Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0333 AM CST Thu Dec 17 2020 Valid 201200Z - 251200Z ...DISCUSSION... Models indicate that an initially intense, zonal mid/upper jet across western and central portions of the southern mid-latitude Pacific will undergo considerable amplification through the extended period. This is expected to lead to amplification within the branching downstream flow, with guidance indicating greatest certainty concerning the evolution of a prominent ridge across the northeastern Pacific into the northern U.S./Canadian Pacific coast early next week. It appears that this ridge will build as far north as the Arctic latitudes of Alaska/northwestern Canada, before large-scale downstream mid/upper trough amplification proceeds, from the lee of the Canadian/northern U.S. Rockies through much of the eastern U.S. by early Christmas Day. There is sizable spread concerning the short wave perturbations contributing to this large-scale trough evolution. However, model output has been indicating the potential for strong cyclogenesis, either with an initial low developing east-northeast of the middle Missouri Valley though the Great Lakes region by mid week, or with a secondary low developing to its southeast by late week. Associated strengthening of deep-layer wind fields within the evolving warm sectors will pose a conditional risk for increasing severe weather potential, given sufficient instability. However, moisture to contribute to appreciable destabilization remains unclear, as deep Gulf of Mexico boundary-layer moistening to support a substantive inland return flow remains hampered, particularly with another mid-level perturbation forecast to pivot across the Southeast early this week. Still, there may be at least some potential for one or two low-topped convective lines to evolve, which could be accompanied by an enhancement of risk for damaging wind gusts, across parts of the Ohio Valley on Wednesday, and parts of the eastern Gulf/south Atlantic Coast states on Christmas Eve.
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