Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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849 FXUS63 KARX 242323 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 623 PM CDT Tue Sep 24 2024 .KEY MESSAGES... - More valley fog expected next couple nights. - Rain chances return to the region late in the week, especially this weekend, as tropical system remnants approach from the south. - A weather pattern early next week could lead to more typical autumn temperatures. && .DISCUSSION... Issued at 140 PM CDT Tue Sep 24 2024 Valley Fog Next Couple Nights Tis the season...for valley fog that is. With several more nights of mostly clear skies, light low level flow, and longer nights, likely will see more bouts of valley fog. Obviously will need to be watching the subtle day to day details for specific impacts, especially as it relates to aviation interests. Rain Chances Return This Weekend Highly amplified upper air pattern and related evolution will be key to future rain chances. This is especially true as upper trough becomes cutoff to our south and tropical system Helene enters the picture. Approaching tropical systems always seem to disrupt flow patterns and often can stall normal progression. As low becomes established more centered on lower to mid Mississippi River valley, upper ridge axis will shift and tilt further east, likely keeping our weather quiet and pleasant. Expecting dry regime for much of the week with above normal temperatures. Interestingly medium range guidance suggest remnants of tropical system wrapping up into cutoff low which could gradually weaken over time, but still looks to push ridge axis back west by late week. Precipitation shield expected to rotate through parts of the mid Mississippi and Ohio River valleys but probabilities decrease as you move northwest based on strength of ridge and drift of reformed upper low from previous system and tropical feature. This is the biggest differences in guidance - how far north this low will get. Cluster analysis shows differences between low locations roughly 250 miles apart with ECMWF the furthest south and GEFS more north. In fact, from late Friday on, the Canadian ensemble members and some GEFS are the main solutions producing rain closer to our service area with the mean and bulk of other outcomes remaining completely dry. This is a lot of detail to say small rain probabilities enter the forecast in our southeast service area over the weekend and gradually work northwestward by Sunday. But confidence in how far north this might get, and when, remains on the lower side. Higher confidence for drier outcomes, especially in west and northern areas. This bears out with 24.12z guidance suggesting even a drier look with lower rain chances and drier ensemble changes loaded in. Probabilities for measurable rain Friday/Saturday went from roughly 30% to 10%. Pattern Shift Early Next Week Nearly a week out but medium range suggesting broad upper ridge sets up across the western CONUS, putting upper Midwest back into a cooler northwest flow regime. Certainly differences in deterministic runs with regards to ridge strength but similiar in idea of temperatures cooling back to more near or below normal for late September via global ensembles. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 623 PM CDT Tue Sep 24 2024 Main taf concern is fog development in low lying areas and river valleys tonight. Light winds/clear skies and strong inversion develops over the region tonight. This will allow for fog to form after 08z in low lying areas and river valleys. The main concern is how widespread fog forms in the Mississippi River Vally to impact the LSE taf site. At this time...kept previous taf mention of IFR visibility potentially from 11z to 14z Wednesday...as confidence is medium on fog impacting the LSE taf site. After 14z Wednesday...VFR conditions should prevail at both taf sites. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Shea AVIATION...DTJ