Fig. 1 – NOAA Storm Prediction Center Filtered Storm Reports for 15 April 2023 as of 1510Z 18 April 2023 (NOAA SPC Archive). Red circles indicate tornado reports, blue circles indicate wind reports, green circles indicate hail reports, black squares indicate large hail (≥ 2” diameter) reports, and black circles indicate high wind reports (≥ 65 knots).
This past weekend, a low-pressure system (LPS) moved through the continental US, bringing with it significant hail, straight-line winds, and 13 tornadoes, with one EF-2 in Maries, Missouri. Hail larger than 2” was common with this storm system, across Missouri down to Texas, and high winds were experienced around the Gulf of Mexico from the outflow of these storm systems. The magnitude of hail reports suggests the presence of strong and concurrent updrafts for these systems in addition to cool, dry air aloft. The magnitude of high wind reports suggests strong straight-line winds associated with a strong outflow boundary. Both require significant upward vertical motion (UVM) in the region over our LPS. In order to diagnose the synoptic forcings at play for this event, we first turn to the 250 hPa jet.
Fig. 2 – 250 hPa jet map plotted for 00 Z 16 April 2023 (Alicia Bentley Real-time GFS Maps). 250 hPa windspeed (shaded, color bar, m/s), mean sea level pressure (solid black isobars),1000-500mb (dashed red and blue lines, blue = below freezing), blue arrows indicate centripetal acceleration, red arrows indicate the ageostrophic wind vectors, orange circles indicate areas of ageostrophic divergence aloft, red “L” indicates surface low-pressure system
The high-amplitude trough within the polar jet stream is just west of our surface LPS (red L), indicating that this system is a westward tilt with height. We also have a strong setup for divergence aloft, with the downstream trough and upstream ridge for both the polar and subtropical jet. The subtropical jet pattern displayed this pattern nearly directly underneath the polar jet, possibly slightly ahead of the polar jet. We can see the ageostrophic wind vectors (red arrow) oriented to the west for the troughs and to the east for the ridges. This creates an area of ageostrophic divergence aloft in the yellow-circled regions. The area of our surface LPS straddles both of these divergence areas, which is likely responsible for the magnitude of the upward vertical motion and rapid development of these storm systems. To confirm the hypothesis that the upper trough-ridge dynamics were the dominant factor in the development of this system, we turn to the traditional form of the Quasi-Geostrophic (QG) omega equation.
We will break down the QG forcing into two main terms. Term A, -(f0/σ)[∂(-Vg·∇ηg)/∂p], is defined as the vertical derivative of the absolute geostrophic vorticity advection by the geostrophic winds. Term B, -(R/σp)[∇2(-Vg·∇T)], is the Laplacian of horizontal geostrophic temperature advection.
Fig. 3 – QG Omega Plots (Thomas Galarneau Real-time QG Diagnostics)
Top: 700 hPa Z, Temperature, Differential Vorticity Advection Term (A) from 00 Z 16 April 2023, 700 hPa geopotential heights (black, every 3 hPa), forcing from differential vorticity advection (shaded, 10^-12 Pam^-2s^-1)
Bottom: 700 hPa Z, Temperature, Thermal Advection Term (B) from 00 Z 16 April 2023, 700 hPa geopotential heights (black, every 3 hPa), isotherms (dashed black contours, every 6 K) forcing from the Laplacian of geostrophic temperature advection (shaded, 10^-12 Pam^-2s^-1)
From the analysis, we can see that term A, the differential vorticity advection term is the dominant forcing for upward vertical motion in our QG analysis, as indicated by the significant region of warmer (yellow-orange) colors in the top image. This indicates that there is positive differential vorticity advection and thus the left-hand side (LHS) of the QG equation would be positive, so omega is minimized and there would be UVM. Under the traditional QG equation, our term B, the thermal advection term, indicates a significant amount of cold air advection or a local maximum, which makes the LHS term negative so omega is maximized, and downward vertical motion would occur in the region of the blue. This term does display the significance of the cold air advection brought by this frontal passage, but it fails to accurately account for the impressive upward vertical motion, which is indicated by term A. It is likely the differential vorticity advection that is driving the upward vertical motion over our LPS. To better understand why term B might be misrepresenting the UVM, let’s take a look at the frontal dynamics at play.
Fig. 4 – GFS 850 hPa frontogenesis map for 18 Z 15 April 2023 (Tropical Tidbits Analysis), temperature (isotherms, °C, red is above freezing, blue is freezing and below freezing) , temperature advection (shaded, color bar, K/hr, red colors indicates warm air advection, blue colors indicate cold air advection), wind barbs (knots), frontogenesis (purple contours, K/100km/3hr)
Here we can see a nice shearing pattern as indicated by the circled areas, with a strong northwesterly wind on the west to a strong southwesterly wind on the east, which will act to tighten the temperature gradient and strengthen this front. Additionally, we see a significant amount of cold air advection (CAA) on the cold side of this frontal passage and some warm air advection (WAA) on the warm side, which will lead to frontogenesis. However, the CAA is much more significant than the WAA here, which is consistent with what we saw in the QG term B plots. While the magnitude of WAA might be less, the CAA provided enough of a strong frontal passage and lifting mechanism to generate initiation. This initiation was then aided by jet stream dynamics and differential vorticity advection to create massive divergence aloft and thus upward vertical motion. While warm air advection might have been weaker, the cool, dry air brought southeastward by this frontal passage was contrast enough to provide explosive initiation. While these synoptic features are only part of the picture, this venting of our LPS allowed it to maintain its strength while providing the strong updrafts and downdrafts we saw with this system.