Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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331 FXUS63 KARX 050245 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 945 PM CDT Thu Jul 4 2024 .KEY MESSAGES... - Moderate to High Confidence for Strong to Severe Storms Later This Afternoon & Early Evening. - Scattered to Isolated Pop-Up Shower & Storm Chances Friday & Saturday. - More Widespread Storm Chances For Sunday, Returning Monday Night and Tuesday Night. && .DISCUSSION... Issued at 1223 PM CDT Thu Jul 4 2024 Ongoing Storms & Precipitation This Morning & Afternoon: Have been monitoring multiple rounds of storms through this morning into the early afternoon with a quasi-closed upper-mid level area of lower heights in the Northern Plains and multiple surface frontal boundaries draped across the Upper Midwest. This area of lower heights is also embedded in a negatively tilted, amplified upper level longwave trough on GOES water vapor imagery early this afternoon. As a result, return flow has been advecting a more moist, more unstable airmass through the Plains into the Upper Mississippi River Valley. Initial, original synoptic forcing lifted through the Upper Mississippi River Valley early this morning, leaving an occluded surface low early this morning, placing the triple point to our west. Further occlusion and closing of the tightened warm sector has been firing off storms through western Minnesota early this afternoon. In the wake of morning precipitation, clearing skies seen on visible satellite imagery and are helping to build diurnal instability. Subsequent Forcing This Afternoon-Evening: With a formation of a subsequent low along the triple point with better synoptic forcing, quick-hitting deepening is expected, causing subsequent storm and precipitation chances later this afternoon and evening. The low is expected to trudge east-southeastward along the baroclinic boundary from west to east from central Minnesota into central Wisconsin, bringing the once stationary cold/dry surface frontal boundary with it. The passage of this frontal boundary will provide sufficient forcing for strong to severe storms. Timing of peak diurnal heating will be primary influence for location of highest impacts. Strong To Severe Storms This Evening: Confidence for strong to severe storms is moderate to high as the low center impacts the local area and the cold/dry surface frontal boundary provides sufficient subsequent forcing to conditionally unstable, nearly straight 7.0C/km soundings in upstream 04.12Z RAOBs. Overcoming any low level PBL inversion has been observed within the upstream clearing skies, shifting northeast early this afternoon through northern Iowa and southern Minnesota. Sufficient multi level speed and directional shear expected to sustain storms through the evening hours. High resolution forecast model soundings and associated hodographs suggest the potential for an initially discrete storm mode, albeit transiently, quickly congealing cold pools eventually making things messy before the mode becomes linear along the frontal boundary as 850-300 mean wind is weak, 10 to 20 kts. Taking the expected cold/dry frontal boundary motion into consideration, storm motion would be 20 to 30 kts to the east. Expected & Possible Hazards Into The Evening: All in all, primary hazard of large hail with strong to damaging winds possible. A quick spin up tornado cannot be ruled out where the low level shear vector best aligns (normal to line). Limiting factor will be initial storms potentially consuming initial low level instability. Shower & Storm Chances Friday: As the low shifts east of the local forecast area, wrap around cold air advection and tightened surface pressure gradient will result in perpetual shower and storm chances through Friday. Increased steepening of low level lapse rates concurrent with peak diurnal heating will allow parcels to tap into a long, skinny SBCAPE profile. The combination of transient saturation and little to no shear will result any storms quickly popping up and down. Could see some very limited opportunity for small hail given the cooler sounding but no shear quickly terminates any updraft sustenance, limiting overall chances and extent. Shower & Storm Chances This Weekend: The active pattern is expected to continue through the weekend. Upper level heights are never able to build, persisting synoptic troughing, perturbations, and precipitation chances. Saturday storm chances seem similar to Friday with waves of low level cold air advection within the upper level cold tongue, wobbles in the upper level low, and peak diurnal heating will cause scattered showers and isolated storms. A spatially limiting factor will be a narrow tongue of increased temperatures, increasing low level lapse rates and weakening any incoming storms. Through Start Of The New Week: Slightly different story for Sunday as diurnal heating advects a more moist airmass farther north into the Northern Plains. In turn, a widespread area of diurnally building instability affects much of Minnesota with long term global ensemble certainty for 500-1000 J/kg of SBCAPE taking shape. Have continued with National Blend which has Likely chances, for Sunday Night, Monday Night, and Tuesday Night. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 945 PM CDT Thu Jul 4 2024 Main taf concerns are IFR/MVFR overnight into Friday. Surface low spins over the region and ceilings will lower to MVFR/IFR at both taf sites later tonight/early Friday morning. However confidence in timing of the MVFR/IFR conditions is low. The lower ceilings linger into the daytime Friday. At this time have MVFR conditions prevailing at both taf sites...as low stratus has not developed upstream underneath surface low. Then...the question is how long MVFR/IFR conditions last at both taf sites during the day Friday. For now have kept ceilings improving after 18z Friday. Scattered showers will rotate around the surface low into both the taf sites Friday. Introduced vicinity showers at both RST/LSE taf sites. && .HYDROLOGY... Issued at 400 AM CDT Wed Jul 3 2024 We continue to monitor river trends with the recent heavy rains with most rivers continuing to fall, however flood warnings continue for the Mississippi River and the Kickapoo at Viola. General rains of .5 to 1.5" are forecast with the greater confidence along and north of I90. Some spots per the 04.00Z HREF could see 2"+ with a 20-40% chance for Winona/Trempealeau Co. 6-hr flash flood guidance is generally 2 to 3 inches for flash flooding. Cannot rule out localized flash flooding with the storms later today and tonight if storms repeat over the same area. Be weather aware with outdoor activities and if traveling, be aware of your surroundings. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAR AVIATION...DTJ HYDROLOGY...Zapotocny