Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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220 FXUS63 KARX 240248 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 948 PM CDT Mon Sep 23 2024 .KEY MESSAGES... - Potential (30 to 50%) for river valley fog again tonight. - Remaining dry through the week, low chances (20 to 40%) for precipitation return Saturday. && .DISCUSSION... Issued at 210 PM CDT Mon Sep 23 2024 Tonight into Tuesday: Passing Clouds, River Valley Fog, A Few Sprinkles? Remnants of a weak low pressure system will be lifting north through the mid-Mississippi River Valley tonight as an upper-level wave drops SE across the Dakotas. A weak surface ridge of high pressure overhead will remain as the dropping system skirts by and the remnant surface low moves into the Ohio River Valley. These two systems will both bring passing clouds overhead tonight, with the biggest impact being low temperatures and fog potential. Should clouds clear to the east quicker, temperatures will likely fall another degree or two with radiational river valley fog developing. Both of these systems will bring a small chance (10% or so) of a brief rain shower, but the low-levels of the atmosphere are dry which will limit hydrometeors reaching the surface. Did keep the prevailing fog forecast for tonight in the river valleys. Midweek Forecast: Dry, With Above Normal Temps (mid 70s to near 80) The system dropping from the Dakotas reaches mid-Missouri by Wednesday afternoon as global ensembles overwhelmingly favor this system cutting off as a western CONUS ridge amplifies. This ridge continues to build and elongates itself from the desert SW towards the Upper Great Lakes region as the cutoff low remains along the mid- Mississippi River Valley and another trough moves into the Pacific NW. This pattern will favor continued dry conditions with above- normal temperatures (5 to 10 degrees above normal). This Weekend: Remaining Warm, Uncertainty for Rain Chances this Weekend Global ensembles show considerable uncertainty as to the evolution of any potential tropical system this weekend and the cutoff low during the middle of the week. Ensembles suggest the ridge axis shifts east over the Upper Midwest as the cutoff low and potential tropical system remain to the south along the Gulf Coast. A 100- member ensemble consisting of 30 GEFS, 50 EPS, and 20 Canadian members suggest a 20 to 30% chance of showers from a remnant tropical system by Saturday across the area. Canadian and ECMWF members suggest this remnant tropical system will bring a low pressure system into the Missouri and Tennessee border by Friday evening, but the GEFS mean is more diffuse with the low pressure fizzling after it moves on shore. Either way, global ensembles all suggest that any precipitation that does shift north, will be fighting a ridge axis that will be along and north of the US/Canada border that will likely keep PoPs on the lower end until models get a more certain handle on upper-level flow evolution. However it shakes out, the ridge overtop of the cut off low pressure system will serve as a quasi-Rex Block with the overall pattern evolution slowing down this week. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 946 PM CDT Mon Sep 23 2024 Fog still appears probable at LSE late tonight into early tomorrow morning, provided ongoing mid-level clouds clear out, as short term models suggest they will. Have thus continued to include LIFR visibility at LSE from 10 to 14z. Amendments may be needed if mid-level cloudiness persists. Otherwise, expect generally light winds through the period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAW AVIATION...Ferguson