SPC 0830Z Day 3 Outlook
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0155 AM CST Thu Dec 17 2020 Valid 191200Z - 201200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... The risk for severe thunderstorms appears negligible across the U.S., Saturday through Saturday night. ...Discussion... Much of North America will remain under the influence of branching westerlies emanating from the mid-latitude and subtropical Pacific, but broad mid/upper ridging developing within this regime may spread inland of the Pacific coast during this period. Downstream, as one short wave trough digs across the southern Rockies and another southeast of the Canadian Prairies, it appears that a short wave trough, initially over the southern Great Plains at 12Z Saturday, will pivot east-northeastward through the lower Ohio and Tennessee Valleys by late Saturday night. A significant surface cyclone, initially developing to the lee of the Canadian Rockies, is forecast to accompany the northern perturbation into northwestern Ontario by the end of the period. Models indicate that this will trail an initially more significant cyclone over Hudson Bay, with a developing cold front within weak surface troughing trailing southward across the upper Great Lakes into the southeastern Great Plains. However, they suggest little in the way of substantive wave development along this boundary as it continues to shift east of the Mississippi Valley. Modest moisture return emanating from the southwestern Gulf of Mexico may reach northwestern Gulf coastal areas by 12Z Saturday. While this may spread inland across upper Texas coastal areas, mostly above a residual surface based stable layer, it appears that it will only contribute to very weak CAPE as far north as the lower Mississippi Valley. ...Southeastern Great Plains into Gulf States... Weak destabilization associated with pre-cold frontal elevated moisture return may support increasing convection capable of producing lightning in a corridor across northeastern Texas through central Arkansas early Saturday. This likely will be aided by forcing for ascent ahead of the approaching short wave trough. As the short wave impulse turns east-northeast of the southern Great Plains, models suggest that one tightening low-level thermal gradient ahead of the cold front (roughly in the 925-850 mb layer), near its intersection, may provide a focus for continuing weak thunderstorm activity. Due to spread among the various model output, how far east remains a little unclear, but low probabilities for thunderstorms may extend as far east as north central Georgia by late Saturday night. Models suggest that another corridor of favorable ascent may support thunderstorm development southeastward into better low-level moisture and instability across southeast Texas and southern Louisiana. However, any intensification into more vigorous thunderstorms capable of posing an appreciable severe threat probably will be confined to northwestern into north central Gulf of Mexico. ..Kerr.. 12/17/2020
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