Severe Storm Outlook Narrative (AC)
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Issued by NWS
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243 ACUS01 KWNS 212007 SWODY1 SPC AC 212006 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0306 PM CDT Sat Sep 21 2024 Valid 212000Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ...SUMMARY... Scattered severe storms capable of all hazards are possible this afternoon and evening across portions of the southern High Plains. ...20Z Update... Severe probabilities have been trimmed behind convection in parts of western Pennsylvania as it appears potential for severe storms will be diminished through the remainder of the afternoon. In north-central/northeast Iowa, temperatures have risen into the low 90s ahead of the cold front and near an outflow boundary in the southeast Minnesota/west-central Wisconsin. This area could see a locally higher potential for large hail/damaging winds. Still, linear forcing along the front and very modest mid-level ascent reduces confidence in a discrete storm mode and sufficient storm coverage, precluding any increase in severe probabilities. ..Wendt.. 09/21/2024 .PREV DISCUSSION... /ISSUED 1127 AM CDT Sat Sep 21 2024/ ...Southern High Plains... A potent upper low and associated 65-75 kt mid-level jet max centered over AZ are expected to quickly move eastward, overspreading the southern Rockies/High Plains this afternoon and evening. At the surface, sustained southerly flow ahead of a deepening surface low and trailing cold front over western NM will support moderate destabilization (MLCAPE 1500-2000 J/kg) with afternoon heating and continued low-level upslope. As the mid-level system approaches, mid-level height falls and flow aloft will intensify, elongating low and mid-level hodographs, allowing for convective development/organization in the form of supercells. One source of continued uncertainty is the ongoing elevated convection and cloud debris centered north of a diffuse surface boundary stretching from central NM to the western TX Panhandle. So far, this boundary has been slowly lifting north with pronounced low-level theta-e advection taking place to the south. As deep-layer ascent intensifies through the late morning, continued convective development to the north may support reinforcement and stalling of the boundary. The northward extent of destabilization remains unclear, though this boundary could serve as a focus for renewed development of more robust convection expected later this afternoon. Most CAM guidance shows scattered thunderstorms initiating off the higher terrain of eastern NM, or ahead of the cold front farther west by early this afternoon. These storms should quickly mature with supportive thermodynamics and kinematics, eventually spreading eastward into the TX Panhandle. The risk for large to very large hail/severe gusts will likely accompany the stronger supercells, and a threat for a couple tornadoes may be realized later this afternoon/early evening from eastern NM into parts of adjacent west TX, especially near the boundary where low-level shear may be locally enhanced. ...Upper Midwest and MO Valley... A second upper low near the international border is forecast to quickly move eastward reaching the western Great Lakes by early this evening. Weak ascent is expected ahead of a surging cold front and diffuse wave cyclone across the northern MO valley and southern MN. Surface moistening with a warm front lifting north near the IA/MN border may act as a focus for more robust convective development along the cold front late this afternoon. Increasingly orthogonal deep-layer flow and steeping low/mid-level lapse rates could support a mix of supercells and multi-cell clusters. A few of the stronger storms may pose some hail/wind threat, which should peak in intensity during the early evening. However, somewhat limited heating and the lack of broader forcing for ascent suggests storm coverage should remain isolated. ...Western PA into the Mid Atlantic... Short-wave troughing across the upper OH Valley within mid-level northwesterly flow is expected to support isolated to widely scattered thunderstorms across western PA/VA later this afternoon. Model forecast soundings with moderate buoyancy and modest deep-layer shear would imply some of the stronger storms being capable of an isolated risk for wind damage (50-60 mph) and perhaps some hail. This activity will likely diminish by early evening coincident with nocturnal cooling. $$