SPC AC 261957 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0157 PM CST Tue Nov 26 2019 Valid 262000Z – 271200Z …THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHEAST MO…EASTERN AR…NORTHWEST MS…WESTERN TN AND SMALL PORTIONS OF EXTREME SOUTHWEST IL/WESTERN KY…
…SUMMARY… Severe storms, with a threat for damaging winds and a few tornadoes, are possible across parts of the lower to mid Mississippi Valley region this afternoon through tonight.
– MS Valley Region… The Enhanced risk area has been shifted southward with the update. This includes the addition of southeast AR and adjacent portions of northwest MS, and the removal of portions of southeast MO and southwest IL. This change is based on latest trends in HRRR and RAP guidance in conjunction with current observational data. As of 1930Z, isolated, likely elevated thunderstorms have developed over northwest AR as ascent increases ahead of the Plains trough. Additional widespread shower and isolated thunderstorm activity was occurring this afternoon in strong warm advection across much of east TX into the lower MS Valley. The greatest instability is centered on the Sabine Valley where broken cloudiness has allowed for better heating and strong southerly flow has transported upper 60s to low 70s dewpoints. Some showers developing over central LA have at times shown signs of weak cyclonic shear. As wind profiles and forcing increase later this evening, convection should become better organized and more robust as dewpoints increase to the north/northeast. While forcing will be somewhat weaker further south as the Plains trough and surface low eject northeast of AR/LA, this area appears to have the best chance for surface, or near-surface-based storms through the evening. This should result in a relative corridor of maximum potential for both damaging winds and perhaps a couple of tornadoes.
Further north, instability will remain limited by a cooler boundary layer and more modest surface dewpoints and modest lapse rates. However, strong shear/fast storm motion and linear storm mode could support storms capable of widely-scattered damaging winds.
…Northern CA Coast… No changes have been made to this portion of the outlook. ..Leitman.. 11/26/2019 .PREV DISCUSSION… /ISSUED 1030 AM CST Tue Nov 26 2019/
…MS Valley region through tonight… A pronounced midlevel shortwave trough now approaching the central/southern High Plains will eject northeastward toward the middle MS Valley by tonight, in response to an amplifying upstream shortwave trough that will reach southwest OR/northern CA this evening. An initial surface lee cyclone in southeast CO this morning will develop east-northeastward to the Kansas City area by this evening, and continue northeastward toward southern Lake MI by Wednesday morning. The surface warm sector will likewise spread northward/northeastward from east TX/western LA/southwest AR this morning to MO this evening, reaching the corridor from IL to MS overnight. A surface trough/dryline will demarcate the west edge of the moist sector this afternoon/evening from southeast KS into northeast TX, though this boundary will be overtaken by a Pacific cold front early tonight across MO/AR. Boundary-layer dewpoints in the 65-70 F range are spreading northward from the upper TX coast to southwest AR and extreme southeast OK as of mid morning, and northward low-level moistening will continue through the day across AR/southern and western MO/extreme southeast KS ahead of the synoptic cyclone and developing dryline feature. Widespread clouds are present within a subtropical moisture plume aloft from the southern Plains to the MS Valley, and low clouds are prevalent from northeast TX into AR along the primary moisture return corridor. The clouds will mute surface heating today, which casts doubt on surface-based storm development during the day in the warm sector. It appears that two general scenarios for convective initiation are more probable this afternoon through tonight. The earlier scenario will be within the moistening warm sector along the warm conveyor belt across AR later this afternoon/evening. The lack of surface heating suggests storms may remain slightly elevated in a strengthening wind profiles that otherwise supports supercells across AR. Late this evening into early tonight, storm initiation is also expected in a band or two across MO/AR after the Pacific cold front overtakes the remnant dryline. Very strong deep-layer vertical shear and long hodographs are expected from I-40 northward along the path of the ejecting midlevel trough and 120-130 kt midlevel jet streak, which may allow some broken cellular characteristics in the storms as they move quickly northeastward toward IL overnight. The thick clouds across the northern portion of the warm sector today, and the primary wave passing a little north of the established warm sector tonight cast doubt on the severe-weather outcome. Buoyancy will be limited where vertical shear is extremely strong from MO into IL, and the more probable area to initiate warm sector supercells across AR (along the warm conveyor) will be plagued by the thick clouds today. There is still potential for forced/shallow convection with damaging winds along the front tonight from MO into IL, and some potential supercells into AR and perhaps northwest MS overnight. However, overall confidence is low in storm evolution and resultant severe-weather threats, so will largely maintain the outlook areas.
…Northern CA coast this evening into tonight… Strong forcing for ascent will overspread northern CA later this afternoon/evening as a rapidly deepening cyclone and associated midlevel trough begin to spread inland. Strong gradient winds are likely with the frontal passage along the coast, but most of the convection is likely to be confined to the post-frontal environment as steeper low-midlevel lapse rates spread inland. Flow will remain strong in the post-front environment, and cold midlevel temperatures may support some small hail. However, the severe storm threat inland is too limited for an outlook area.
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