Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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283 FXUS63 KLOT 251948 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 248 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 /18Z TAFS THROUGH 00Z FRIDAY/... Issued at 1159 AM CDT Wed Sep 25 2024 The only minor concerns are: - Timing a light east/northeast wind shift with a lake breeze late this afternoon/evening at the Chicago-area sites. - Potential for shallow BR/FG tonight/Thursday morning, mainly near RFD. Fair weather cumulus with bases near 040-050 will continue to develop today. VFR conditions will prevail at the terminals. Northwesterly winds will become east/northeast this afternoon at ORD, MDW, and GYY with the passage of a weak lake breeze. Timing at ORD/MDW remains unchanged from previous forecasts. Tonight, winds will become calm/light and variable. Some shallow BR/FG will again be possible, with the strongest signal at this time near RFD. Some potential this could extend towards DPA and GYY, but confidence is too low to include a mention with the outgoing 18z TAFs at this time. Thereafter, winds will become northeasterly through Thursday morning. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago