Severe Storm Outlook Narrative (AC)
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Issued by NWS
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910 ACUS01 KWNS 121952 SWODY1 SPC AC 121950 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0250 PM CDT Wed Jun 12 2024 Valid 122000Z - 131200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN NORTH-CENTRAL/NORTHEAST MN TO FAR NORTHWEST WI... ...SUMMARY... Scattered severe thunderstorms capable of very large hail, damaging wind gusts, and a few tornadoes are expected across parts of the Upper Midwest during the late afternoon and early evening. ...20Z Update... No appreciable changes were made to the categorical outlook across the Upper Midwest, beyond a minor adjustment over the Red River Valley, with the previous forecast appearing to be largely on track. Given destabilization trends and impact of early-day/ongoing convection, the southern extent of the severe threat in the IA vicinity might be a bit overdone. Still, some severe wind/hail risk may spread back into northern portions of the state later this evening. For additional short-term discussion regarding the severe threat across the Upper Midwest, see MCDs 1238 and 1239. Over south FL, low-level winds have both slightly weakened and veered relative to late morning when transient supercell structures occurred. Still, with deep convection persisting across the southern peninsula into early evening, will maintain the level 1-MRGL risk for another outlook cycle. ..Grams.. 06/12/2024 .PREV DISCUSSION... /ISSUED 1131 AM CDT Wed Jun 12 2024/ ...Upper Mississippi Valley/Upper Midwest... Bands/clusters of convection are ongoing at this time, from the Minnesota Arrowhead southward across northern Wisconsin, and southwestward across southern Minnesota into the Mid Missouri Valley region. This convection continues moving eastward, toward a less unstable airmass, which should temper severe risk over the next few hours. Greatest potential for a couple of marginally severe storms appears to exist over southwestern Minnesota and adjacent portions of southeastern South Dakota and northwestern Iowa, where the greatest instability resides. Greater severe risk is forecast to evolve later this afternoon, north of the ongoing storms. Here, daytime heating ahead of an advancing cold front (associated with a low-amplitude short-wave trough embedded in fast westerly flow aloft, and moving eastward along the international border) will result in moderate to strong instability (2500 to 3500 J/kg mixed-layer CAPE) within the pre-frontal warm sector. Weakly veering but strengthening flow with height across the region will provide favorable shear for organized/rotating storms. Initial convection -- expected to initiate in the 12/20Z to 12/21Z time frame -- should quickly organize/become supercellular, with very large hail likely to be the initial/primary severe risk. A couple of tornadoes will also be possible, though veered low-level flow should temper overall tornado risk. With time, storms are expected to cluster/grow upscale, with small-scale bowing expected, which should yield a gradual increase in damaging-wind potential. Storms will spread rather quickly eastward/southeastward with time, with some severe risk likely to linger through the evening before gradually diminishing overnight. ...South Florida... Widespread convection is ongoing across southern Florida, within a zone of low-level warm advection south of a weak cold front lying across central portions of the Peninsula. Modest lapse rates aloft, but a very moist/tropical boundary layer is resulting in deep CAPE, which will continue to support the widespread convection. While mid-level flow remains rather weak, flow veers/increases with height through the lower troposphere. This is providing ample shear for generally weak/transient updraft rotation. Given the associated potential for a brief tornado or two, low (2%) tornado probability/MRGL risk is being introduced across portions of South Florida. For additional short-term information, please refer to SWOMCD #1237. $$