


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
451 FXUS63 KLOT 182355 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 655 PM CDT Fri Jul 18 2025 .KEY MESSAGES... - There is a chance of thunderstorms on Saturday, some of which could be strong to severe, though uncertainty is high in the specific details. - Several rounds of thunderstorms may occur early next week, with strong/severe thunderstorms possible along with torrential rain and localized flash flooding, particularly south of I-80. - Dangerous heat and humidity possible next Wednesday and Thursday. && .DISCUSSION... Issued at 305 PM CDT Fri Jul 18 2025 Through Saturday Night: The bottom line up front for Saturday`s convective forecast is there are no significant changes in thinking from our previous forecast package issuance early this morning (Friday). There`s too much uncertainty to offer much in the way of specific details, so we`ll continue to highlight some of the (still wide range) of plausible outcomes. Another suggestion for those assessing the latest high resolution convective allowing model data is to not necessarily put too much stock in any particular solution being offered, as these models are forecasting convection and the subsequent effects of ongoing and prior convection that mostly haven`t materialized yet as of Friday afternoon. Most of this evening and early overnight will be quiet and pleasant with weak surface high pressure over the area, so there are no concerns for outdoor activities. The forecast then becomes quite challenging overnight through Saturday. An embedded wave within overall zonal mid-level flow across the northern Great Plains and Great Lakes is loosely phasing with an impulse originating from the northern periphery of the southwest monsoon today. Effects from this wave and convection over the northern Great Plains tonight will have substantial implications on the potential for severe convection into our area on Saturday. Four general (somewhat more simplified) scenarios still appear possible at this time. 1) Upstream convection that is expected to develop over southern Minnesota this evening tracking southeast with a veering 40-50 knot LLJ into eastern Iowa and southern Wisconsin overnight. Convection could congeal into a loosely organized MCS given modest effective shear, but the thermodynamic environment will become less favorable with eastward extent. This would bring outflow-dominant convection into portions of northwest and north central Illinois toward or after daybreak, with continued weakening over northern Illinois through the late morning and early afternoon (showers and perhaps embedded storms with isolated pop-up convection). Gusty winds would be possible during this time. Then, depending on how far south the outflow tracks, an incoming cold front will interact with a moisture rich airmass and seasonably strong mid-level flow to produce renewed upscale growth of convection over eastern Iowa or far western Illinois during the afternoon. This would then potentially yield at least a broken line of convection or possibly an organized MCS into central Illinois and Indiana and by late afternoon, clipping the southern CWA with a risk for damaging winds. 2) Capping breaks to the southwest of the expected convection over southern Minnesota, allowing a more-organized MCS to develop across western Iowa late this evening. This MCS would then possibly encroach on the northwest half or western half of the forecast area around or shortly after sunrise. Gusty winds, potentially damaging, would be possible across at least the western half of the CWA through mid-morning before renewed strengthening early afternoon past the diurnal convective minimum. In this case, the overall potential for severe convection may end up being highest toward northwest Indiana. The effective outflow would then limit or prevent additional convection before the cold front reaches the area Saturday evening. 3) Upstream convection tonight does materialize and become organized into an MCS, but dives farther southwest (into west and southwest CWA and points west-southwest) and weakens quickly during the morning. This would maintain a play for robust convection/potentially severe, to develop ahead of the cold front mid-late Saturday afternoon. 4) The convection well to the northwest tonight fails to organize, with little impact on the forecast area Saturday morning. The cold front would become the main driver for convection over the area mid-afternoon and early evening, with some surface-based capping remaining. The modest shear noted above would support organized convection, with initial storm motion favoring clusters or semi-discrete supercells with damaging winds and hail quickly transitioning into multicell segments with damaging winds. The first scenario still appears more likely based on the overall pattern and more-favorable thermodynamic environment remaining south and west of the forecast area. This would mean that much of the CWA could be split by the convective severe threat on Saturday. With that said, thorough mesoscale assessment will be needed through tonight to ascertain whether one of the other scenarios becomes more favorable. How the rest of Saturday evening and night plays out will be substantially modulated by the effects of prior convection with respect to outflow/effective front position. Some CAMs indicate training convection firing just north of the effective frontal boundary across central IL and central IN. This particular scenario would entail a potential flash flood threat within the CWA, mainly near and south of the Illinois and Kankakee River Valleys (where our highest PoPs/50-60% are depicted) Saturday night. There`s an alternative scenario where the effective front gets shunted far enough south to force showers and storms safely south of the CWA with time. Kluber/Castro Sunday through Friday: Primary forecast concerns are thunderstorm chances/timing and building heat/humidity midweek. A frontal boundary is expected to be across the southern cwa or central IL Sunday morning and its possible thunderstorms will be ongoing Sunday morning across these areas. The main threat during this time would be torrential rain and possible localized flooding, as alluded to above. This activity would then shift southeast and dissipate into the mid/late morning. With the boundary nearby, there would remain a chance of showers and a few thunderstorms Sunday afternoon into Sunday evening, but much of northern IL may remain dry on Sunday and have trimmed pops across the northern cwa. Northeasterly winds will bring in an other period of cooler temps, perhaps only reaching the upper 70s for northern areas and near Lake Michigan, though dewpoints may still be in the 60s for much of this time period. The northeasterly winds may be strong enough to build waves into the 2-4 foot range at Lake Michigan beaches, resulting in a moderate swim risk but still some uncertainty for how strong the winds will become. The upper ridge is still expected to build into the region Monday night into Tuesday. This time period was looking to be a concern for possible thunderstorms, along with heavy rain, but much of the models and ensembles are now rather dry during this transition with really only the NAM showing any convection. Blended pops have followed these trends and are now either dry or just low slight chance pops. Made no changes, but this time period will need to be monitored. The lack of precipitation may allow highs on Tuesday to reach 90 for most areas. No real changes for midweek, Wednesday is still looking like the hottest day with highs in the mid 90s and dewpoints in the upper 70s/near 80. Mature crops will likely push some dewpoints into the lower 80s. Peak heat indices on Wednesday could be in the 110-115 range. And with these conditions, the atmosphere should be sufficiently capped for dry conditions. Thursday will also be hot and humid with highs at least in the lower 90s and dewpoints still in the upper 70s/near 80. A cold front will slowly be moving south across the western Great Lakes Thursday into Thursday night. Still quite a bit of uncertainty regarding when this front moves into the local area and how long it persists, as it may slow/stall. The net effect is that the best precip chances next week may end up being at the end of the week. cms && .AVIATION /00Z TAFS THROUGH 06Z SUNDAY/... Issued at 655 PM CDT Fri Jul 18 2025 Aviation forecast concerns for the 00Z TAFs: - TSRA potential Saturday. Timing and coverage continues to be of low confidence. Early evening surface analysis shows an area of weak low pressure over eastern South Dakota, with a warm front extending southeast through western IA into MO and far southern IL. To the east, weak high pressure was centered over the eastern Great Lakes, drifting slowly to the east. In the near term, light east lake breeze winds will linger across ORD/MDW/GYY early this evening, before eventually becoming light south by predawn Saturday morning as the surface high continues to move off to the east and the warm front to our south slowly evolves northward. The surface pressure gradient is weak enough that winds may just go light and variable for a time overnight. Scattered TSRA were developing along the SD/MN border early this evening in the vicinity of the weak surface low/warm front. While CAM guidance continues to show considerable spread in the details, the general expectation is that these storms will continue to develop into southern MN/IA and western WI overnight, which would then move east-southeast into northern IL after sunrise Saturday morning. This would likely be during a period of diurnal convective weakening, especially if a consolidated cold pool/outflow boundary does not develop or is weak. The best timing for these storms to impact our terminals looks to be roughly 11-17Z at RFD, and in the 14-19Z time frame for the Chicago sites, though again confidence remains low with coverage and exact timing. Most of the CAM guidance suggests relatively little/no significant coverage later into the afternoon, thus have maintained dry TAFs beyond prob30s for the above windows for now. If morning activity is of lower coverage, and there is no significant outflow however, the chance of TS later into the afternoon can`t be ruled out. Most guidance is in agreement that winds will turn west- southwest and may be slightly breezy during the afternoon, eventually turning more northwest Saturday night ahead of an approaching cold front. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago