Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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953 FXUS63 KLOT 181126 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 626 AM CDT Wed Sep 18 2024 .KEY MESSAGES... - Today and tomorrow will be similar to the past few days with sunny skies, highs in the upper 80s, light winds, and daily lake breezes. - A few showers and storms may move into northwestern Illinois Thursday night. - Friday through the weekend will be noticeably more humid. Chances for widespread rain return this weekend. && .DISCUSSION... Issued at 327 AM CDT Wed Sep 18 2024 Through Tuesday: Patches of dense fog continue to meander through northern IL near the Fox and Kankakee River Valleys within a westward- traveling moisture axis. A Dense Fog Advisory is in effect until 800 AM for Lake (IL), McHenry, Kane, and DeKalb counties where webcams, ASOS/AWOS observations, and nighttime microphysics satellite imagery shows a solid "slug" of dense fog. Fog (in the Dense Fog Advisory and elsewhere) will erode quickly after sunrise. National infrared satellite imagery depicts an upper-level trough digging along the Pacific Coast upstream of an upper-level shortwave lifting into the northern Plains. Meanwhile, an upper-level low comprised of the remnants of two tropical systems continues to slowly meander through the Carolinas. Our area remains locked in a relative stagnant zone of the atmosphere between all features of interest. Just to illustrate the banality of our current pattern, the mean of all sampled winds from the surface to 200mb (basically the depth of the troposphere) from the past 9 RAOBS from ILX (7 AM Saturday through 7 PM Monday) is just shy of 12kt. This has been quite the slow pattern. With little change expected in the synoptic pattern over the next 24 to 48 hours, the weather today and tomorrow will be nearly the same as the past few days with sunny skies, highs in the mid to upper 80s, and daily lake breezes. The one departure today from the past few will be passing high clouds this evening and overnight, though they should clear in time for sunrise on Thursday. Thursday night into Friday, showers and storms should develop along and ahead a weakening cold front across parts of Iowa and Wisconsin. With the moisture axis feeding the convection lagging behind and supportive upper-level forcing departing further into Canada, the showers and storms may struggle to make much inroads into Illinois. Blended NBM guidance offers 30 to 50% chances for showers and storms before daybreak Friday focused along and west of I-39, which seems fair. After daybreak on Friday, low-level moisture characterized by surface dew points in the mid to upper 60s will finally slosh into our area as winds turn westerly. (Another gee whiz fact...The last time Chicago O`Hare logged a wind with a westerly component greater than 5 knots appears to have been on September 11. Again, that speaks to the persistence of the pattern this week). Anyway, the air will feel noticeably heavier by Friday afternoon, especially when combined with highs in the mid to upper 80s. While forecast soundings depict minimal capping by mid-afternoon, a dearth of large- scale forcing mechanisms suggests that development of any shower or storm Friday afternoon would be isolated at best, consistent with a 10 to 20% chance. This weekend, ensemble model guidance continues to favor a return of chances for welcome rainfall seemingly associated with an approaching upper-level shortwave emanating from the Plains. However, exactly how and when is not exactly discernible at this point given ensemble clustering exhibiting several forecast scenarios related to phasing opportunities with upper-level shortwaves parading along the jet stream near the US/Canadian border. At this point, blended NBM chances for thunderstorms ranging from 20 to 40% seems fair from Sunday onward keeping in mind they will change as time goes on. Borchardt && .AVIATION /12Z TAFS THROUGH 18Z THURSDAY/... Issued at 626 AM CDT Wed Sep 18 2024 Forecast concerns include... Areas of fog early this morning. Lake breeze this afternoon. Chance of fog late tonight. Areas of fog extend just west of the Chicago terminals this morning and will continue for a few more hours before lifting. Conditions will likely improve quickly at DPA as the fog drifts west, but may lower at RFD. Only medium confidence for how low the vis may drop at RFD, prior to quickly improving and trends will need to be monitored. There will be another chance of fog late tonight into early Thursday morning, but confidence remains low and no mention in any of the current tafs. If fog does develop, it may be in similar areas as it currently is this morning. Light/calm winds to start may end up becoming southerly or even south/southwest by mid/late morning, with speeds 4-7kts. A lake breeze will then move inland, shifting winds east/northeast for the Chicago terminals with speeds to 10kts. Timing of the lake breeze may need some refinement with later forecasts. Winds will diminish under 5kts this evening and likely become light and variable again overnight. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Dense Fog Advisory until 8 AM CDT this morning for ILZ005- ILZ006-ILZ011-ILZ012. IN...None. LM...None. && $$ Visit us at weather.gov/chicago