Upper Level Dynamics Associated with a Strengthening Low Pressure System (March 24-25th, 2023) (Author: Nadiyah Williams)

On March 24th, 2023 at 12Z, a longwave trough persists over continental United States, with its axis digging into the north-northwest edge of Mexico (Figure 1). A straight jet streak exists east of the axis over New England with wind speeds of over 200 knots. The placement of the jet streak aids in the dissipation of the trough by March 25th and may have lead to instability within the upper atmosphere that aided the strengthening of the 997 hPa surface cyclone situated at the intersection of Arkansas, Tennessee, and Mississippi.

Figure 1. 250 hPa upper level map from March 24, 2023 at 12Z. The longwave trough is situated over western US and the jet streak east of the trough. The placement of the jet streak will help weaken the trough. The surface low is at the intersection of Arkansas, Mississippi, and Tennessee. (SPC Upper Air Maps Archive).

Following the traditional four-cell model of straight jet streaks in the Northern Hemisphere, upward vertical motion is expected in the right jet entrance and left jet exit regions due to the divergence of ageostrophic winds (k/f × dv/dt) and mass continuity (∂p/∂t+▽⋅(ρu)). By March 25th, significant ageostrophic divergence can be seen in both regions, as denoted by the irrotational wind vectors within the vicinity of the surface cyclone (Figure 2). Instability within the region was also encouraged by the lifting of south winds by the cold frontal passage due west of the surface low. Both of these factors contributed to strengthening the surface low in Arkansas.

Figure 2. 300-200 hPa map initialized at 00Z March 25, 2023. The irrotational winds show that the ageostrophic winds are diverging over the area of interest (within the vicinity of the 997 hPa surface cyclone). Ascent from 600-400 hPa is shown in red. (GFS Archive – Alicia M. Bentley).

Implementing the Trenberth (1978) version of the quasi-geostrophic equation, which is given by σ(▽2+f02/σ ∂2/∂p2)ω=2f0 [-(vT) ⋅▽(ζg+f)], there is positive absolute geostrophic vorticity advection by the thermal wind. Evaluating the right-hand side of the equation, the thermal wind can be found using a 1000-500 hPa thickness map initialized at the same time. Because there is no absolute geostrophic vorticity at the surface and the Coriolis parameter f is positive in the Northern Hemisphere, the 500 hPa map of relative vorticity can be used directly to determine the sign of the equation. In Figure 3, it is shown that the southwesterly thermal wind is advecting positive values of absolute geostrophic vorticity within the vicinity of the surface cyclone. This makes the overall sign of the left-hand side of the equation positive, and this indicates upward vertical motion, which strengthens the surface cyclone to 989 hPa by March 25, 2023 21Z (Figure 4).

Figure 3. 500 mb relative vorticity map initialized at 00Z on March 25, 2023. Ascent values are shown in blue, and the thermal wind vector is shown with a black vector. The thermal wind is advecting positive values of absolute geostrophic vorticity within the vicinity of the surface cyclone, which leads to upward vertical motion. (GFS Archive – Alicia M. Bentley).
Figure 4. Surface analysis taken at 21Z on March 25, 2023. The surface cyclone has strengthened just north of Michigan over Lake Huron to 989 hPa. (WPC Surface Analysis Tool).