Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
919 ACUS03 KWNS 191941 SWODY3 SPC AC 191940 Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0240 PM CDT Thu Sep 19 2024 Valid 211200Z - 221200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS PARTS OF NM INTO THE SOUTHERN HIGH PLAINS...PARTS OF THE UPPER MIDWEST INTO THE CENTRAL PLAINS...CENTRAL APPALACHIANS VICINITY... ...SUMMARY... Isolated severe thunderstorms are possible on Saturday afternoon to early evening in New Mexico and the southern High Plains, mid Missouri Valley to upper Mississippi Valley, and central Appalachians region. ...Southern High Plains... In advance of a southern-stream shortwave trough reaching the Four Corners, a strong midlevel jet is forecast to eject over NM by early evening Saturday. This will yield favorable deep-layer shear for organized storms across NM into parts of northwest TX. Strengthening large-scale ascent within a low-level warm/moist conveyor should overspread this region by Saturday morning. This should yield early-period convection and renders moderate uncertainty over the degree of destabilization by afternoon. Still, with ample boundary-layer heating to the south of early-day activity, a plume of at least weak-surface based buoyancy should develop. Renewed afternoon to early evening thunderstorms could become severe. Steep mid-level lapse rates should be confined along and west of the Sangre de Cristo Mountains, rendering further uncertainty over the intensity and coverage of the severe threat. The eastern extent of the threat should be limited by the confined buoyancy plume and decreasing instability deeper into TX/OK. ...Central Plains and Lower/Mid-MO Valley to the Upper MS Valley... A relatively complex and uncertain evolution to convective potential remains evident on Saturday afternoon/evening. This region will largely remain within a low-amplitude mid-level ridge. This may flatten somewhat in between a southern-stream shortwave trough in the Southwest and a northern-stream trough over the southern Prairie Provinces to along the ND/MN international border. In association with the latter wave, a surface cold front should push southeast in the north-central states. Guidance varies with the degree of destabilization ahead of it, and especially with the spatial extent of afternoon/evening convection. There is sufficient confidence in destabilization and storm coverage to introduce a 5% area from the mid MO Valley to the upper MS Valley, though uncertainty remains rather high. ...Parts of the Northeast/Mid Atlantic... A low-amplitude midlevel shortwave trough is forecast to move through northwesterly flow aloft across parts of the upper Ohio Valley into the Mid Atlantic on Saturday. Isolated to scattered thunderstorm development will be possible across the region during the afternoon. Veering wind profiles within the northwesterly flow regime will support effective shear for some storm organization, though it remains somewhat uncertain if instability will become sufficient to support an organized severe threat. Guidance has trended toward somewhat greater instability and storm coverage, and a small Marginal Risk has been included where confidence is currently highest in some severe threat. ..Dean.. 09/19/2024 $$