Severe Storm Outlook Narrative (AC)
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Issued by NWS
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110 ACUS01 KWNS 171628 SWODY1 SPC AC 171626 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1126 AM CDT Tue Sep 17 2024 Valid 171630Z - 181200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE CENTRAL HIGH PLAINS... ...SUMMARY... Thunderstorms capable of producing scattered to numerous severe wind gusts and isolated large hail should occur this afternoon and evening across much of the High Plains. Some of the wind gusts may be significant (75+ mph). ...High Plains and Central/Southern Rockies... Expansions have been made to the Slight and Enhanced Risks across the northern/central High Plains to account for latest trends in guidance and observations. The Marginal Risk has also been expanded westward to include more of western CO and western/central WY, where isolated strong to severe winds are already ongoing with high-based convection. A strong upper trough/low over the Great Basin will pivot east-northeastward across the northern/central Rockies through this evening. An associated 50-60 kt mid-level jet will move eastward across the central/southern Rockies this afternoon, and over parts of the adjacent High Plains through this evening. As large-scale ascent with the upper trough and mid-level jet overspreads these regions, a surface low is forecast to deepen over eastern MT through this evening, while another low also develops over the central High Plains. A surface trough will extend southward from the central High Plains low, and it should sharpen as it develops eastward across the central/southern High Plains through the day. Greater low-level moisture, characterized by generally mid 50s to low 60s surface dewpoints, will gradually increase today along/east of the surface trough, while remaining more limited across the higher terrain of the central/southern Rockies. Moderate instability should develop to the east of the surface trough across much of the High Plains through peak afternoon heating. Strengthening deep-layer shear with the approach of the mid-level jet will support organized convection, including some potential for initial supercells across the northern High Plains (eastern MT and vicinity). Related threat for large hail will exist with any of this more discrete convection. Otherwise, thunderstorms will continue to develop this afternoon over the central/southern Rockies while producing isolated strong to severe wind gusts in a weakly unstable but strongly sheared environment. This activity will eventually spread off the higher terrain and into the central/southern High Plains. Steepened low-level lapse rates with robust daytime heating, along with more limited/shallow low-level moisture along/west of the surface trough, will likely support a gradually increasing severe wind threat with the high-based convection spreading east-northeastward across the southern/central Rockies. As these clusters and linear segments encounter greater low-level moisture and related instability along/east of the surface trough across the High Plains late this afternoon into early evening, the threat for scattered to numerous severe wind gusts should increase. This may particularly be the case across parts of the central High Plains, where the best combination of steepened low-level lapse rates, moderate instability, moderate to strong deep-layer shear, and greatest linear convective signal exists in 12Z guidance. Some chance for isolated gusts of 75+ mph may also be realized across this region with the more intense clusters spreading east-northeastward in tandem with the mid-level jet. The threat for severe winds is expected to gradually decrease with eastward extent across the Plains this evening as convection encounters increasing MLCIN with nocturnal cooling of the boundary layer. ...Florida... A weak mid/upper-level low will remain centered over the Southeast today. Modest mid-level westerly flow will persist over FL on the southern periphery of the upper low. Scattered to numerous thunderstorms should develop later this afternoon along/south of a remnant front, and along the Atlantic Coast sea breeze. Even with daytime heating of a rather moist low-level airmass, poor mid-level lapse rates should temper updraft intensities to some extent, even as moderate instability develops. That said, the stronger cores may be capable of producing isolated strong to damaging winds, with 20-25 kt of deep-layer shear aiding in modest thunderstorm organization. Some adjustments have been made to the Marginal Risk across parts of the FL Peninsula, based on latest observational trends and 12Z guidance. ...Southeast Virginia/Northeast North Carolina... A weak convective band is ongoing across far northeast NC into southeast VA, on the northeast side of a mid/upper-level low over the Southeast. Occasional strong/gusty winds may occur with the more robust cores this afternoon as the convective band pivots further into southeast VA. Even so, weak instability, modest low/mid-level flow, and related weak shear should temper overall thunderstorm organization. Accordingly, severe wind potential should remain low. ..Gleason/Moore.. 09/17/2024 $$