Day 4-8 Outlook

Day 4-8 Outlook Image

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0330 AM CST Wed Jan 20 2021

Valid 231200Z - 281200Z

Medium-range models are in reasonably good agreement with respect to
the synoptic-scale features across the U.S. through roughly Day 5
(Sunday).  Beyond that, increasing divergence in solutions suggests
insufficient predictability to make any meaningful assessment of
convective potential.

Within the Day 4-5 time frame, the primary feature of interest will
be the large trough situated over the West at the start of the
period.  More specifically, a short-wave feature moving through the
broader-scale cyclonic flow -- which is progged to lie near the
central California coast at the start of Day 4 -- will substantially
influence convective potential in the medium range.

Day 4 (Saturday), this short-wave feature is forecast to shift
southeastward, moving into the southwestern U.S./northern Baja
during the overnight hours.  As this occurs, an increase in
southerly low-level flow is progged over the southern Plains.  This
trend will continue into Day 5, as the upper system shifts eastward
and crosses northern Mexico/Arizona/New Mexico -- but while
weakening steadily as it progresses eastward.  In response, rather
ill-defined cyclogenesis is expected to occur over the Texas
vicinity.  As ascent increases and low-level moisture advection
continues, an increase in convective potential will occur across the
southern Plains and into the lower Mississippi Valley.  However,
despite favorable shear that would otherwise support severe
potential, some likelihood for persistent/weak boundary layer
stability beneath low-level capping is a concern.  Still, some
severe potential could evolve, possibly focused along a weak
east-to-west warm frontal zone across east Texas and the Arklatex
region later Sunday and into Monday.  However, degree of risk
remains questionable at this time, such that an outlook area will
not be introduced at this time.  

By Day 6 (Monday), the weakening upper system is progged by the GFS
to essentially dampen out with time, as it crosses Oklahoma and
Kansas, and shifts into Missouri.  Meanwhile, the ECMWF maintains a
much more well-defined feature, that moves quickly across the Ozarks
and Mid Mississippi Valley by evening, and then into/across the
Carolinas and Virginia by the end of the period.  Along with this
more pronounced upper feature, a corresponding/well-developed
surface cyclone is also progged to cross the southeastern quarter of
the country Monday, suggestive of at least some severe potential
spreading eastward across this region should the pattern evolve more
similar to the ECMWF solution.  

At this time however, given the increasing model differences beyond
Day 5/Sunday, no severe-weather assessment will be attempted through
the remainder of the medium-range period.

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