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Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0355 AM CDT Sun Sep 12 2021 Valid 151200Z - 201200Z ...DISCUSSION... A large-scale upper trough should continue to move eastward from Ontario and the Great Lakes region across eastern Canada and the Northeast on Day 4/Wednesday. Medium-range guidance continues to indicate that rich low-level moisture will be in place ahead of a surface cold front across the Northeast by Wednesday afternoon. Diurnal heating of this moist low-level airmass should foster at least moderate instability. Even though the strongest ascent and mid-level winds associated with the upper trough may be focused to the north in eastern Canada, there should be enough deep-layer shear and low-level convergence along the front to support isolated to scattered severe storms Wednesday afternoon and evening. If storms can congeal into one or more lines along/ahead of the front, then severe/damaging winds would probably be the main threat. There has been enough consistency in guidance to support the introduction of 15% severe probabilities for Day 4/Wednesday across parts of the Northeast, where the most favorable overlap of instability and shear is forecast ahead of the front. Some medium-range guidance suggests that another upper trough may develop eastward across central Canada and the north-central CONUS on Day 5/Thursday. There may be enough low-level moisture return ahead of this feature to support surface-based storms and perhaps an organized severe risk across parts of the northern/central Plains into the Upper Midwest ahead of a cold front. Regardless, there is still far too much uncertainty regarding the evolution of the upper trough, placement of the surface front, and overall storm coverage to include a 15% severe area for Thursday at this time. From Day 6/Friday through the end of the forecast period, differences in the synoptic-scale upper pattern across the CONUS become apparent in medium-range guidance, suggesting low predictability.
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