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
211 FXUS63 KLOT 170849 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 349 AM CDT Wed Jul 17 2024 .KEY MESSAGES... - A few isolated showers/thunderstorms early this morning mainly southeast of I-57. Slight chance of a few showers or sprinkles along the lake breeze in northeast IL this afternoon. - Potential for hazardous swim conditions at Lake Michigan beaches tonight into Thursday. - Period of dry, seasonably cooler and less humid weather Thursday into the weekend. - Pattern trends toward slightly warmer temps and a return of slight shower/thunderstorm chances by early next week, though does not look particularly stormy at this distance. && .DISCUSSION... Issued at 348 AM CDT Wed Jul 17 2024 Through Thursday... Early morning GOES water vapor imagery depicts a rather deep upper trough (by July standards) across Ontario and the upper Midwest. A couple of short wave disturbances were noted within the trough, one low-amplitude impulse and mid-level jet streak tracking across northern IL/IN, and another more amplified wave rotating southeast across the upper Mississippi Valley. At the surface, the weak cold front which sagged across the forecast area on Tuesday appears to have become nearly stationary from west-central IL into northern IN, while a subtle surface trough/cold front stretched from northeast WI into central IA. A fairly moist low-level air mass lingers in the vicinity of the stationary front, depicted by 70 degree surface dew points lingering across the southeast half of the WFO LOT cwa. Farther northwest, dew points gradually decrease into the mid-upper 50s ahead of the cold frontal trough to our north. A few showers and isolated thunderstorms have festered overnight within the moist low-level air near the southern front, where objective analysis depicts an axis of MLCAPE ~500 J/kg and weak moisture convergence from south of the Chicago metro into northern IN. These may continue to bubble in isolated fashion through sunrise or so, before eventually shifting off to the east-southeast. The morning hours otherwise should be dry, with light northwest winds providing modest dry advection. Isolated shower chances do look to increase again by this afternoon however, as the upper Mississippi Valley short wave (and larger scale trough axis) shift across the region. Guidance soundings and CAM output vary on degree of shallow instability and shower coverage, though there`s some signal that isolated showers may focus along the lake breeze boundary over the Chicago area. Even the more bullish model soundings suggest equilibrium levels to around -5C, suggesting little/no thunder threat with these. Have kept inherited slight (<25%) pops for the afternoon hours inland of the lake in northeast IL along the lake breeze. The upper trough axis shifts east of the area later tonight, with strong height rises developing across the western Great Lakes region into Thursday. At the surface, this area of subsidence manifests as a broad area of surface low pressure which will settle across the area through the end of the week. Temperatures will cool from the upper 70s/low 80s today to the mid-70s in most spots Thursday, with slightly cooler readings along the lake with onshore winds. In fact, increasing north- northeast winds on the southeast flank of the surface high will lead to building waves on southern Lake Michigan tonight into Thursday, which will likely make for hazardous swimming conditions at Lake Michigan beaches, especially from Cook county southeast into IN. Ratzer Thursday night through Tuesday... The upper pattern is forecast to remain fairly amplified into the weekend, with a deep upper trough across the eastern CONUS and a strong upper ridge across the west. We`ll be in between these features, in a region of northwest flow aloft, and lingering surface high pressure ridging settling across the Midwest/Ohio Valley. This will maintain a period of dry and pleasant weather conditions across our area into the weekend, with temperatures generally in the upper 70s to lower 80s. Models continue to be in decent agreement in a mid- level trough eventually developing southeast across the region by later in the weekend and early next week, with indications of a weak surface trough perhaps allowing at least some chance of precip to return as well as a gradual moderation in temps by Monday and Tuesday. Ratzer && .AVIATION /06Z TAFS THROUGH 12Z THURSDAY/... Issued at 1222 AM CDT Wed Jul 17 2024 The primary aviation weather concerns for the 06Z TAF period include: * At GYY, intermittent LIFR cigs tonight Light and variable winds will take us through the rest of tonight. Near GYY, patches of IFR cigs have found their way over the airfield from time to time. This may continue through the night, along with some instances of MVFR BR. VFR is expected at all other TAF sites tonight. Winds on Wednesday will blow in from the northwest moving at 10-20 kt by late morning. A lake breeze is expected to move onshore tomorrow, but timing for reaching the airfields is somewhat uncertain. The boundary is currently expected to stay east off the Chicago sites for most of the day before making a late day surge sometime around 00Z. However, there is notable support for NNEs to arrive by mid-afternoon, as early as around 21Z. Expecting light NEs across all sites for the night. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago