Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0330 AM CST Wed Jan 20 2021 Valid 231200Z - 281200Z ...DISCUSSION... 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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