


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
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --770 FXUS63 KLOT 151925 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 225 PM CDT Tue Jul 15 2025 .KEY MESSAGES...-- Changed Discussion --- Isolated thunderstorms remain possible south of I-80 through this evening. - Strong/severe thunderstorms are expected Wednesday afternoon into Wednesday evening with damaging winds the main threat. Torrential rain and localized flash flooding are also possible. - Hot and humid conditions are expected Wednesday with peak heat indices around 100 degrees. - High waves and dangerous swimming conditions are possible at Lake Michigan beaches Thursday through Friday. - After a brief break Friday, high humidity levels and occasional bouts of storms are expected to return over the weekend.-- End Changed Discussion --&& .DISCUSSION...-- Changed Discussion --Issued at 225 PM CDT Tue Jul 15 2025 Through Wednesday Night: The primary forecast concern remains severe thunderstorm potential Wednesday afternoon/evening. Isolated showers and an occasional isolated thunderstorm will remain possible through this evening, mainly south of I-80 as a weak wave slowly lifts northeast. Lack of organization, lack of shear and warm temps aloft suggest this activity is going to remain isolated but will likely slowly drift north through the late afternoon. And it may continue into tonight for the Chicago Metro area, which has been a somewhat consistent trend from guidance today. Have included slight chance pops for just showers into the early overnight hours, though most areas will remain dry. There may also be some lingering showers Wednesday morning, across northwest IN. There could be some light/patchy fog again late tonight into early Wednesday morning. Set up is a little different than this morning. Its possible there may be a bit more cloud cover and low level winds may be a bit stronger, helping to keep the lower levels better mixed. Low temps might be a few degrees higher, though so will the dewpoints. Opted to leave out fog mention from the forecast, with the best chance outside of the Chicago metro area, if it were to happen. Attention then turns to convection expected to develop over NE this evening which will weaken overnight, sending an MCV east toward the local area Wednesday afternoon, combined with a deepening surface low expected to be moving across WI Wednesday afternoon into Wednesday evening. While storm coverage remains uncertain, with perhaps the best thunderstorm coverage just north of the local area, into southern WI, trends support at least scattered thunderstorms developing across far eastern IA/ northwest IL early Wednesday afternoon and then spreading across northern IL through late afternoon/early evening. Best instability is maximized across northwest IL in the early/mid afternoon, where storms are expected to develop. Soundings show winds aloft steadily increasing through the afternoon, perhaps peaking in the 45kt range in the late afternoon or early evening, just a bit after the best instability timing. However, given the set up, whatever thunderstorms do form Wednesday afternoon are likely to become severe and perhaps rather quickly after forming with the severe threat extending to the lake through early evening. While the main severe weather threat will be damaging winds, the initial storm mode could favor a few supercells which may lead to large hail and an isolated tornado, with the most favored area across northwest IL. Storm coverage south of of I-80 is even more uncertain, but damaging winds would remain possible. While the severe threat looks to end by early/mid evening, there will be a chance for showers and a few thunderstorms Wednesday night into early Thursday morning, especially with the cold front slowly moving south across the local area. With dewpoints in the lower/mid 70s and precipitable water values again into the 2 inch range Wednesday afternoon, torrential rain can be expected with any thunderstorms that form. However, these storms may be fairly progressive which would likely limit the overall flash flooding threat. Though rainfall amounts of 1-2 inches in a short time in urban areas can lead to some flooding issues. cms Thursday through Tuesday: Thursday morning may start on the wet side as upper-level ripples propagate atop a southward-drifting frontal zone. Forecast soundings show appreciable depth to low-level moisture through mid-morning, suggesting that heavy clouds and even pockets of drizzle may prevail. During the afternoon, the building surface high into the Great Lakes should lead to a renewed push of a northerly winds into northern Illinois and northwestern Indiana. As a result, the frontal boundary and accompanying shower chances should get pushed further southward and out of our area by evening. The secondary push of winds down the spine of Lake Michigan should also build waves heights particularly at northwestern Indiana beaches. The Thursday through perhaps even Friday morning timeframe will be a candidate for a Beach Hazards Statement as we get closer. Finally, the mostly cloudy start to the day as well as north to northeasterly cold air advection will lead to a noticeably cooler day compared to Wednesday, with highs in the lower 70s near the Wisconsin state line to around 80 near US-24. A push of dry air (dew points dipping toward the upper 40s in spots?) should help clouds erode Thursday night. Overnight lows should hence drop toward the mid 50s north to around 60 south. Such a pleasant night and a continuation of a surface pattern dominated by the surface high pressure system will lead to an equally pleasant Friday with relatively low humidity levels, partly sunny skies, and highs around 80 degrees. This weekend, the upper-level pattern will undergo a transformation toward pronounced ridging across the southern US and quasi-zonal flow across the north. As this occurs, the surface high pressure system across the Great Lakes will shift eastward toward the northeastern US. The net result will be the development of deep south to southwesterly flow across the central US, which will encourage the frontal boundary responsible for showers and storms this week to creep back northeastward toward the Great Lakes this weekend. With upper- level shortwaves poised to parade along and atop the boundary, the stage will be set for episodic MCSs to form and propagate from the Upper Mississippi River Valley toward the Lower Great Lakes. Now, timing out the cadence and locations of MCSs in such a pattern is often a futile effort since the frontal boundary delineating the instability axis will no doubt be modulated by the storms themselves. It is also worth noting that eventual convection often verifies further south and west than depicted in model guidance several days out in these types of patterns. With that said, ensemble model guidance is starting to highlight Saturday through early Sunday morning time period as the one to watch for one or more MCSs in our neck of the woods before the front is shunted southwestward by Sunday afternoon. With PWATs nosing toward 2" and 0-8km shear inching toward 40kt, damaging winds and flash flooding would be pronounced threats wherever eventual MCSs develop. It is for this reason that WPC currently already has our area in a relatively rare "Day 5" Level 2 threat level for flash flooding, valid for Saturday. We`ll see if the ensemble signal holds as we get closer. Beyond this weekend, ensemble model guidance continues to advertise the "heat dome" strengthening across the central US leading to a corresponding increase in hot and humid conditions into our region (particularly July 22-24). It`s tough to ignore a continued signal in both deterministic and ensemble guidance for dew points in the general region to creep above 80F at some point next week, which would translate to stifling humidity. Of course, we will remain within striking distance of progressive (severe) MCSs roaming around the perimeter of the heat dome, as well. So, hazardous weather looks to continue in the region in some form or fashion next week. Borchardt-- End Changed Discussion --&& .AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...-- Changed Discussion --Issued at 1236 PM CDT Tue Jul 15 2025 Main Concern: - Potential for gusty TS late Wednesday afternoon. Varying south to south-southwest winds may shift to southeast or east-southeast for a time this evening at MDW and ORD as the lake breeze pushes inland and then washes out. Expect light southerly to VRB/calm winds the rest of the night thereafter. The light winds and humid conditions may set the stage for minor VSBY reductions at RFD, DPA, GYY early Wednesday. On Wednesday, VFR conditions and southwest winds (gusts to ~20 kt PM hours) are expected until the mid-late afternoon, when a broken cluster of TS may be approaching from the west. There`s uncertainty on the exact timing and coverage, but felt the signal was strong enough for possible TS impacts in the last couple of hours of the ORD/MDW 30-hour TAFs to add a PROB30. Downburst/outflow winds with any TS would likely be from a west/west-northwest direction, with gusts to 30 kt or higher certainly possible. Castro-- End Changed Discussion --&& .LOT WATCHES/WARNINGS/ADVISORIES... IL...Air Quality Alert until midnight CDT Wednesday night for ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108. IN...Air Quality Alert until midnight CDT Wednesday night for INZ001-INZ002-INZ010-INZ011. LM...None. && $$ Visit us at weather.gov/chicago