Day 4-8 Outlook

Day 4-8 Outlook Image

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0333 AM CST Thu Dec 17 2020

Valid 201200Z - 251200Z

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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