SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0745 AM CDT Mon Aug 02 2021 Valid 021300Z - 031200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF CENTRAL/EASTERN OREGON... ...SUMMARY... Strong to isolated severe downburst winds will be possible over central Oregon this afternoon and early evening. ...Synopsis... In mid/upper levels, a large-scale western-ridge/eastern-trough configuration will persist over the CONUS. Height falls will occur across the Southeast as a series of mostly low-amplitude shortwaves traverse the cyclonic flow aloft, while a strong shortwave trough crosses, then departs, New England. West of the mean ridge, a shortwave trough -- apparent in moisture-channel imagery over northern CA -- will move slowly northward over OR today, while weakening. At the surface, 11Z analysis showed a cold front from near the southern NC Outer Banks southwestward over north-central GA, becoming wavy and slow-moving to quasistationary over northern MS, southern AR, southeastern OK, north-central TX (behind outflows), and a cold front again across the Big Bend/Trans-Pecos region of southwest TX. This boundary will undergo a net southward drift through the period, but very unevenly in the absence of substantial forcing aloft. A weak frontal/wave low may ripple northeastward across parts of GA/SC tonight in response to shortwave activity in the area of cyclonic flow and height falls aloft. ...OR... Isolated to widely scattered, high-based thunderstorms are expected to form over portions of central OR today as: 1. Large-scale lift increases somewhat with the approaching shortwave trough, and 2. Strong surface heating/mixing remove MLCINH, mixing lower- elevation dew points from the 50s and 60s into the 30s and 40s F, while leaving just enough low/middle-level moisture to support the convection. Modified RAOBs and forecast soundings suggest the mixed layer may extend as high as around the 600-mb level, with MUCAPE in the 250-500 J/kg range, atop deep inverted-v thermodynamic profiles. Though low/middle-level winds will be modest, with weak deep shear, isolated downdrafts may accelerate to severe limits. This regime also will pose a threat for fire starts from dry lightning; see the SPC day-1 fire-weather outlook for more details. ...Gulf Coast States to southern Atlantic Coast... Widely scattered to scattered thunderstorms are expected to develop episodically throughout the period, with greatest coverage from mid-afternoon into early evening, following the weakest MLCINH of the diurnal cycle. Activity should form in the frontal zone and southward, including along Gulf Coast and FL sea-breeze boundaries and convective outflows. Rich low-level moisture, strong surface heating, and high tropopause/equilibrium levels will contribute to areas of 1500-3000 J/kg MLCAPE, despite modest midlevel lapse rates. Vertical shear and low-level winds will be weak. In such an environment, brief water-loaded downdrafts near severe limits cannot be ruled out anywhere from south TX to FL and the southern Atlantic Coast. Severe potential appears too isolated, conditional and disorganized for a categorical outlook area at this time. ..Edwards/Leitman.. 08/02/2021
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