SPC Sep 13, 2017 Day 4-8 Severe Weather Outlook

By Newsroom America Feeds at 13 Sep 2017

Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0400 AM CDT Wed Sep 13 2017 Valid 161200Z - 211200Z ...DISCUSSION... In mid/upper levels, a longwave pattern characterized by western troughing, and ridging from TX to the Great Lakes, should prevail through most, if not all, of the outlook period. Ensemble guidance reasonably indicates a gradual, partial low-level moisture-return process in advance of the western trough, given a pattern characterized by persistent surface ridging from the northern Appalachians to the northern Gulf. Day 4/16th-17th: The ejecting northwestern mid/upper low/trough discussed in the day-3 outlook should dampen as it moves northeastward across the Dakotas. Timing/amplitude differences already are apparent amongst ensemble members and deterministic spectral/ECMWF/UKMET models regarding shortwave timing/amplitude of this process by late day-4. Regardless, a strong consensus of guidance supports movement of the associated surface cold front out from beneath both the most favorable associated deep-layer flow and large-scale cooling/lift aloft by this time, with possible exception of near the surface low over MN. Strong, perhaps isolated severe convection may develop near that surface low and proximal cold front. However, given the aforementioned moisture-return/upper-support concerns, organized severe potential currently appears too uncertain for a 15% unconditional severe area. Days 5-6/17th-19th: Potential appears low for severe with the leading mid/upper-level shortwave trough having ejected into Canada, and ridging to flat southwest flow aloft (with little upper support) in advance of a deepening northwestern-U.S. cyclone. A strong lee trough should set up over the central/northern High Plains in advance of the northwestern mid/upper low. Days 7-8/19th-21st: A high-amplitude mid/upper trough should develop over the West, with cyclogenesis/frontogenesis probable over the northern Plains and central High Plains. The general configuration of the pattern (positive tilt on the synoptic scale) suggests a good deal of front-parallel flow over the central/northern Great Plains, which would be deleterious to organized severe potential. However, given the strength of flow aloft, shortwave differences are apparent that could influence severe potential on smaller scales. Read more



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