SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0656 AM CST Mon Jan 25 2021 Valid 251300Z - 261200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY FROM PARTS OF THE ARKLATEX EAST-NORTHEASTWARD INTO PARTS OF KENTUCKY AND TENNESSEE...AND TONIGHT OVER PARTS OF THE DEEP SOUTH... ...SUMMARY... A marginal severe threat is expected today from parts of the Arklatex east-northeastward into parts of Kentucky and Tennessee, and tonight over parts of the Deep South. ...Synopsis... In mid/upper levels, mean troughing will be maintained over the western CONUS as one perturbation exits, and a trailing one amplifies. The leading shortwave trough -- evident in moisture- channel imagery over the central High Plains to the TX Panhandle -- will eject east-northeastward and deamplify across portions of northeastern KS, southeastern NE, northern MO and IA into this evening, reaching the mid/upper Mississippi Valley overnight. The trailing perturbation currently is manifest as a negatively tilted, elongated trough extending along much of the West Coast with primary vorticity max over central CA. A closed cyclone should evolve from that vorticity lobe today, with its 500-mb center near DAG by 00Z, then eastward to near GCN by 12Z tomorrow. At the surface, 11Z analysis indicated a low near MLC with cold front southwestward across the DFW Metroplex to near DRT. The warm front was drawn over southeastern OK and southern AR, becoming diffuse eastward over MS. The low is progged to move east- northeastward to eastern KY through the period, while the warm front remains poorly defined to its east and southeast. The cold front should reach central AR, east TX and deep south TX by 00Z, moving only slowly eastward from there. By 12Z, the front should reach eastern TN, northern/central MS, and southwestern LA. ...Arklatex to parts of Ohio/Tennessee Valleys... A near-frontal band of convection, with widely scattered embedded thunderstorms, is apparent on the western rim of the outlook area. This activity will continue to shift eastward through the day in an environment of favorable deep/speed shear but marginal instability, offering the threat for isolated severe wind gusts and marginal hail. A tornado cannot be ruled out. In the near term, sporadic hail and strong to marginally severe gusts will be possible from the most intense cells embedded in the band, as it moves through marginally but sufficiently unstable airmass to sustain the convection. A relative lull in convective organization may occur through mid/late morning across the Arklatex region eastward to near the Mississippi River, until the activity encounters a boundary layer gradually destabilizing from both muted diurnal heating and low-level theta-e advection, in and south of the ill-defined warm-frontal zone. Deep shear will be favorably strong, but with the 55-70-kt effective-shear vector aligned only slightly rightward of the convective band. As such, the mode should remain predominantly quasi-linear, but with embedded LEWPs, small bows and perhaps occasional supercells. The foregoing airmass will be moist, with low LCL but also poor lapse rates, keeping MLCAPE generally 500-1000 J/kg (locally/briefly higher, especially this afternoon). ...AL, northwestern GA, eastern MS overnight... Widely scattered thunderstorms are possible ahead of the front this evening and overnight, in a regime of favorable vertical shear. Any sustained convection may pose a threat of damaging gusts or a tornado, but coverage and duration of sufficiently deep convection remain too uncertain for more than marginal unconditional probabilities. Still, at least isolated supercell potential exists. Forecast soundings indicate a deep, high-RH to saturated boundary layer above the surface with weak lapse rates, but also, low LCL and surface-based effective-inflow parcels. Peak MLCAPE may reach the 500-1000 J/kg range, in a corridor of relatively maximized moisture/ buoyancy extending southwest/northeast ahead of the cold front. MLCINH will be weak, and a collocated zone of persistent low-level warm advection, moisture transport and modest convergence is expected. Substantial upper support will remain northwest of the area as the shortwave trough ejects, and gradual veering of surface winds with time will keep convergence broad and on the margins for initiating/sustaining supercells. Accordingly, guidance is inconsistent with longevity and coverage of strong-severe convection. However, tight mid/upper height gradients will contribute to favorable deep shear, with effective-shear magnitudes around 50-60 kt possible, and 250-350 J/kg effective SRH even amidst veering surface flow. ..Edwards/Gleason.. 01/25/2021
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