SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0649 AM CST Fri Nov 13 2020 Valid 131300Z - 141200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Severe thunderstorms are not expected through tonight. ...Synopsis/Discussion... In mid/upper levels, the pattern over most of the CONUS will be nearly zonal through the first half of the period. Cyclonic-flow amplification will occur in much of the West after 00Z, shifting eastward to the High Plains late overnight. This will occur as a strong shortwave trough -- now apparent in moisture-channel imagery just offshore from the Pacific Northwest and southern BC -- moves inland after about 18Z. Multiple closely associated vorticity maxima will contribute to general strengthening toward more of a synoptic-scale trough over the Rockies by 12Z tomorrow, extending south-southeastward from a closed 500-mb low over southwestern SK. In the meantime, cold air aloft and related steepening of low/ middle-level lapse rates -- above a cool but favorable moist marine layer -- are expected over coastal WA/OR. This will contribute to general, isolated thunder potential until the low-topped convection leaves behind its supportive low-level air mass inland. Cooling aloft, steepening lapse rates and weak moisture also may contribute to a nonzero potential for lightning over parts of the northern Great Basin tonight, under the left-exit region of a cyclonically curved upper speed max digging across OR to central parts of NV/UT. However, weak low-level theta-e should limit coverage/duration of convection, and the potential appears too isolated/uncertain to maintain an unconditional area. As heights fall tonight across much of the West and Plains, and the center of a large surface anticyclone retreats slowly northeastward up the Ohio Valley, a broad, strong swath of low-level warm advection will spread northeastward over the southern Plains to mid Mississippi Valley. Accompanying moist advection/transport in the 700-850-mb layer will increase RH under steep midlevel lapse rates. leading to the development of effectively uninhibited MUCAPE in the 100-500 J/kg range. Parcels will be isentropically lifted to LFC. Forecast soundings suggest the buoyancy may extend into thermal layers supporting lightning production in the deepest cells. Lack of both greater moisture and shear should limit organization/ intensity of this convection. ..Edwards/Leitman.. 11/13/2020
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