Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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260 FXUS63 KARX 250712 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 212 AM CDT Wed Sep 25 2024 .KEY MESSAGES... - Favorable radiation/steam fog (cool nights/warm water) conditions for valley/river valley fog each morning. - Threat for rain decreases as mid-tropospheric low/remnants of tropical storm Helene currently forecast to remain south of the area. - Warmer than normal temperatures through early next week, then a swing to more seasonable weather. && .DISCUSSION... Issued at 204 AM CDT Wed Sep 25 2024 Overview: Water vapor satellite imagery, heights, and lightning showed a closed low over the Mid-Mississippi Valley with a ridge over the Rockies and another trough approaching California. Tropical storm Helene was located over the Caribbean. At the surface, weak high pressure was located over the region with surface low pressure over Michigan and also over the western High Plains. Under mostly clear skies, the cool locations already had low dewpoint depressions. We`ve had valley fog/river valley fog the last two mornings and fog is developing about three hours ahead of the previous mornings. Mississippi River temperatures are in the 70s...thus with temperatures in the 40s to around 50...in addition to favorable radiational conditions/calm winds at KLSE, the water/air temperatures are conducive to fog formation. Nighttime microphysics satellite imagery is also showing the smaller Wisconsin river valleys with fog. Valley fog continues each morning through Friday with above normal temperatures and mainly dry conditions into early next week: Conditions continue to remain favorable for valley fog through Friday morning. Calm to light winds in the low levels, cool nights with few clouds, a low level inversion, and warm Mississippi River water temperatures in the 70s. Some hints at patchy clouds Thursday morning and less of an inversion Friday morning, thus extent of fog could be affected by these factors. The closed 500mb low is forecast to remain over the Illinois with the trough extending toward the Great Lakes. Over time, the closed low retrogrades westward as tropical storm Helene heads toward the Florida Coast. Meanwhile, a ridge of high pressure builds into the Upper Mississippi Valley. The current deterministic models appear to be in good agreement with this evolution and that the two areas of low pressure merge into one over the Mid-Mississippi/Tennessee Valley Friday. Some differences by Sunday where the storm system will edge closer to the forecast area or remain to the south and how quickly the ridge breaks down with colder air arriving next week. With the ridge building; rising heights and warmer 850mb temperatures, we have highs in the 70s to lower 80s forecast for the rest of the week into early next week. The 25.00Z cluster analysis had shifted the rain south of the forecast area. The 24.12Z probability for measurable rain was 0-10%, so would think when the 00Z data comes in it would be even lower. We`ll need to continue to monitor the storm evolution with remnants of tropical storm Helene as the track could change over the next few days. There are still some differences in when the ridge breaks down, but it does look like a return to seasonable or cooler than seasonable highs in the 60s could occur early next week. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1035 PM CDT Tue Sep 24 2024 Main aviation weather forecast concern this period will be potential for valley fog impacting LSE late tonight. Skies should remain mostly clear with light winds at the surface, allowing for a strong surface inversion. On the plus side for fog development, recent forecast soundings show a trend toward a deeper light wind layer just off the surface, and 03Z dewpoint depressions of 5 degrees is very favorable. On the negative side, although winds will remain near calm at LSE, the prevailing direction is more westerly and not the favored south/southeast that typically brings thicker fog from the channel into the airfield. Some model guidance isn`t as "sold" on fog at LSE as they were last night, but their dewpoint depressions at this hour are also larger than current observations. So, medium to high confidence that fog is likely and have steered TAFs that direction, but lower confidence on just how low visibility may tank. Have gone low end IFR visibility and LIFR ceilings and will amend as necessary. VFR conditions for the remainder of the TAF period at both RST/LSE. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...Kurz