Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
907 ACUS02 KWNS 161732 SWODY2 SPC AC 161730 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1230 PM CDT Mon Sep 16 2024 Valid 171200Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL HIGH PLAINS INTO NORTHEAST MONTANA... ...SUMMARY... Isolated to scattered severe gusts and hail are possible Tuesday across a large swath of the High Plains region, mainly from late afternoon into the evening. The greater concentrations of severe storms may occur over the central High Plains into northeast Montana. ...Synopsis... A mid/upper cyclone is forecast over the Great Basin Tuesday morning. This system will pivot northeast across the central Rockies and into the northern High Plains through the forecast period. This will bring a belt of strong south/southwesterly deep-layer flow across CO/WY and adjacent portions of the High Plains. A surface low will deepen over MT through the period, with a surface trough extending southward through the central/southern High Plains. A warm front is forecast to extend west to east from southeast MT into north-central SD during the morning. With time, this boundary will lift northward toward the International border during the nighttime hours. As the system deepens and the low-level baroclinic zone over the Plains strengthens, 850-700 mb flow is forecast to markedly increase during the afternoon/evening to 30-50 kt. While deeper boundary-layer moisture is forecast to remain east of the surface trough axis and north of the warm front, steep midlevel lapse rates will support pockets of weak to moderate instability. An arc of thunderstorm activity, with a mix of line segments and discrete cells, are expected to develop in the vicinity of the surface low/warm front south/southeast along the surface trough during the afternoon and evening hours. ...Eastern MT... Strong ascent will overspread the region as the upper trough pivots northeast during the late afternoon/evening. Boundary-layer moisture will be greatest to the north of the warm front across north-central/northeast MT, but capping will likely remain in place. Storm motion will likely result in convection remaining elevated to the cool side of the warm front. Nevertheless, steep midlevel lapse rates and favorable vertical shear (elongated/straight hodographs) will support some risk for large hail. As stronger forcing and sufficient erosion of capping can overlap during the evening, strong to severe wind gusts also will be possible. ...Northern/Central High Plains Vicinity... Convection is expected to develop over the higher terrain of CO/WY during the afternoon. Given strengthening mid/upper flow and weak instability, some locally strong gusts could occur with this initial activity near the core of the upper low over western/central CO. As convection shifts east/northeast through the afternoon and into the evening, strong heating is expected to result in a steep low-level lapse rate environment across the High Plains. While deep boundary-layer moisture will remain further east across the Plains, steep midlevel lapse rates and surface dewpoints in the 50s F will result in a corridor of around 1000-1500 J/kg MLCAPE from far eastern CO into parts of western NE/SD. Strengthening midlevel flow atop a deeply mixed boundary-layer will likely support severe gusts (isolated gusts to 75 mph possible) with linear segments spreading northeast across the central High Plains during the late afternoon/evening. ...Southern High Plains Vicinity... Further south, large-scale ascent will be weaker, with stronger height falls focused north of the region. Nevertheless, low-level convergence along the surface trough and modest instability (up to 1000 J/kg MLCAPE) will support initial thunderstorm development over the higher terrain of south-central CO into north-central NM. Forecast soundings show supercell wind profiles across the adjacent High Plains by afternoon. Midlevel lapse rates will be somewhat modest (around 6.5-7 C/km), but elongated/straight hodographs and plenty of instability through the 700-300 mb layer suggest isolated hail is possible. Steep low-level lapse rates also will support isolated strong to severe gusts. ..Leitman.. 09/16/2024 $$