There was a significant history of severe thunderstorms and tornadoes across the central United States for several days prior to May 11 (May 7-10). Synoptic and mesoscale settings over the Midwest on May 10 are compared to those of May 11 over the eastern United States in order to identify notable similarities and differences. The degree of low-level shear was similar on both days, indicating that the wind shear profiles on the 11th were supportive of severe, organized linear convective systems and/or supercells. However, the strong shear on May 10th was well balanced by substantial convective available potential energy (CAPE), while the CAPE on the 11th was much less, due in part to the presence of a strong mid-tropospheric capping inversion. In addition, evidence is shown that a deep layer of large-scale forcing for ascent occurred with the storms over the Midwest on May 10th, while the forcing for upward motion was restricted to the lower-troposphere on the 11th. Meanwhile, strong subsidence was indicated over the mid-troposphere on the 11th, associated with the cap. Several possible reasons for the development of this cap are outlined. Thermal and moisture parameters within the boundary layers of the warm sector environments on May 10th and 11th are also contrasted.
Numerical model performance over the eastern states on May 11 are assessed. For some model runs, middle and upper-tropospheric jet streak features were misplaced, leading to an incorrect forecast of where the strongest jet induced vertical motions would develop. The misplacement of the jet induced subsidence likely led to errors in the strength and placement of the cap, while forecast errors regarding the jet induced ascent resulted in problems assessing associated factors such as the amount of environmental destabilization and forcing for convection.
Lastly, radar imagery from New York State and Pennsylvania on May 11 is investigated. This data show that a few thunderstorms over central New York and northeast Pennsylvania developed significant low-level rotation, however there was little associated severe weather. It is hypothesized that one reason for the lack of severe weather was that weak updrafts associated with the storms could not support strong reflectivity cores aloft. Another possible factor was that the warm sector associated with this event was very narrow. As a result, the rotating storms that developed within the warm sector quickly moved east of the warm sector into a region where the lower-troposphere was relatively stable. The low-level stability may have been sufficient to keep strong winds from reaching the surface, and may have inhibited tornadogenesis.