On Tuesday, April 18th, 2023, a weak area of low pressure moved over the Rocky Mountains. The motion of the associated air column over the mountain range created increased potential vorticity. When an air column is over mountains, it is squished or flattened, and will spin slower – much like a figure skater with their arms out; when an air column clears the mountains, it is stretched and will spin faster – much like a figure skater with their arms tucked in.
One day later, at 12Z on Wednesday, April 19th, 2023, two 1004mb areas of low pressure formed east of the Rockies (Figure 1). Both centers of low pressure were roughly positioned between a trough to the west and a ridge to the east, an optimal spot for ageostrophic divergence. At the base of a trough, the acceleration vector points into the axis of rotation. Crossing the acceleration vector and k-hat, yields an ageostrophic wind vector pointing west. At the crest of a ridge, the acceleration vector points into the axis of rotation. Crossing the acceleration vector and k-hat, yields an ageostrophic wind vector pointing east. Given that these ageostrophic wind vectors are pointed opposite, it can be determined that there is ageostrophic divergence and upward vertical motion by mass continuity. Since the season is transitioning from spring to summer, the jet stream is weakening and there is not much in the way of jet streak dynamics.
The Traditional form of the QG Omega equation can be used to further diagnose upward vertical motions in the vicinity of the surface lows. Since the severe weather occurred late on April 19th, April 20th maps will now be used for analysis. Specifically, maps from April 20th at 0Z will be used. Figure 2 shows the forcing of Term A or the vertical derivative of absolute vorticity advection by the geostrophic wind (-(f0/σ)[∂(-vg·∇ηg)/∂p]). Since the magnitude of geostrophic wind at the center of the surface cyclone is zero, geostrophic vorticity advection is also zero. Therefore, it can be inferred that positive absolute vorticity advection is increasing with height (shown by the yellow and orange shading). Neglecting Term B, a positive term A means that omega is being minimized and there is upward vertical motion. However, the positive absolute vorticity advection with height contributes much less forcing than the geostrophic temperature advection (Figure 3; -(R/σp)[∇2(-vg·∇T)]). Areas with positive term B, neglecting Term A, imply that omega is being minimized and there is upward vertical motion. In other words, the warm sector is getting warmer, and the cold sector is getting colder. Both vorticity advection and temperature advection contributed to significant upward vertical motion in the vicinity of the surface lows.
