Severe Storm Outlook Narrative (AC)
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Issued by NWS
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007 ACUS01 KWNS 181258 SWODY1 SPC AC 181257 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Sat May 18 2024 Valid 181300Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF NORTHERN FLORIDA AND THE FLORIDA PANHANDLE TO PORTIONS OF THE SOUTHERN ATLANTIC COAST... ...SUMMARY... The greatest severe-thunderstorm potential today appears to be from the Florida Panhandle eastward to the southern Atlantic Coast, with damaging gusts the main concern. ...Synopsis... Distinct northern- and southern-stream features over nearly opposite parts of the CONUS will continue to exert the greatest influence on severe-thunderstorm potential. In the North, a well-developed, zonal to cyclonic mid/upper-level jet core was evident from the coastal Northwest to the northern Plains. Some amplification in that belt is likely overnight, as a shortwave trough digs southeastward across the Pacific Northwest. Downstream, a compact but strong shortwave trough -- evident in moisture-channel imagery over western ND and southeastern SK -- should move east-northeastward to southern MB and northern MN by 00Z, forming a closed 500-mb low over the Lake Winnipeg vicinity. The attached trough should eject northeastward to northwestern ON by 12Z tomorrow. The associated surface cold front -- drawn at 11Z over eastern MN to near FSD, EAR, GLD and PUB -- should reach Lake Superior, northern WI, central IA, and south-central KS by 00Z, stalling across southwestern KS into eastern CO. By 12Z, the front should reach northern Lower MI, northern to west-central IL, and northern MO, while moving northward as a warm front near the I-70 corridor in KS. Meanwhile, a positively tilted mid/upper-level shortwave trough -- with several associated vorticity maxima -- was evident in moisture-channel imagery from KY across the Mid-South to south-central TX. The main vorticity lobe -- now over the northeastern AR/western TN area -- should shift eastward to middle TN by 00Z, with trough southwestward to TX shelf waters of the Gulf. By 12Z, the trough should extend from the western Carolinas across southern AL. The associated surface low -- analyzed at 11Z near MEM -- should move eastward today and devolve into a weak trough. An outflow-reinforced marine front was analyzed over shelf waters off the middle Texas Coast to just southeast of the Mississippi River mouth to near PNS, then across southern parts of GA and SC. This boundary should drift eastward over the north-central Gulf Coast region and across GA. ...Southeast... Scattered thunderstorms in clusters -- including occasional/embedded supercells -- have been persistent this morning just offshore from the LA/MS/AL coastline to near the western FL Panhandle, reinforcing the marine/outflow boundary on the southern margins of that convection. That, and persistent rainfall north of the boundary across southeastern LA to the AL Gulf Coast, have led to decreasing unconditional severe potential over those areas. Instead, the greatest threat today should be with the eastern/inland extent of the loosely organized Gulf convective plume, which extends along and north of the boundary over the FL Panhandle and southern GA. Damaging wind will be the main concern, with a tornado or two possible, and isolated large hail. This region should continue to exhibit the greatest low-level moisture/instability through the day as the convection undergoes a net eastward shift, with surface dewpoints commonly in the 70s F, areas of cloud-tempered diurnal heating, weak MLCINH and MLCAPE reaching the 2000-2500 J/kg range. Deep shear will favor organized convection, with effective-shear vectors continuing to be commonly in the 40-50-kt range -- albeit aligned with a substantial component parallel to the main swath of convective lift. As such, mode should remain somewhat messy, clustered to linear, with some embedded supercell/bow structures possible. Elsewhere, less-organized convection (mainly due to weaker shear) should occur near the peninsular FL Atlantic Coast, and ahead of the midlevel vorticity lobe and surface low/trough, from the Mid-South across the Tennessee Valley. Isolated damaging gusts and marginally severe hail will be possible. ...Midwest... Thunderstorms should develop from mid/late afternoon into early evening along the cold front -- with coverage diminishing from scattered across the Lake Superior and WI/northeastern IA regions to widely scattered or isolated from southern IA to portions of KS. Damaging gusts will be the main concern, though any sustained, relatively discrete cell may rotate and produce locally large hail as well. Development should occur later over the MO/KS segment than farther north, due to the presence of capping related to an EML. The strongest flow aloft and deep shear should remain behind the surface front, with effective-shear magnitudes generally ranging from around 45-50 kt over the MN Arrowhead to less than 30 kt over southeastern KS. Low/middle-level lapse rates and boundary-layer moisture also should increase with southward extent, but the opposite will be true for deep-layer lift/forcing. A narrow corridor of favorable MLCAPE should develop today ahead of the front, ranging from around 1000 J/kg either side of western Lake Superior to 2000 J/kg over east- central KS. These offsetting factors and the probable quick evolution to quasi-linear mode in northern areas -- combined with a narrow east-west and temporal window for the most vigorous convection before nocturnal stabilization contribute to weakening -- support keeping unconditional probabilities at marginal levels for this update. ...Central High Plains... Widely scattered, high-based, somewhat skeletal thunderstorms are expected to develop over the mountains and foothills of central/ south-central CO this afternoon near the front, and move east-northeastward over the adjoining High Plains from late afternoon into parts of tonight. The main concern will be isolated strong-severe downbursts. Activity will be supported initially by diurnal heating of higher terrain, with marginal but adequate moisture to support convection at those altitudes. While strong gusts are possible in the mountains, the activity should move atop a deep, well-mixed subcloud layer over the downshear central High Plains. That deep boundary layer will be characterized by nearly dry-adiabatic lapse rates, and enough moisture to support 200-500 J/kg MLCAPE. Eastward extent of the threat is uncertain, but in general, should diminish overnight as nocturnal stabilizing of the near-surface profile proceeds. ..Edwards/Goss.. 05/18/2024 $$