Severe Storm Outlook Narrative (AC)
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Issued by NWS
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104 ACUS01 KWNS 270606 SWODY1 SPC AC 270604 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0104 AM CDT Thu Jun 27 2024 Valid 271200Z - 281200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN MONTANA TO NORTHERN AND CENTRAL HIGH PLAINS.... ...SUMMARY... Severe thunderstorms producing gusts to 80 mph, large hail greater than 2.5 inches, and a few tornadoes are possible across parts of eastern Montana into western portions of the Great Plains this afternoon and evening. Additional severe storms are possible over parts of the western US and the Southeast. ...MT and the Dakotas... An upper ridge centered over the Rockies will weaken under the influence of a deepening mid-level low over the Pacific Northwest. Ascent from the deepening low and strengthening flow aloft will aid in the development of a surface lee cyclone over eastern MT. Surface-pressure falls will increase low-level advection of rich boundary-layer moisture over the central and northern Plains northwest into eastern MT and the western Dakotas by midday. Steepening mid-level lapse rates, increasing large-scale ascent, moist advection, and diurnal heating will rapidly destabilize the air mass, allowing for scattered thunderstorm development. Convective development appears likely over several areas of the northern Rockies/High Plains by early to mid afternoon. A cold front trailing the low in central MT will likely initiate first, followed by the surface low in eastern MT and western ND, and along a trailing dryline into portions of western SD and far northeastern WY. With moderate buoyancy and increasingly strong deep-layer shear of 40-50 kt, convection should rapidly organize into supercells. Steep mid-level lapse rates and the initial discrete mode will favor large to very large hail with these supercells. Backed southeasterly upslope flow and large low-level CAPE near the surface low also suggest the potential for a couple tornadoes, especially with supercells that remain discrete into the early evening. Eventual upscale growth into a cluster or MCS with a threat for damaging gusts is likely later this evening, and could persist overnight with ample buoyancy and a 40-50 kt southerly low-level jet over the northern Plains. ...Central and southern High Plains... Widely scattered storm development is expected late this afternoon along the dryline from western SD into western NE, as temperatures warm and inhibition is removed from upslope flow and the approach of the upper low. Rich moisture (dewpoints in the upper 60s to low 70s F) and steep mid-level lapse rates will support large buoyancy by early afternoon. Albeit weaker, on the southern fringes of the better flow aloft, adequate deep-layer shear will support supercells capable of all hazards, with eventual upscale growth occurring this evening. Farther south, more isolated storm coverage is expected into parts of eastern CO and western KS. Likely higher based, owing to very warm temperatures in the upper 90s to low 100s F, damaging winds and hail will be possible with a mix of mutlicells and transient supercells. Some clustering is expected into the evening hours with the risk for damaging gusts shifting into parts of central KS and northern OK. However, the exact eastward extent remains unclear, given stronger capping and weakening upper-level support to the south and east. ...Eastern UT into Western CO and southern WY... Modest ascent from the approaching trough combined with diurnal orographic circulations will support scattered thunderstorm development across parts of the Intermountain west from mid morning through the afternoon. Relatively high dewpoints in the low 50s F and steep lapse rates will aid in afternoon destabilization, with MLCAPE of 1000-1500 J/kg forecast. Scattered organized storms, including supercells, should form over the higher terrain and along the trailing cold front/wind shift eastern UT into southwest WY and northwest CO. Damaging winds and hail are likely with the more robust storms. While low-level shear is not overly strong, favorable terrain interactions and the supercell mode could also support a tornado or two. ...Southeast... An upper trough over the eastern COUS will phase with a modest subtropical vort max rounding the eastern periphery of the rapidly flattening ridge over the southeastern CONUS. Subtle ascent from the phasing of the two troughs will support scattered afternoon thunderstorms over portions of AL/GA and into the Carolinas along a diffuse surface boundary. A very warm and moist air mass will allow for moderate to large buoyancy supporting strong updrafts despite poor mid-level lapse rates. While effective shear appears to be only modest (generally less than 15-20 kt), heavily water-loaded downdrafts may produce isolated pulse-type damaging gusts. A remnant MCV/weak upper-level support may also allow for the development of a loosely organized cluster of storms with time over portions of eastern GA and southern SC. A localized corridor of more sustained severe potential may develop, though confidence in exactly where remains low. ..Lyons/Wendt.. 06/27/2024 $$