Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0645 AM CST Sat Jan 07 2017 Valid 071300Z - 081200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorms are expected across central and southern Florida today but without appreciable severe-weather risk. Isolated thunder also is possible tonight in parts of west-central California. ...Synopsis... The middle/upper-level pattern will be dominated by three primary features: 1. A progressive synoptic-scale trough, currently anchored by a strong, positively tilted shortwave perturbation extending from southern OH over middle/eastern TN to southeastern AR. The shortwave trough will eject across the Mid-Atlantic Coast around 00Z then reach the Canadian Maritimes by the end of the period. 2. Ridging from Baja northward across Idaho to the northern Rockies. Some amplification is forecast as... 3. A closed cyclone offshore from BC continues to cut off from its initiating flow branch over northwestern/central Canada. This feature should retrograde southwestward toward a 500-mb-low position 700-800 nm west of the WA coast by 12Z. To its south, a long southwesterly fetch of moist/subtropical marine air in low/middle levels, juxtaposed with large-scale destabilization from embedded/low-amplitude shortwave perturbations, may support thunder potential tonight over parts of west-central CA. At the surface, an elongated frontal-wave cyclone was located offshore from the Carolinas, with a cold front southwestward across northern Florida to the central/southern Gulf. The low is forecast to deepen and move in a curving path toward Nova Scotia through the period. The trailing cold front will continue to sweep southeastward down the FL peninsula, departing the Miami-Homestead area and the Florida Keys by 00Z. ...FL... Widely scattered to scattered thunderstorms are expected to move east-northeastward across the peninsular warm sector and near the cold front itself through mid/late afternoon. Appreciable convective potential will end from northwest to southeast generally in tune with cold frontal passage. The severe-weather threat, while not yet zero, has diminished substantially and become too isolated and conditional to introduce any categorical-threshold probabilities. An increasing share of the prefrontal sector has become thermodynamically favorable thanks to veering boundary-layer flow related to the deepening Atlantic low and associated isallobaric forcing offshore. That is bringing relatively high-theta-e Gulf trajectories across the peninsula and making effective-inflow parcels surface-based, based on modified RAOBs and model soundings. The resulting surface dew points reaching upper 60s to low 70s F and related 1.5-1.75-inch PW, combined with diabatic surface heating later this morning, will contribute to MLCAPE 1000-1500 J/kg, locally higher. However, the aforementioned veering also will continue to reduce directional shear, low-level hodograph size, and storm-relative boundary-layer flow with time, as well as temper convergence and lift along/ahead of the cold front. Nearly unidirectional deep-layer wind profiles will help to maintain quasilinear convective modes. These factors suggest the severe threat has peaked and should continue to decrease through the morning, despite the boost in buoyancy from inland heating. A few cells may produce gusts approaching 50-kt severe limits and capable of minor damage, but the organized severe threat appears minimal. ..Edwards/Mosier.. 01/07/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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