Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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418 FXUS63 KLOT 252321 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 621 PM CDT Wed Sep 25 2024 .KEY MESSAGES... - Patchy shallow ground fog again tonight into early Thursday. - Windy on Friday and Friday evening, which will also cause large waves and dangerous conditions on Lake Michigan. - Diminishing chance for any meaningful rain Friday night through early next week. && .DISCUSSION... Issued at 248 PM CDT Wed Sep 25 2024 Through Thursday Night: Following a pleasant afternoon today, the only item of note tonight will be the potential for patchy shallow ground fog once again. The main limiting factors will be the very shallow inversion height, and modest northeasterly flow above it, as well as dry air above the inversion. However, with overnight temperatures dipping down into the upper 40s (locally mid 40s) to lower 50s outside of the heart of the Chicago metro, these temps are generally at or below the afternoon crossover temps. Calm winds, clear skies, and a bit more moisture due to recent rain, should support patchy ground fog development. It seems like a less favorable setup than last night, so the coverage of any dense fog should be more limited. Thursday will be another seasonably warm day after any lingering early morning fog erodes, with mostly sunny skies and highs in the 75-80F range away from the lake, and lower 70s along the immediate shore. High clouds well ahead of Helene`s remnants will filter in from the south Thursday night and keep overnight temperatures propped up a bit vs. tonight`s. Friday through Wednesday: The main change in the forecast message for Friday is the increasing likelihood of strong northeasterly winds, especially south of I-80 and along the Lake Michigan shore. Helene`s strong extratropical remnants, possibly still technically partially tropical into Friday morning, will track to the Ohio Valley by Friday evening, still at 990 mb or a bit less. Given the unusually strong surface low pressure butting up against 1015+ mb high pressure over the Great Lakes, the pressure gradient will quickly tighten through the day on Friday. Exceptionally dry air in the 700-500 mb layer and down to the low levels will be tough to erode. This will not only slow the northward progress of (and eat away at) the northwest fringe of the precip shield, but also present supportive conditions for efficient mixing, as the drier low levels will keep lapse rates fairly steep. Model forecast soundings today were in solid agreement depicting 35-45 kt flow atop the mixed layer, highest in our southern CWA, closest to the center of Helene`s remnants. The wind field should peak in magnitude in the mid afternoon through the early evening hours. There`s some uncertainty on how thick the mid and high cloud cover will be north of Helene`s remnants, which could modulate how warm temps get, how much dew points mix out, and how effectively the higher momentum aloft can be tapped into. With that said, the signal was consistent across the guidance today (along with showing a general uptick in the robust EPS suite) to support ramping up wind speeds to 15-25 mph and gusts into the 35-40+ mph range. The best chance for any gusts approaching 45 mph advisory criteria is, as noted earlier, primarily pegged for our southern CWA. Occasional gusts up to 40 mph appear probable near the lake, though there it`s looking like more of a stronger sustained wind off the frictionless lake, and gusts not much higher than the sustained winds. A Marine section will be added below with details on the large waves and dangerous conditions expected on the lake. The interesting "fujiwara" interaction and then merger with a larger upper low does still look likely to take place with Helene`s remnants, but limiting factors for appreciable rain in our area discussed the past few to several days have not materially changed. Strong mid-level ridging extending across the northern Great Lakes will likely be a road-block and prevent the gradually weakening, but still very large extratropical cyclone, from getting much farther north than the general Ohio Valley area. With very dry air needing to be eroded and stronger forcing held to our south, some scattered showers (30-40% PoPs) focused southeast of I-55 may be the best we can do Friday night. The potential showery regime over the weekend into early next week is trending toward more of a occasional light rain showers and sprinkles setup Saturday and Sunday, with the best (but still low/~30%) chances of measurable rain in our southeast half or third of the CWA. Saturday will be another breezy, but not quite as windy day as Friday, with winds further stepping down on Sunday as the cyclone to our south steadily weakens. Seasonably warm daytime temperatures are expected over the weekend, in the 70s, and possibly approaching 80F in our far western CWA on Sunday. A cold front will likely sweep across the area in the Monday PM-Tuesday timeframe, bringing a more substantial cool-down from near summer-like warmth to start the work week. Signs do not currently point to this being an anomalous cool-down for this time of year into mid next week, so it looks to remain comfortable by day and chilly overnight. It does appear we`re headed into another well below normal precip. stretch, particularly with dwindling chances of beneficial rain this weekend. Castro && .AVIATION /00Z TAFS THROUGH 06Z THURSDAY/... Issued at 621 PM CDT Wed Sep 25 2024 Forecast concerns for the terminals... - Patchy fog possible late tonight into early Thursday morning, mainly in northwest IL A pleasant evening weather wise is underway as a surface high swings overhead. Though the lake breeze continues to make steady progress towards DPA. While the lake breeze should begin to washout after sunset, it looks as if the boundary will reach DPA between 01z and 0130z resulting in an easterly wind shift and slight uptick in wind speeds. Winds will become light and variable later this evening with skies expected to clear out as the high moves in. The combination of clear skies, light winds, and lingering low- level moisture from yesterday`s rain does look to support another night of patchy fog, especially across northwest IL. However, the shallow moisture depth does pose questions as to the coverage and intensity of said fog. For now, have decided to maintain the TEMPO for MIFG at RFD, but have left fog out of DPA and GYY due to lower confidence. Regardless, any fog that does develop should be generally in the 3 to 5 SM range at worst and will erode by 13z Thursday morning. As for Thursday, expect another quiet weather day with winds becoming easterly through the morning with otherwise VFR conditions. Though wind speeds will be slightly stronger, in the 7 to 10 kt range, as deeper mixing is expected due to lesser cloud cover. Yack && .MARINE... Issued at 300 PM CDT Wed Sep 25 2024 A prolonged period of strong northeast winds is expected Friday into Saturday as the large and unusually deep (pressure wise) remnants of Hurricane Helene stall near the Ohio Valley. The air mass over the lake will be seasonably mild for this time of year, so neutral to fairly stable conditions should somewhat limit the upper echelon of potential wind speeds/gusts. However, the high pressure to the north, slow moving nature of the pattern, and only gradual weakening of Helene, will yield a lengthy period of dangerous conditions on the lake. Expecting northeast winds to steadily increase through Friday, peaking in the afternoon and evening hours at about 30 kt (35 mph) with occasional gale force gusts (up to ~40 mph). Waves may not become quite as large as in an unstable setup, though the persistence of the strong northeast winds should compensate. The next shift will likely be able to issue a long duration Small Craft Advisory, along with a Beach Hazards statement, for the likely life-threatening conditions for anyone who ends up in the water. Can`t rule out a gale watch/warning if the wind forecast trends up, though this is currently appearing less likely. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago