![National Oceanic and Atmospheric Administration](/build/images/header/noaa.d87e0251.png)
![National Weather Service](/build/images/header/nws.4e6585d8.png)
![United States Department of Commerce](/build/images/header/doc.b38ba91a.png)
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
770 FXUS63 KLOT 111131 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 631 AM CDT Thu Jul 11 2024 .KEY MESSAGES... - Scattered non-severe thunderstorms again today, primarily south of I-80. - Hot and humid conditions expected early next week. Heat indices may approach or exceed 100 degrees. - Periodic thunderstorm chances Saturday through mid next week, including the potential for a couple nocturnal/early AM t-storm complexes. && .DISCUSSION... Issued at 320 AM CDT Thu Jul 11 2024 Through Friday Night: A mid-level trough axis centered across eastern IA early this morning, is expected to gradually slide eastward over northern IL through the day. As it does so, several embedded smaller scale impulses are expected to pinwheel into the area through the day. Each of these impulses is likely to foster at least some isolated to widely scattered showers and thunderstorms today, particularly to the south of I-80, in closer proximately to where a surface boundary resides. Similar to yesterday, mid-level lapse rates and deep layer shear are expected to remain poor, which will largely curtail the threat for severe thunderstorms today. However, some locally gusty winds and very heavy downpours may accompany these storms, particularly during the afternoon. Significant widespread hydro concerns are not anticipated, though rather slow storm motions (towards the east at 10-20 mph) could result in some very localized heavy rainfall totals in excess of an inch. Widely scattered showers and storms may linger across eastern/southeastern sections of the area into tonight as the mid-level trough axis gradually shifts into northwestern IN. However, the coverage of storms is expected to wane with time overnight, and end altogether prior to daybreak Friday morning. Shower and thunderstorm chances then going into the daytime hours of Friday look rather low (20% or less), owing to rising mid-level heights and associated capping in the wake of the departing mid-level trough. Temperatures today will be a hair cooler than those on Wednesday, owing to more cloud cover and cooler temperatures under the mid- level trough. Accordingly, inland temperatures are expected to top out in the lower 80s, while onshore northeasterly flow keeps areas along the lakeshore in the 70s. On Friday, conditions will warm a bit with more sunshine. Temperatures are thus forecast to top out in the mid 80s inland, with a bit cooler conditions expected along the lakeshore areas again. Surface high pressure will remain over the area through Friday night, making for a quiet and seasonably mild night. KJB Saturday through Wednesday: Forecast thinking has not changed beyond Friday night. For more on the long term period please reference the previous discussion below. The first potential episode of convection in the humid and at times stormy pattern expected this weekend through early next week may arrive Saturday morning in parts of the area, probably in a weakening state. This will occur in concert with a warm front lifting north and a large instability reservoir advancing eastward across the mid and upper MS Valley. Large scale forcing mechanisms will be lacking through the afternoon, though any convectively induced impulses, and convergence tied to lingering outflow and the northward advancing warm front may be enough for widely scattered afternoon pulse-type thunderstorms (30-40% PoPs). Highs on Saturday will be in the mid to upper 80s, except slightly cooler lower 80s along the Illinois shore due to lake influence backing winds east- southeasterly. Dew points in the upper 60s to lower 70s at peak heating will yield peak heat indices in the low-mid 90s, a harbinger of things to come Sunday through Tuesday. Expansive mid-upper 500s DAM 500 mb ridging centered over the inter-mountain west and mean troughing centered over Hudson Bay will set up a hot and humid northwest flow aloft locally in the Saturday night-Tuesday timeframe. Unlike previous occurrences this warm season of overnight arriving convection weakening quickly as instability diminishes, the impressive instability reservoir (3000-4000+ J/kg of MUCAPE) will largely remain in place through the period. This entails a period of "ring of fire" conditions, where our position on the northeast flank of the ridge puts us in the threat zone for southeastward moving convection, including potentially well-organized severe MCSs, and episodes of flash flooding due to PWATs up in the ~1.75-2" range at times (150-200% of normal). Aside from operational and ensemble members hinting at the likelihood of this type of activity, specific timing predictability in these types of patterns are commonly low confidence. This maxes out PoPs only in the chance (30-40% range) during windows of MCS favorability at this lead time. The first window may be Saturday night, prior to stronger deep layer shear being present, followed by Sunday night into early Monday when 40+ kt of deep layer bulk shear is forecast. Monday night into early Tuesday could end up being another window for parts of the area as well. Classic in these type of patterns, there will be many dry hours, and a relative daytime lull/capping Sunday and Monday PM will support highs in the lower to possibly mid 90s with dew points in the 70s yielding peak heat indices flirting with heat advisory (105+F) criteria. Tuesday afternoon will be similar thermally to Sunday and Monday, though perhaps less capped with a bit better chance of widely scattered PM thunderstorms as a weak cold front advances south. It`s unclear whether the appreciable instability axis will be shunted far enough south by Wednesday to limit another round of convection, especially south of I-80, as the cold front takes on back door characteristics. What is of higher confidence is an end to the hot and humid conditions, with forecast high temps on Wednesday in the low-mid 80s inland and upper 70s near Lake Michigan. Castro/KJB && .AVIATION /12Z TAFS THROUGH 18Z FRIDAY/... Issued at 630 AM CDT Thu Jul 11 2024 Key Aviation Impacts for the current TAF period: - Less than a 30 percent chance for light showers late morning and afternoon - Chances for lower vis/fog to the west overnight, including KRFD Winds are light and variable and slowly become northeasterly and up to 10 knots through the morning and into the afternoon. A few non-impactful sprinkles are around the city, but a larger complex is providing -SHRA around KRFD, expected through 15Z. For the rest of the day, there remains a limited chance for pop-up showers late this morning or into the afternoon, but with the probability less than 30 percent, it was kept out of the TAF and shown as just a midlevel SCT group. The chances and confidence in thunder development at terminals is just as low. The higher confidence in scattered shower and thunderstorm development later today is south of I-80 (over the southern approaches). Cloud cover will gradually diminish into tonight and winds will once again become light and variable. Model guidance is starting to key in on a better fog signal, though it looks better to the north and west. MVFR vis was introduced in KRFD for tonight, but fog is not currently expected at the Chicago terminals. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago