SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0650 AM CST Sat Jan 23 2021 Valid 231300Z - 241200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... A few non-severe thunderstorms may develop across parts of southern California near the coast today, and late overnight over northern Louisiana. ...Synopsis/Discussion... A complex, mid/upper-level cyclone -- now covering parts of the Great Basin westward to coastal north-central CA -- is expected to consolidate in favor of its western lobe through the period. This will occur as an eastern vorticity max and original cyclone center -- now apparent on moisture-channel imagery over central NV -- ejects northeastward to the central Rockies, and weakens in confluent flow. Meanwhile, the western portion of the gyre contains vorticity maxima over inland northern CA and southwest of MRY. Those will phase enough to form a new, dominant cyclone moving southeastward down the southern CA coast through this evening. The new cyclone and attendant trough should pivot eastward across southern CA to near the lower Colorado River Valley and northern Baja between 06-12Z. At the surface, mostly cool to cold and stable conditions prevail across the interior CONUS from the Rockies to the East Coast. A wavy/quasistationary frontal zone was drawn at 11Z from central FL across the northern Gulf, bending southwestward/southward off the lower TX Coast. ...Southern CA to southern AZ... Midlevel cooling and related steepening of low/middle-level lapse rates will occur atop a moist marine layer, as the cyclone develops and approaches. This should result in sufficiently deep buoyancy to extend at least locally/briefly through a rising inversion and into icing layers suitable for lightning production near the coast. Forecast soundings suggest 100-300 J/kg MUCAPE, with similar MLCAPE over water and immediate coastal areas as the cyclonic cold core moves nearby. Farther east, an area of widely scattered to scattered showers may develop over northwestern MX and the northern Gulf of CA tonight, moving/spreading north-northeastward over parts of southern AZ. Although forecast soundings suggest CAPE will remain too shallow for an areal thunderstorm threat north of the international border, very isolated lightning cannot be ruled out. ...Northern LA and vicinity late tonight... A few thunderstorms are possible within the last few hours of the period -- roughly 09-12Z -- embedded within a growing area of precip rooted in an elevated inflow layer. A broad area of low-level warm advection and isentropic lift will continue to expand and strengthen through the period over the western Gulf States, beneath broadly anticyclonic mid/upper flow and north of the surface baroclinic zone over the Gulf. Though forcing aloft will be minimal, gradual moistening within the warm advection plume is expected above a relatively stable boundary layer. The increase in theta-e from about 800-900 mb results in MUCAPE 300-800 J/kg off the east rim of the EML after about 09Z, with isentropic lift to an LFC. Modest lapse rates and lack of buoyancy should preclude a severe threat. ..Edwards.. 01/23/2021
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