/or cloud cooled/stabilized air north of I-80 will likely continue to limit the threat for deep convection into the early evening. The only exception to this is well north of the IL/WI state line in the Milwaukee metro (tied to differential heating), and this activity isn`t expected to approach far northern IL. Otherwise, the better overlap of less "mucking up" from existing clouds/light precip, deeper moisture sloshing back north, and the MCV trajectory generally to the south of I-80 to favor higher thunderstorm coverage this afternoon. In the gridded forecast, we`re continuing to highlight this (I-80 and south) area for scattered showers and thunderstorms, with isolated to widely scattered showers and perhaps isolated thunderstorms north of I-80. Despite the moist adiabatic lapse rate and weak deep layer shear environment, up to or more than 1500 J/kg of uncapped MLCAPE, and perhaps slightly enhanced 0-3km shear from the MCV may contribute to isolated strong to marginally severe storms capable of localized downbursts (50-60 mph gusts). Swath of seasonably high 1.5 to 1.7" PWATs also sloshing eastward may support localized corridors where torrential rainfall rates could cause some flooding. We`ve already seen this behavior with some of the more robust cells (non-severe microburst and ~1.25" of rain in ~25 minutes near Braidwood IL during the 2-3pm hour). The primary threat timing window for scattered gusty storms, localized torrential rainfall, and occasional to frequent cloud to ground lightning strikes is roughly through 9pm CDT timeframe, again favoring areas near and south of I-80. To the north, with ongoing weak convection and perhaps widely scattered showers this afternoon (~30% coverage but only isolated/20% chance of storms), the main threat will be occasional localized downpours and isolated sporadic lightning, winding down in the ~6-8pm timeframe. As weak forcing pinwheels overhead through the overnight at a diurnally unfavorable time, can`t rule out spotty showers and maybe an isolated thunderstorm or two, with the best chances of this primarily east of I-57. Forecast low temps are in the mid- upper 60s outside of Chicago and around 70F in the city. If there`s enough clearing well west of Lake Michigan later tonight, can`t rule out some patchy fog development. On Wednesday, in addition to isolated to perhaps widely scattered afternoon showers/storms primarily inland of Lake Michigan, our other forecast concern is at Lake Michigan beaches. A wind shift to lake enhanced northerly winds behind a backdoor front/strong lake breeze should quickly build waves during the afternoon and into the evening. This will be driven by ~1020 mb high pressure building down from the northern Great Lakes with the cooler air mass in northerly fetch also bringing slightly unstable conditions given the now 70+F water temps in the nearshore waters. Regarding the somewhat low end convective threat, residual low- level moisture and the warming boundary layer may allow for an isolated shower or storm to develop by early afternoon, particularly near the lake breeze/backdoor front. However, coverage may be quite limited owing to increasing mid-level subsidence and very dry air as well. Hence capped PoPs in the 20-30% range, with the focus shifting well inland of the lake by the mid to late afternoon. Highs will range from the 83 to 87F inland, but only the mid to upper 70s lakeside. While the cooling will not be nearly as pronounced as with springtime lake enhanced ("pneumonia") fronts, a more noticeable lake cooling footprint (for early July) will spread inland during the afternoon and early evening. Wednesday night will be fairly pleasant with lows in the low-mid 60s outside of Chicago and upper 60s in/near the city. Castro Thursday through Tuesday: The influence of an area of high pressure will begin to wane across the region on Thursday as it starts to shift to the east. That being the case, instability largely looks to remain displaced to the west and south of the region, with some degree of lingering subsidence evident in area forecast soundings. The northern fringes of a returning EML plume will begin to advect into our far south and west locales through the morning and afternoon. Suppose it`s not entirely out of the question that a few elevated showers/storms attempt to fire along this axis, although any coverage at this point looks like it`d stay quite limited. Guidance does also suggest that one or several thunderstorm complexes may develop across central Iowa at the nose of a modest 35-40 kt LLJ as it intersects a returning plume of steeper mid-level lapse rates. Steering flow/upper-level winds should safely guide any complex south and west of the CWA, however. Blended guidance re-introduced some slight chance PoPs south of I- 80, and based on the latest guidance, didn`t see a big reason to remove these, but planning on mentioning a largely dry day. An afternoon lake breeze will hold temperatures in the mid-upper 70s lakeside. A more robust shortwave trough is forecast to shift across parts of the Upper Midwest Friday afternoon and into Saturday. Timing, intensity, and location differences continue across the guidance suite today, however, with the GFS still somewhat of a south and stronger outlier. This has implications on any strong-severe weather threat in the region, along with timing of storm chances. At this point, there`s still too much variability in the guidance suite to to say one way or another if an organized severe weather (and localized flash flood) threat will materialize, mainly across our northwest, late on Friday into the evening, but this something we`ll be keeping a close eye on. Regardless, gradually increasing large scale forcing and returning moisture and instability should result in increasing shower and storm chances, particularly late on Friday afternoon and into the overnight hours, with at least some attendant threat for strong storms and locally torrential rainfall. Flow will turn northwesterly across the area on Saturday, but it doesn`t seem like we`ll totally scour out boundary layer moisture. With a sharp shortwave in the vicinity, additional scattered convection may develop through the late morning and afternoon as things destabilize with heating. By Sunday, current indications point to moisture finally getting shunted south of the region, with things looking largely dry. Semi-active quasi-zonal flow will continue into next week, however, with additional shower/storm chances returning during the Monday-Wednesday timeframe. Carlaw && .AVIATION /06Z TAFS THROUGH 12Z THURSDAY/... Issued at 1232 AM CDT Wed Jul 9 2025 Mvfr cigs, possible ifr cigs around daybreak. Ifr/mvfr vis with fog northwest IL this morning. Wind shift to northeast early this afternoon. Chance of showers this afternoon. An isolated shower will remain possible, but the rest of tonight through late morning appears to be dry. A cold front will move inland from Lake Michigan early this afternoon, switching winds to the northeast and providing the potential for a few showers. An isolated thunderstorm is also possible but current trends would suggest any precipitation later today will be isolated and current prob shower mention seems on track. Light south/southwest winds will turn westerly or northwesterly this morning, then shift northeast with the cold front. There may be a short period where gusts into the 20kt are possible with and behind the front. East/northeast winds will then slowly diminish into this evening. A few locations are reporting mvfr vis with fog/mist currently. Fog is most favored across northwest IL and the Fox Valley. Have included tempo mention at RFD/DPA for now. Guidance is also now showing the potential for ifr cigs across northwest IL and mvfr cigs across much of the rest of the area, around daybreak this morning, which may limit how widespread any fog becomes. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago