Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Tue Sep 12 2017 Valid 121300Z - 131200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF CENTRAL CALIFORNIA... ...SUMMARY... Isolated damaging gusts are possible from thunderstorms over parts of central California this afternoon and early evening. ...Synopsis... The upper-air pattern is characterized largely by two substantial cyclones -- a long-lived, cold-core low currently offshore from southern/central CA, and a large, increasingly baroclinic vortex over parts of the Tennessee Valley region associated with the remains of Irma. The western upper cyclone is forecast to pivot eastward toward the central CA coast late in the period as height falls (related to a shortwave trough over BC) spread across the Pacific Northwest. The southeastern deep-layer cyclone is expected to turn northward across western/middle TN today, then north-northeastward over western KY tonight, with a positively tilted 500-mb trough southwestward to near the TX coast by 12Z. At the surface, an 11Z analysis showed the surface low (former center of Irma) near BHM, with occluded front to coastal northeastern SC, warm front across extreme southern NC then offshore the southern Outer Banks, and cold front arching southwestward over central FL to the central Gulf. The warm front should move very slowly northward today, while the cold front continues moving eastward with some weakening likely as the parent cyclone fills. ...Central CA... Isolated to widely scattered thunderstorms may develop this afternoon over western parts of the Central Valley and coastal mountains, moving northward to northwestward with the potential for strong/isolated severe gusts under the most intense cells. The environment will be characterized by weakening MLCIN though the afternoon, related to a combination of seasonally rich boundary- layer moisture and strong surface heating. Morning analysis shows a pocket of mid/upper 60s F surface dew points from N of BFL through FAT to SFO. The 12Z OAK sounding indicates the moist layer is very shallow and prone to mixing, but also, with enough moisture in low/middle levels to support nearly 1.25 inch PW -- a value consistent with RAP-based objective analyses southeastward through much of the San Joaquin Valley. This will support MLCAPE rising to around 500 J/kg inland through the afternoon, amidst 30-40-kt effective-shear magnitudes. That buoyancy will reside atop a well-mixed subcloud layer conducive to evaporative downdraft acceleration, beneath any sustained convection that can form. The potential appears strongly diurnally driven, and should wane quickly after sunset. Convective coverage and lack of more robust deep-layer forcing are limiting factors. ...Coastal NC... A southeasterly low-level fetch associated with the weakening remnants of Irma will persist across this region today east of the cold front and south of the warm front. Embedded/weak convergence zone will supporting plumes of mostly weak convection, with isolated to widely scattered embedded thunderstorms, moving northwestward off the Atlantic. A very moist air mass will remain in place near the coast and offshore, characterized by surface dew points in the mid 70s F. However, with relatively warm air aloft and only a small wedge of mostly precip-modified warm-sector air ashore, instability will remain very limited, with substantial MLCAPE only near the immediate coast and over the ocean. As the deep-layer cyclone center remains well west of this area and continues to fill, the weakening of peripheral kinematic fields -- as noted the past several hours of VWP from Wilmington -- should continue. While the supercell threat has not entirely disappeared near the coast, the risk for tornadoes appears to have become too small and conditional to maintain categorical-level probabilities this period. ..Edwards/Bunting.. 09/12/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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