SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0636 AM CST Sat Jan 09 2021 Valid 091300Z - 101200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Isolated thunder may occur this afternoon and evening over parts of central/south Texas and New Mexico, and tonight over coastal areas of northwestern California and southwestern Oregon. ...Synopsis/Discussion... A complex, progressive, somewhat split-flow pattern will prevail in mid/upper levels. Continental/polar air in the boundary layer, left behind the last substantial cold-frontal passage, will render the low-level air mass by itself unsuitable for supporting thunderstorms over the CONUS from the Rockies eastward. However, isolated thunder -- from convection rooted well above the surface -- may develop over parts of the southern Rockies region (perhaps associated with winter precip) and parts of central/south TX, this evening and overnight. Isolated thunder also is possible late tonight over parts of the OR and nearby CA coastlines. The main two upper-air features relevant to convective potential will be: 1. A strong shortwave trough now apparent in moisture-channel imagery over the Great basin. This feature will dig southeastward across the Four Corners region today, evolving a closed (or very nearly so) 500-mb low this evening and overnight over NM. By 12Z, the trough should extend from the central High Plains across far west TX to north-central MX, with primary vorticity lobe likely positioned between FST-ELP. A preceding swath of strong large-scale DCVA/lift in midlevels will cross NM overnight, beneath strongly difluent mid/upper flow, steepening 700-500-mb lapse rate quickly to near 8 deg C/km. Where saturation occurs near the base of that layer, CAPE may develop, straddling the -20 deg C isotherm. Forecast soundings indicate MUCAPE will remain less than 100 J/kg, and any resulting thunder should be very sporadic. This outlook area appears the most marginal and uncertain of the two associated with this perturbation, in terms of lightning-coverage potential, due to a lack of more-robust theta-e beneath the zone of intense midlevel cooling. Meanwhile, a very immaturely modified, yet still marginally moist, return-flow air mass will spread over the relatively stable boundary layer across parts of southwest through central/southeast TX tonight. Lapse rates steepened by warm advection and moistened at the base of the buoyant layer will yield 100-400 J/kg MUCAPE, rooted largely in the 600-750-mb layer. Buoyancy sill be activated by parcels isentropically lifted to LFC, within a large area of precip (some of it becoming snow in northwestern areas) and embedded convection. 2. A weaker shortwave trough that now forms the basal perturbation of a synoptic cyclone over the northeastern Pacific, south of AK. The basal trough -- currently located west of CA between 143W-146W -- is progged to split eastward from what is now the coldest, most lightning-active part of the cyclone. This southern lobe then should penetrate the mean larger-scale ridge as it reaches the northwestern CA/southwestern OR coastline around 09Z. In the few hours before this trough reaches the coast, sufficient midlevel cooling may occur to foster MUCAPE up to about 200 J/kg, including some icing layers potentially supporting lightning generation. Any thunder threat will be isolated, decreasing quickly with inland extent. ..Edwards.. 01/09/2021
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