Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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903 FXUS63 KARX 150008 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 708 PM CDT Sun Jul 14 2024 .KEY MESSAGES... - Storm chances persist for this evening into Monday evening. A few storms may be strong to severe with damaging wind gusts and localized flooding being the primary threats primarily on Monday. Large hail is a secondary threat. - Hot and humid conditions continue into Monday with highs in the upper 80s to near 90. Widespread heat indices in the lower to upper 90s with some areas reaching 100 on Sunday. - Cooler and drier conditions beginning Tuesday and into much of the upcoming week with highs in the middle to upper 70s. && .DISCUSSION... Issued at 304 PM CDT Sun Jul 14 2024 Humid With Storm Chances This Evening, Some Strong to Severe: Muggy conditions are the name of the game this afternoon with observed dewpoints across much of the region well into the 70s. Air temperatures maintaining into the lower to mid 80s are maybe a touch cooler than expected with some cloud cover around but still relatively on track to see highs reach into the upper 80s in spots by late afternoon and early evening with heat indices in the middle to upper 90s in some locales. As we head into the evening hours, our convective environment is essentially a powder keg with guidance showing the potential for the cap to erode across southern portions of our area during the evening hours (shown in the KDBQ 14.13z RAP sounding at 6pm). When considering MLCAPE values of 2000-3000 J/kg across our south would expect storms to initiate very quickly with anything that provide any kick in the atmosphere. Looking at GOES-16 water vapor imagery, a remnant MCV can be noted across western and central MN that is slowly trekking southeast accompanied with a subtle shortwave. As a result, many of the CAMs rapidly develop a cluster of storms across portions of northeast IA and southwestern WI later this evening. Currently, the main question with this is the aforementioned cap which tries to erode with this weak forcing. How much the cap erodes will determine convective coverage. Shear profiles appear fairly weak with 0-6km shear in the 14.16z RAP being fairly minimal, around 20-30 kts. As a result, would expecting a more multi-cell configuration with individual cells struggling to maintain. Consequently, would only expect to see hail with more robust updrafts. However, with fairly robust DCAPE values collocated with the MCV, would think that cores could collapse fairly vigorously which could lead to isolated downburst gusts to 60 mph. Storms will quickly exit the region to the southeast with the window for storms being likely a few hours (through about 8-9pm at the latest). Severe Weather Potential Early Monday into Monday Evening: As we head into the overnight, forecast soundings show in the 14.16z RAP has low-level saturation and fairly light winds throughout the low-levels. Consequently, with much of the recent rainfall we have seen over the past few days, could see areas of fog initially during the overnight, with questions remaining as for how long it maintains as an incoming MCV approaches. As this MCV and accompanying 500mb shortwave approaches, expecting to see convection as it approaches the local area during the morning hours. As it then progresses eastward a surface cold front is draped across our region and pushes south with additional convective initiation possible. As a result, with severe regimes in play for Monday several different scenarios may be possible. Starting with the morning MCV, as the shortwave moves through the local area (Which also still maintains some uncertainty in timing amongst deterministic guidance) during the morning hours, modest MLCAPE of 2000-3000 J/kg would certainly provide enough of a boost to get convection going. However, deterministic soundings (14.15z RAP) would suggest a cap is still largely in place during the morning hours which would be a serious hindrance to convection as noted in the 14.12z HRRR which has storms fizzling out as they approach the local area. However, if storms can form an organized cold pool with the higher effective bulk shear in the vicinity of the MCV. This type of convective complex (as illustrated in the 14.12z ARW) would likely pose some damaging wind threat if it can utilize the instability gradient and get through the cap. As we head into afternoon, the aforementioned cold front pushes through the region during the afternoon. The key question as we approach this time of day is how the cap will behave, the CAMs vary considerably on this with the 14.12z HRRR keeping the cap in place throughout the day leading to minimal convective development as the front progresses through. This is contrasted heavily by the most aggressive solution in the 14.12z ARW which would favor an organized MCS along the boundary pushing south with relatively uncapped thermodynamics and plenty of 0-3km shear (roughly 35-50 kts in the 14.15z RAP) for forward propagation and potential bowing structures. As a result, if this scenario were to manifest would expect damaging winds to be the primary threat with heavy rainfall potentially leading to flash flooding concerns where soils are already very saturated. Consequently, the SPC has a slight risk (Threat level 2 of 5) across much of the local area. The last caveat to this whole forecast is an organized MCS across central IA that some of the recent CAMs have been trying to pick up on. While much of the CAMs mainly keep it south of the area through the afternoon and evening hours, fairly favorable 0-3km shear profiles and MLCAPE of 3000-5000 J/kg in the 14.15z RAP would enable a bow echo type structure to push east with the potential for damaging wind gusts (perhaps approaching 70-80 mph). Furthermore, this is highlighted in the 14.12z HRRR neural network probabilities which has modest probabilities (40-50%) for winds over 50 kts across portions of northeastern IA. With some uncertainty in exactly where the MCS positions itself the SPC has put extreme southern portions of our forecast area across northeast Iowa and southwestern WI in an Enhanced risk (Threat level 3 of 5) to address this potential. However, many questions remain such has how any prior convection`s outflow could affect the environment and available instability for this MCS. As a result, will be very important to stay tuned to upcoming forecast updates as mesoscale details become more fine tuned. Regardless, if storms materialize will want to watch the heavy rain potential as well with precipitable waters of around 2" and fairly respectable warm cloud depths (around 3.5 km in the 14.12 NAM) which would lead to some efficient rainfall makers. While the 14.12z EC/GEFS ensemble generally only has medium probabilities (30-60% chance) or over 0.5" through Monday night, the 14.12z HREF ensemble max paints spots of 2-3" over the next 48 hours which would suggest areas that receive repeat rounds of storms could see locally heavy rainfall amounts. As a result, when considering prior rainfall from yesterday of anywhere 2-5" leading to saturated soils, localized flash flooding may be possible. Cooler and Dry Pattern Begins Tuesday: Going into Tuesday, a broad scale synoptic trough pivots its way into our region, aiding in the shove cold air advection further south resulting in a cooling trend through the middle of next week. As a result, the 14.00z grand ensemble (GEFS/EC ensemble/Canadian ensemble) with an inter-quartile range of highs at La Crosse for Wednesday of 71 to 76 with a median of 74. Median temperatures in the grand ensemble generally trend in the middle 70s to lower 80s. Additionally, with increasing subsidence with surface high pressure building in mid-week, would expect minimal precipitation chances through Friday. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 707 PM CDT Sun Jul 14 2024 Much of the showers and storms will remain south and southeast of the TAF sites for tonight. On Monday, there will be 2 areas of showers and storms moving southeast through the region. The first will move through the morning and the other during the afternoon. Still some uncertainty on the strength of these storms, so just went with vicinity showers at the tAF sites. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Naylor AVIATION...Boyne