Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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447 FXUS63 KARX 261658 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1158 AM CDT Thu Sep 26 2024 .KEY MESSAGES... - Favorable conditions for morning river valley fog through Friday. - Forecast remains dry (85 to 100% probability) as mid- tropospheric l ow/remnants of Hurricane Helene continue to be forecast southeast of the area. - Warmer than normal temperatures through Monday, then a pattern change to more seasonable weather. && .DISCUSSION... Issued at 255 AM CDT Thu Sep 26 2024 Overview: Water vapor satellite imagery, heights, and lightning showed a closed low over the Mid-Mississippi Valley with a ridge from the Plains into the Southwest U.S. Helene is now a category 1 hurricane and was located over the Gulf of Mexico. At the surface, broad high pressure was located over the region. Similar to Wednesday morning at this time, under mostly clear skies with calm or light winds and low dewpoint depressions, river valley fog has already begun to form. Mississippi River temperatures are still in the 70s. Morning river valley fog continues through Friday with above normal temperatures and mainly dry conditions into early next week: The closed 500mb low over Illinois is forecast to settle south today as Hurricane Helene is forecast to increase to a category 3. Meanwhile, a ridge of high pressure continues to build into the Upper Mississippi Valley. The current deterministic models continue to be in good agreement with the evolution of the closed area of low pressure sagging southward and merging with remnants of Hurricane Helene, gradually lifting north and east toward the Ohio River Valley and eventually the Mid-Atlantic States this weekend. Still some differences for the weekend as the operational European is slightly farther northwest with the track and slightly slower with it exiting the region. The ECS/GEFS/GEPS probabilities for rain through the weekend remain quite low with 0-15% through Monday. With the ridge building; rising heights and warmer 850mb temperatures, we have highs in the 70s to lower 80s forecast into early next week. At this time, the ridge breaks down Monday with colder air arriving Tuesday, where highs dip back into the 60s. The 25.12Z probability for measurable rain Monday night was 5-20%. The 25.12Z cluster analysis for early next week has a variety of solutions related to the strength of the trough/ridge across Canada and the northern U.S. This plays into a widening of solutions for Max and Min temperatures from Tuesday and beyond. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1159 AM CDT Thu Sep 26 2024 Light winds and generally VFR conditions will continue through the TAF period. Like most days this week, conditions will be favorable for early morning fog development in river valleys and low-lying portions of central Wisconsin on Friday. Given the climatologically favored time of year combined with conducive environment (clear skies, deep light wind layer), have included a period of LIFR fog/stratus at KLSE Friday morning. 1/4 SM fog may be added in subsequent updates if trends hold. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...JM