497 FXUS63 KLOT 170523 AFDLOTArea Forecast Discussion National Weather Service Chicago/Romeoville, IL 1223 AM CDT Wed Sep 17 2025
.KEY MESSAGES...
- Patchy fog is expected early this morning, some of which could become locally dense.
- Warm and dry conditions persist through the work week.
- Periodic shower and thunderstorm chances return this weekend into early next week (20-40%).
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.DISCUSSION... Issued at 233 PM CDT Tue Sep 16 2025
Quiet conditions will persist through at least Thursday as a somewhat blocked large-scale pattern with a strong ridge over the Great Lakes begins to break down. Cloud cover will remain quite low with only some cirrus filtering over the area from a pair of upper-level lows situated well to the west and southeast. Meanwhile, the southwest extent of a surface ridge will remain entrenched over the area through Wednesday before another ridge noses southward toward the area on Thursday. This will favor an extended period of generally E to NE winds.
Temps will continue to gradually warm each day with inland locations experiencing highs in the mid 80s and lows in the mid to upper 50s. Close to the lake, reinforcement of lake-modified air will result in highs in the mid 70s and lows in the 60s. While precip is unlikely during this period, the longer nights, light winds, and mostly clear skies could support shallow fog (primarily tonight) given marginally favorable dew points in the mid 50s. Have included patchy fog late tonight through sunrise for areas along the north of the Illinois River and west of the I-294/94 corridor in the Chicago metro.
Kluber
While quiet and dry weather will persist in this blocked pattern through the rest of the work week, the upper level ridge begins to break down heading into the weekend as a western trough (and/or potential cut-off low) begins to slide eastward towards the Great Lakes. This will lead to increasing chances for periodic showers and thunderstorms over the weekend into early next week.
KJB
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.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/... Issued at 1223 AM CDT Wed Sep 17 2025
- MVFR VSBY reductions in BR early this morning
The combination of calm/variable winds, low-level moisture drifting inland off Lake Michigan, and radiational cooling under clear skies may lead to fog development again early this morning. In contrast to recent foggy mornings in the suburbs, low-level moisture may be sufficient for at least MVFR VSBY reductions in BR at the Chicago terminals (ORD/MDW). For now have maintained MVFR TEMPOs but have nudged VSBYs down to 4SM (3SM at DPA/GYY). There remains about a 5-10% chance for near LIFR VSBYs in some spots toward daybreak and observational trends will be monitored closely.
Winds return to a prevailing light northeast to east direction behind a lake breeze late morning into the afternoon then ease again after sunset. Another round of patchy fog can`t be ruled out early Thursday morning but will plan to address that with later updates if confidence increases.
Petr
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.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None.
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