


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
180 FXUS63 KLOT 060501 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1201 AM CDT Sun Jul 6 2025 .KEY MESSAGES... - Shower and thunderstorm chances return this evening especially across northwest IL. Waves of widely scattered showers and storms are expected in the general region overnight into Sunday. Torrential downpours will provide localized relief to ongoing drought conditions, and some areas will stay completely dry. - A period of dry and slightly cooler weather is on track to start the upcoming week. && .DISCUSSION... Issued at 307 PM CDT Sat Jul 5 2025 Through Sunday: A hot well mixed boundary layer has allowed surface dewpoints to mix out into the mid 60s as air temperatures have warmed into the low to mid 90s. The lower dewpoints have acted to hold heat indices in check, with readings generally right around the actual surface temperature. The poor low-level moisture has also helped cap the airmass to thunderstorm development this afternoon. While this is expected to continue to be the general trend through the remainder of the afternoon, we will be monitoring the increasing threat for scattered thunderstorms across northwestern IL into early this evening (after 6 PM). A recent surface analysis indicates that a much richer low-level airmass (dew points in the low to mid 70s) currently resides to our west in the vicinity of an a surface frontal trough and mid- level impulse over IA. The eastern periphery of the richer moisture is already beginning to work into far northwestern IL near the Mississippi and has recently helped spark some showers. These showers look to move into western parts of Lee Ogle and Winnebago counties prior to 4 PM, though the threat of thunder looks to remain low as they move into a increasingly capped envionrment with eastward extent. The main area of storms we are watching is the line of ongoing storms over IA. These storms are expected to continue increasing in coverage as they shift east-northeastward into northwestern IL late this afternoon and early evening. There continues to be a low threat for a few of these storms to organize enough to produce instances of localized strong wind gusts, though this threat is expected to largely remain across northwestern IL (generally along and west of I-39) through 10 PM this evening. Overall, poor mid-level lapse rates and the loss of daytime heating should ultimately result in these storms weakening with eastward extent across northern IL later this evening. While this will curtail the severe threat and decrease the coverage of showers and storms after 10 PM, some widely scattered activity may continue to fester in the area overnight, especially in advance of the next impulse expected to move out of northern MO. The main threat with any showers or storms overnight would be locally heavy rainfall given the notable increase in moisture (precipitable water values ~2"). Shower and storm coverage looks to be the lowest for a period Sunday morning, before activity begins to increase in coverage along the surface frontal boundary by midday/early afternoon. Rich moisture overhead (precipitable water values ~2") in advance of this front will support very efficient rainfall production, and thus support torrential downpours within these showers and storms. Relatively slow storm motions and possible training cells may thus support some localized hydro concerns. There continues to be differences in the timing of the cold front dropping down southern Lake Michigan, and this ultimately leads to lower confidence with the extent of the heavy rain and hydro threat over much of the Chicago metro area. If the frontal timing ends up slower and holding off until mid afternoon, then the heavy rain and hydro threat will be higher in the Chicago area. Otherwise, an early frontal timing should keep these threats largely south of I-80 Sunday afternoon. KJB Sunday Night through Saturday: The mid-level trough responsible for potential convection over the area on Sunday will clear the area to the southeast by Sunday evening. A trailing and slow-moving cold front will continue to support scattered showers and some thunderstorms south of a Dixon to Chicago line Sunday evening before gradually suppressing activity farther to the south overnight. Poor mid-level lapse rates will limit coverage of thunderstorms, but slow storm motions combined with PWATs to around 2" will yield locally heavy rain with any thunderstorm or stronger shower. The cold front is expected to clear the CWA to the south no later than Monday morning as a weak surface ridge builds across the western Great Lakes. This will favor dry conditions with notably less humidity for much of the area on Monday. Similar conditions should persist through the day on Tuesday as consensus guidance stalls the front well south of the area. Persistent onshore and stronger gradient flow behind the front may also result in higher waves and dangerous swimming conditions on Monday. A low-amplitude trough drifting across the Upper Mississippi River Valley on Wednesday will bring increasing chances for showers and some storms Tuesday night into Wednesday night. Mostly dry conditions are then favored Thursday and Friday, though decaying convection could survive eastward into the CWA on the eastern periphery of a central CONUS ridge and within a more favorable moisture reservoir. Kluber && .AVIATION /06Z TAFS THROUGH 12Z MONDAY/... Issued at 501 PM CDT Sat Jul 5 2025 Primary aviation concerns through the TAF period include: - Scattered showers and a few storms through about 13-14Z this morning - Redevelopment of showers and storms between 19-23Z this afternoon - Wind shift from southwesterly to northeasterly after 21Z this afternoon - Potential for MVFR cigs, first for a few hours after sunrise and again toward the end of the TAF period Discussion: Showers and a few thunderstorms are expected to move across the airspace overnight and into tomorrow morning within a broad frontal zone. For now, will maintain the inherited structure of advertising VCSH at ORD/MDW/GYY/DPA and -SHRA at RFD at times through the overnight hours. However, will be proactive with issuing AMDs as focused areas of showers or thunder become established and threaten specific terminals. Winds overnight should remain southwesterly. Depending on coverage of showers overnight, do still see some potential for MVFR cigs to develop by around daybreak at all the terminals. Confidence on whether this occurs remains low. There is a signal in model guidance for a break in shower coverage from mid morning to noon before showers and storms redevelop by early afternoon in advance of a secondary front approaching from the north. Will maintain PROB30 groups at all but RFD for now, though anticipate needing to upgrade to TEMPOs in later TAF packages. Winds will turn north to northeasterly behind the front, perhaps accompanied by MVFR cigs particularly at ORD/MDW/GYY/DPA. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement from Sunday afternoon through Monday morning for ILZ006-ILZ103-ILZ104. IN...Beach Hazards Statement from Sunday evening through Monday morning for INZ001-INZ002. LM...Small Craft Advisory from 3 PM Sunday to 9 AM CDT Monday for the IL nearshore waters. Small Craft Advisory from 6 PM Sunday to 9 AM CDT Monday for the IN nearshore waters. && $$ Visit us at weather.gov/chicago