On May 14th I posted several images representing our (Juston Drake and I) Spring 2013 chases up till that day. The following day, May 15th, we documented a large tornado East of Ringold, TX. The period of significant severe weather activity over the Great Plains the following two weeks, May 15-31, surpassed all other days from the Spring of 2013 combined. Several substantial tornadoes developed on May 18 near Rozel, KS including a tornado rated EF4 by mobile radar data. An outbreak occurred over Central Oklahoma on May 19, which included two violent tornadoes affecting the towns of Carney and Shawnee. The following day we documented a violent and deadly tornado with damage rated EF5 over Moore, OK. The following week continued to be very active with several significant severe weather and tornado days topped by possibly the most intense tornado ever documented near El Reno, OK on May 31st, which tragically took the lives of several storm chasers and other victims.
The large-scale global pattern during the early-mid Spring of 2013 was not conducive for severe weather. However, a powerful MJO wave developed over the Indian Ocean during the first two weeks of May and propagated eastward, which significantly changed the pattern over North America. A long-term drought over the Southwest United States provided a substantial temperature inversion, which played a significant role in several strong tornado days. The significant temperature inversion (CAP) present near 700mb on May 18, 19, 20, 28, and 31 combined with very deep moisture, relatively cool upper-level temperatures and strong deep-layer shear provided environments for powerful tornadoes. The environments, which developed on May 19th and 31st were especially volatile and conducive for powerful violent tornadoes with near-storm estimated effective-layer EHI’s approximately 12-18 and 18-23 respectively (values calculated by Juston Drake and Simon Brewer). Had the Shawnee or the El Reno tornadoes moved over heavily populated neighborhoods catastrophic damage would have likely resulted. Luckily these two tornadoes passed over relatively less populated areas, which minimized their impact.
Below are several images from May 15th through June 14th:

Incredible Supercell Structure looking South at early Multivortex Stage of El Reno EF5 Tornado May 31 2013