Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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945 FXUS63 KARX 270846 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 346 AM CDT Fri Sep 27 2024 .KEY MESSAGES... - Above normal temperatures and mainly dry conditions into early next week. After this morning, the valley fog should be more limited this weekend due to the stronger winds aloft. - 10-20% probability for a shower north of I94 with the cold front Tuesday night...then turning cooler for Tuesday. && .DISCUSSION... Issued at 330 AM CDT Fri Sep 27 2024 Overview: Water vapor satellite imagery, heights, and lightning showed a closed low over the Tennessee River Valley/northern Mississippi with Hurricane Helene, a category 4, already dropping to a category 2 and centered over southern Georgia. A ridge of high pressure is over parts of the Southwest U.S. into the Upper Mississippi Valley with a trough over Manitoba with zonal flow farther west. At the surface, high pressure was located over the region, however the trough over Manitoba was dragging a cold front through the Dakotas. With the clear skies,calm winds, and low dewpoint depressions, river valley fog had re-developed. Of note also is the cirrus shield making it into southern Wisconsin and parts of eastern Iowa. Above normal temperatures and mainly dry conditions into early next week. After this morning, the valley fog should be more limited this weekend due to the stronger northeast winds aloft: Hurricane Helene is forecast to merge with the 500mb low to the west. This area of 500mb low pressure stalls over the Ohio/Tennessee Valleys through Sunday, gradually lifting northeast into the Mid- Atlantic region Tuesday. Meanwhile, a cold front over the Dakotas will moves through Minnesota and wash out. Mid-tropospheric ridging builds back in for Saturday. Cirrus will increase, but with time, patchy low and possibly mid level moisture will try to work into parts of southern WI. The strengthening northeasterly flow is forecast to mitigate fog development for Saturday and Sunday morning. That being said, the low levels do decouple and could see some isolated valley fog develop, thus will still need to monitor this through the weekend. The ECS/GEFS/GEPS probabilities for rain through the weekend remain quite low, only 0-10% through Monday. The 850mb thermal ridge is more over MPX`s area with similar temperatures today locally compared to yesterday. For Sunday, the 850mb temperatures rise, but then some cooling occurs at 850mb for Monday. Highs in the 70s to lower 80s remain on track into early next week. 10-20% probability for showers north of I94 then turning cooler for Tuesday: The deterministic EC/GFS are consistent in bringing a cold front through Tuesday night. The ECS/GEFS/GEPS probabilities for rain through 01/12Z are 10 to 20% north of I94. A more zonal flow pattern develops for Wednesday through Friday with chances for light rain returning sometime Wednesday night through Thursday night. Only one day of the cooler highs in the 60s Tuesday, then back to seasonable temperatures to end the week. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 1108 PM CDT Thu Sep 26 2024 Valley fog remains the main aviation concern with LIFR/IFR fog and low-stratus at KLSE for the approaching morning hours. Conditions appear to remain favorable for valley fog development across much of the local area with a light wind layer to around 5-6kft in model soundings and falling dewpoint depressions. Minimal cloud cover is expected through early morning, although may need to watch if any cloud cover can sneak (10-20% chance) from the southeast which may hinder fog development. Otherwise, trends appear similar to previous nights with dewpoint depressions at KLSE being 6 degrees F as of 27.03z. As a result, have introduced a tempo for 1/4 SM at KLSE, favoring later into the morning when considering potentially drier soils hindering fog formation some and also seasonal climatology allowing fog to linger later into the morning. After fog/low-stratus lifts, VFR conditions are expected through the remainder of the TAF period with some high clouds and generally light winds under 8 kts. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...Naylor