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Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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139 FXUS63 KARX 151750 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1250 PM CDT Thu Aug 15 2024 .KEY MESSAGES... - Widespread showers with a few thunderstorms will sweep east this morning. Some areas may receive more than 1" of rain (15-60% chance) with a small chance (10%) for an isolated pocket with 2"+, mainly south of Highway 18. - A second round of thunderstorms may develop this afternoon and evening. If thunderstorms occur, a few may be strong to severe storms, particularly if a bit of clearing occurs after the morning rain departs. - Friday through next Wednesday looks to feature temperatures just a bit below normal with potential for afternoon showers and storms. && .DISCUSSION... Issued at 355 AM CDT Thu Aug 15 2024 This morning: Widespread rain 08z composite radar shows a large area of showers and thunderstorms ongoing across MN/IA with this advancing eastward toward our CWA. Strongest activity and heaviest rain rates remain to our south and east in IA, in line with the trend seen in the past few runs of guidance to take the main body of the upper wave responsible for this precip farther south. Similarly, 2"+ PWATs should remain just to our south through the morning hours. Thus, while widespread rainfall is still expected, final rain amounts will likely be less than previously thought. Indeed, NBM/HREF probabilities now suggest a 15-60% chance to exceed 1" and a 10-15% chance to exceed 2", with higher probabilities residing in southern portions of the forecast area, mainly south of US Highway 18. That said, deep warm cloud depths suggest efficient rain production should still occur, so will need to remain vigilant, particularly if an embedded thunderstorm takes aim at a more urbanized area. This afternoon and evening: Additional rain, potential severe Next shortwave looks to eject downstream this afternoon, opening the potential for a round of thunderstorms. While progged MLCAPE values will likely climb into the 1500-2000 J/kg range in the presence of 0- 6km bulk shear increasing to around 30 knots, two major limiting factors for the development of strong to severe thunderstorms look to be present as well - convective inhibition and suboptimal shear profiles in the mid-levels. In the case of convective inhibition, if more clearing in cloud cover occurs late this morning and early this afternoon, the aforementioned MLCAPE values would be realized. As for shear, progged hodographs suggest deeper rotating updrafts may struggle to sustain themselves due to relatively weak winds above 3km. Thus, while sfc-500m SRH values of 50 to 100 m2/s2 - sufficient for a tornado risk - look to develop in SW WI, would need to have everything come together for that risk to be realized. Primary concern with any thunderstorms this afternoon instead looks to be damaging winds given modeled DCAPE of around 1000 J/kg. Friday through Wednesday: Seasonable, potential for afternoon showers/storms Friday, as the upper low becomes centered over north central WI, shortwave rotating around this feature should, in concert with peak afternoon surface heating, lead to numerous showers and a few thunderstorms in a textbook high PoP, modest QPF setup. With the upper low potentially directly over Taylor/Clark Counties, will need to watch out for any remnant surface boundaries, as a shower becoming rooted on one of these boundaries could provide enough stretching potential for a funnel cloud or non-supercell tornado to develop. Saturday through next Wednesday, with an upper ridge firmly in place over NM, northwesterly flow aloft will likely be predominant. Thus, expect temperatures to run just a bit below normal for mid to late August with potential for a stray afternoon shower or thunderstorm, particularly if a fortuitously timed upper shortwave helps cool the mid levels. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1245 PM CDT Thu Aug 15 2024 IFR to low MVFR conditions persist east of the Mississippi River for the rest of the day with MVFR to VFR ceilings west of the river by mid-afternoon. Scattered showers and storms move through the region this afternoon and into this evening, but impacts will be isolated to scattered in nature (20-30% probability of rain). Forecast confidence is low for tonight with two scenarios possible. If clouds clear, LIFR fog could quickly develop overnight along and east of the Mississippi River. If clouds persist, fog will have a harder time forming. MVFR ceilings are expected to develop again Friday by midday. Winds will generally be from the SW to W at 5-10kts, increasing to 10-15G20kts for Friday midday. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Ferguson AVIATION...Skow