This year has started with a bang for severe weather season, with a rare double high-risk being issued on the last day of March. We’re going to look at a case that almost mirrored the setup of March 31, 2023, only a few days later on April 5, 2023. The defining feature of this event was a strong NE-SW jet streak (Figure 1) that helped to sustain a surface cyclone of 985mb across most of the US. The surface low for this cyclone is positioned near the southeasternmost tip of Minnesota (Figure 2) at 06 UTC on April 5, right in the left jet exit region (a known promoter of upward motion due to ageostrophic divergence). We’re going to focus on the Trenberth form of the QG-Omega equation today, using it to diagnose upward vertical motion as a result of the advection of vorticity.
Taking a look at Figure 3, we see a large strip of positive relative vorticity extending from the panhandle of Oklahoma to the western edge of Minnesota, and that is what we are going to focus on. First, we must introduce the Trenberth form of the QG-Omega equation. Simplified so it is easier to diagnose operationally, this is given by σ(∇^2+f0/σ ∂2/∂p2 )=2f0 [-VT∙∇(ζg+f)], where we care the most about the right-hand side. This is the absolute geostrophic vorticity advection by the thermal wind. The nice thing about the Trenberth term is we only need one component of it since it neglects the deformation terms found for the traditional form. We can look at our region of high vorticity in Figure 3 and compare it to the flow of the thermal wind in Figure 1. Thermal wind (-VT) flows parallel to thickness contours with lower values to the left of the direction of the flow. Analyzing this in Figure 1, we can assess that the thermal wind is southwesterly. Since the thermal wind is pushing areas of relatively high vorticity to areas with little to no vorticity, there will be positive vorticity advection.
So, what does this mean in terms of synoptic forcing and the life cycle of this system? Because we are maximizing absolute geostrophic relative vorticity advection by the thermal wind, we also maximize the left-hand side of the equation. This, due to the second derivative test, will minimize omega and force upward vertical motion. We can see this in Figure 4, where there is a solid red blob over Minnesota, indicating the analysis is correct. Figure 3 also has a tool to analyze upward motion, the ascent at 500mb in blue contours. There is a healthy amount of blue contours over the overlapping region of red in Figure 4, so it is safe to say that there will be upward motion as a result of vorticity advection. What does this mean for the surface cyclone? It was forecasted to be supported below a 990mb pressure level across the entire US and up into Canada by this synoptic forcing. The surface low also stayed in the left jet exit region throughout it’s travel across North America. It is expected that this storm will create strong severe weather in a wake similar to that of the outbreak of March 31, 2023.
