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Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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211 FXUS63 KARX 111726 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1226 PM CDT Sun Aug 11 2024 .KEY MESSAGES... - Showers are possible (20-40% chance) south of I-90 late tonight into Monday, but otherwise precipitation chances are low through Tuesday for most areas. - More active weather is likely later in the week, with shower/storm chances trending upward (40-60%) by Thursday. - Temperatures will be fairly seasonable for mid-August for much of the week. && .DISCUSSION... Issued at 215 AM CDT Sun Aug 11 2024 Today-Tuesday: Early this morning, surface high pressure extended from the northern plains southeastward into the Ohio Valley with mostly clear skies locally. Atmospheric moisture profiles are quite dry, with precipitable water values in the 10-25% percentiles relative to climo, per MPX/DVN 00Z soundings. Today should be similar to yesterday, with scattered diurnal cumulus and otherwise plenty of sunshine and temps a few degrees warmer than yesterday as thermal profiles modestly warm. A very low (10-15%) chance for shower exists across north-central Wisconsin where mid-levels remain a bit cooler behind the departing trough across eastern Canada. However, very shallow, weak instability should keep the thunder threat negligible. A shortwave trough across the northern Rockies early this morning will approach the Upper Mississippi Valley region by Monday morning. Instability continues to look minimal and the stronger moisture transport and warm advection are expected to remain farther south across the Mid-Mississippi Valley. As a result, any rain brushing mainly northeast Iowa or southwest Wisconsin late tonight into Monday should be light. An isolated diurnal shower/storm could not be ruled out north of I-94 on Monday with some weak instability forecast and soundings showing skinny CAPE profiles. Otherwise, through Tuesday night the mid-level ridge will build eastward with temps slowly moderating closer to seasonal norms and rain chances low. With lighter boundary layer flow, early morning valley fog would be possible Monday/Tuesday mornings if skies are sufficiently clear. Wednesday-Saturday: As the surface ridge passes east into the Great Lakes by later Wednesday, southerly low-level flow will increase along with a return of a seasonably moist airmass. There has been a reasonably consistent signal in the global ensemble suite for a more pronounced mid-level trough to cross the region, resulting in increased shower/storm potential, especially towards Thursday. NBM rain chances have trended upward over the past 24 hours into the 40-60% range, although among all EPS/GEFS/CPC members, chances for rain at some point Wednesday night through Thursday night are greater than 80%. Differences remain with the amplitude and timing of this trough, limiting confidence in details, but it seems probable that rain chances will increase as model spread decreases over the next few days. It at least continues to look more likely that this will be an opportunity for meaningful rains across the region with NBM probabilities for at least 1/2 inch of rain Thursday-Friday of 40-60%. With predictability remaining lower with the evolution of the trough, confidence in severe weather potential is also low. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1225 PM CDT Sun Aug 11 2024 Predominantly VFR conditions are expected for the TAF period with a low (20%) risk for light showers south of a AEL to DBQ line early in the morning Monday. Confidence in valley fog formation tonight remains on the low side given that high clouds will be passing through during this time. Most other variables do point towards fog, so cloud coverage and thickness will need to be monitored this evening. Winds this afternoon will be 5-10 kts from the W/NW, becoming light and variable this evening into Monday morning. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JM AVIATION...Skow