SPC 0830Z Day 3 Outlook
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0207 AM CST Thu Dec 10 2020 Valid 121200Z - 131200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Showers and scattered thunderstorms will affect portions of the eastern U.S. Saturday, mainly east of the Mississippi Valley and south of New England. ...Discussion... Broadly cyclonic flow aloft will prevail across much of the U.S. this period, as eastern U.S. ridging is shunted steadily eastward by the advance of a strong short-wave trough across the Mid South/Ohio Valley. Short-wavelength ridging will also shift onshore over the West during the day, and into the Great Basin overnight ahead of a large northeastern Pacific upper low moving steadily toward the western NOAM coast. At the surface, the primary feature of interest will be a low initially progged to lie over the central Illinois vicinity, with a cold front trailing southward across the Mid Mississippi Valley into Louisiana. With time, the low is expected to move northeastward toward/into the Great Lakes, reaching Lake Huron/Georgian Bay late in the period, with the cold front reaching the Appalachian crest. Showers and thunderstorms should be ongoing at the start of the period, particularly from Illinois/Indiana southward to Louisiana in the vicinity of the cold front. With time, convection will advance/spread eastward, in tandem with frontal progression. At this time, it appears that instability will remain largely minimal, and elevated, and thus severe weather appears largely unlikely. One possible exception could conceivably evolve from coastal Louisiana eastward into central and southern Alabama and western portions of the Florida panhandle. Here, very modest (around 100 J/kg) surface-based CAPE may precede the front, with thunderstorm coverage possibly aided by a weak/secondary vort max progged to move eastward into the central Gulf Coast states during the afternoon. While the lack of more substantial instability will likely act as a strong limiter to any potential risk, shear ahead of the front would be sufficient to support organized/possibly rotating updrafts. At this time, will opt not to introduce 5% probability/MRGL risk for severe weather, but if it becomes apparent that slightly greater CAPE may evolve within the warm sector, low probabilities may be required in later outlooks. ..Goss.. 12/10/2020
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