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Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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674 FXUS63 KARX 121126 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 626 AM CDT Mon Aug 12 2024 .KEY MESSAGES... - Although a few showers could occur in spots today, chances for more widespread showers/storms continue to increase later this week, highest (60-80%) Wednesday night and Thursday. - Temps will be warmer, near seasonable values, for most of this week. && .DISCUSSION... Issued at 246 AM CDT Mon Aug 12 2024 Today-Tuesday: A few ripples in the flow were evident in GOES water vapor imagery early today across the Dakotas into Iowa/Minnesota. Regional radar mosaics showed scattered weak returns across southern Minnesota and northern Iowa with a few surface reports of light rain. However, the bulk of showers/storms should pass farther south across central/southern Iowa into Missouri today where the warm advection/moisture transport signal is focused. A few showers or sprinkles could graze southeast Minnesota and northeast Iowa. Otherwise, much like the past few days, isolated to scattered showers/storms are possible diurnally across northern into central Wisconsin this afternoon where weak destabilization is expected to occur. Although the airmass remains dry, a favorably deep light wind layer, relatively clear skies, and surface ridge axis crossing the region may lead to some fog development tonight, especially in valleys, but patchy fog could not be ruled out area-wide in the scenario. Gradual warming will continue into Tuesday as mid-level ridging spreads eastward from the plains. Wednesday-Friday: Surface ridging will slide east into the Great Lakes on Wednesday with return southerly flow developing and deep layer moisture beginning to increase. Some showers/storms could develop (20-40%) through the day with the increase in moisture transport/warm advection, especially with any weak lead shortwaves. However, the higher shower/storm potential is expected for Wednesday night and Thursday as a broad mid-level trough crosses the region. NAEFS precipitable water values increase above the 90th percentile relative to climatology during this time and among all GEFS/EPS/CMC ensemble members, probabilities for at least 1/2 inch of rain through Thursday are generally 40-60% with 20-40% chances for 1" or more. Thus, although there is uncertainty how this system will evolve, especially with convection involved, confidence in a more widespread precipitation event continues to grow, with NBM rain chances continuing to increase (60-80% chance). Global model spread in temps is larger on Thursday, likely owing uncertainty in storm track and impacts of clouds/rain. No strong signal for severe weather is evident given the greater uncertainty in details of the trough/surface pattern evolution and potential for clouds/showers to limit destabilization. The trough may linger across the area on Friday with some continued shower chances, but spread in potential outcomes for the trough increases in time. Next Weekend: Heading into next weekend, guidance generally suggests ridging expanding across the southern Rockies/plains downstream from a larger trough across the Pacific Northwest. Our late week trough will move east across the Great Lakes. Lower end rain chances (10-30%) linger due to potential proximity to the larger trough, although this evolution remains unclear. Overall, seasonable temps are favored with global ensemble interquartile ranges for high temps in the 70s to mid 80s. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 626 AM CDT Mon Aug 12 2024 Valley fog will be the main TAF concern towards the end of the forecast period with light surface winds, lessening cloud cover, and a fairly deep light wind layer in the 11.09z RAP. Consequently, already have a high degree of confidence that valley fog will occur at KLSE for Tuesday morning to introduce a BCFG beginning at 09z with category reductions potentially required in upcoming forecast issuances. Another possibility that will need to be considered with surface high pressure directly overhead is the possibility of more widespread fog outside of the river valleys which may bring visby reductions at KRST for Tuesday morning. However, with mixed signals in model visby guidance, have opted exclude any fog mention at KRST at this time. Otherwise, VFR conditions are expected through much of today with decreasing cloud cover and with winds remaining light and variable. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JM AVIATION...Naylor