Continuing the trend of this March, another severe weather outbreak affected the Southeastern United States primarily on March 25th. On this day, there were 42 reports of tornadoes and 51 hail reports (Figure 1). One of these tornadoes reached EF-4 strength in Newnan, GA. A low-pressure system with a central pressure of 1004 mb was the reason for this convective activity. Centered over Arkansas around 11 AM on March 25th, this low was situated in a very favorable region for surface cyclone strengthening and maintenance. Analysis of the synoptic environment at this time can provide more information on why this low strengthened.
Figure 1: SPC Storm Report of the United States for 25 March 2021. This plot displays tornado reports (red), wind reports (blue), hail reports (green), and total reports (black). Image courtesy of the Storm Prediction Center.
The 300 mb level can provide valuable information about the synoptic environment by analyzing the jet stream. At 15Z (11 AM EDT) March 25, a deep trough is present over the Central/Western US extending from the Dakotas & Montana southward to New Mexico & Texas (Figure 2). With respect to the trough, the surface low is located in the region downstream (east of the trough). This region is associated with ageostrophic divergence aloft due to the orientation of the ageostrophic wind vectors. This divergence aloft will induce upward vertical motions in this region via mass continuity. These upward vertical motions will strengthen the surface low. Additionally, 2 jet streaks are present, a smaller one over east Texas and a larger one over the Midwest and Great Lakes Region (Figure 2). With respect to these jet streaks, the surface low is in the left-exit region and right entrance regions. Both of these regions are associated with upward vertical motions due to ageostrophic divergence aloft. As a result, both jet streaks will strengthen and maintain the surface low.
Figure 2: This plot displays 300 mb height (contours, black) , wind speed (fill pattern), and divergence (contours, purple). This plot is from 15Z 25 March 2021. Image courtesy of the SPC Mesoscale Analysis Archive.
Moving vertically downward in the atmosphere, vorticity at the 500 mb level and 700 mb level can be used to further assess vertical motions in the atmosphere. The reason this analysis is possible is due to the QG Omega Equation. Term A (the first term) of this equation is the vertical derivative of absolute geostrophic vorticity advection by the geostrophic wind. More simply put, this term analyzes rotation and how it is moving in the atmosphere. At the 500 mb level, positive vorticity is being advected into the region where the low is centered (Figure 3A). In addition, at the 700 mb level positive vorticity is also being advected into the region where the low is centered (Figure 3B). Increasing positive vorticity advection with height is indicative of mid-tropospheric upward vertical motions, and these motions will strengthen the surface low.
Figure 3: 2A (left) is a plot of 500 mb height (contours, black) and relative vorticity (fill pattern) from 15Z 25 March 2021. 2B (left) is a plot of 700 mb height (contours, black) and relative vorticity (fill pattern) from 15Z 25 March 2021. Images courtesy of Pivotal Weather.
Term B of the QG Omega equation is the Laplacian of geostrophic temperature advection. This a complicated way of saying this term looks at cold versus warm air advection and its role in atmospheric vertical motions. The counterclockwise flow associated with the surface low will advect warm, moist air from the Gulf into the region where the surface low is located (Figure 4). This warm air advection over this region is indicative of mid-tropospheric upward vertical motions due to the QG omega equation. Overall, this surface low was strengthened by upward vertical motions from a variety synoptic factors such as trough/ridge dynamics, jet streak dynamics, and QG Omega dynamics. The synoptic scale environment played a crucial role in strengthening this surface low, which allowed it to take full advantage of the primed mesoscale environment leading to another spring severe weather outbreak.
Figure 4: This plot displays 850 mb height (contours, black) and temperature (fill pattern) from 15Z 25 March 2021. Image courtesy of Pivotal Weather.