2. MOTIVATION Goal: An increased understanding of the types of severe weather systems (lines/clusters) that prompt large warnings. Concern is large polygons with few if any reports: Training? Verification? Population? Radar sampling? Workload? Fear? Conclusion: Environment + Radar Signatures  Better Decisions

3. NARROW THE FOCUS Consider “LARGE” SVR polygons (> 7000 sq km). Consider “SPARSELY VERIFIED” polygons (< 20% coverage). Over 72% of large polygons from GL offices (139 of 191) were sparsely verified from 2008-2013.

4. LSR 15KM BUFFER Polygon Area: 2088 sq km 1 LSR and 43% Areal Verification Polygon Area: 12538 sq km 5 LSRs and 21% Areal Verification

5. QUESTIONS Can we better distinguish between high-end and low-end lines or clusters of storms? ** Use both environment and radar signatures ** For lower-end lines/clusters, can we focus the warning area? Are we willing to take a few hits to avoid over-warning? Is there an SPS or IBW approach? Is over-warning even a problem??? Are we missing something else?

8. LARGE (>7000 SQ KM) POLYGONS: 08-13

9. 2008-2013

11. WHAT DO WE KNOW ABOUT DAMAGING LINES/CLUSTERS? Wind is main concern with large polygons? Favorable Environments Marginal to large (sfc-based?) instability. Marginal to large shear (0-6km or 0-3km?). Cold pool potential, LFC height, wind/shear orientation… Radar Characteristics Fast motion, tight reflectivity gradients, rear-inflow jets, rotational couplets, MARC signatures, cell intersections, leading/trailing/parallel stratiform, bookend vortices, reflectivity tags….

12. FAMILIAR RESEARCH Corfidi, 2003

13. MORE RECENT RESEARCH Schaumann and Przybylinski, 2012

14. 2008 AUG 4 VS 2012 JUN 29

15. 20080804 23 UTC20120629 14 UTC 3km VGP and 0-3km Shear Vector

19. RESULT 4-5 August 2008: Warning 163 from 0018z-0130z 68% areal verification Widespread damage 29 June 2012: Warning 54 from 1544z-1645z 8% areal verification (Still a HIT!) ~60 mph gust, power lines down

21. VGP and 0-3km Shear Vector 06/12/2013 23Z

22. VGP and 0-3km Shear Vector 07/03/2012 01Z

23. CONCLUSIONS Use large polygons based on morphology, not geography. Large polygons best suited for derecho / bowecho types. A large but ordinary line or cluster may warrant warnings, but only for smaller portions. Environmental factors: Orientation of 3km Shear Vector to lines of cells Strength of 3km shear relative to cold pool