It is clearly spring in the United States as another round of severe weather affected a large portion of the Midwest and Southern US. The catalyst for this round of severe weather was an extratropical cyclone making its way easy across the country (Figure 1). This system displayed classic extratropical cyclonic features as a cold front extended from the center of low pressure over the Iowa/Missouri border south to Louisiana and Texas and a stationary front extended west/northwest from the center of the low. (Figure 1). The main lifting mechanism that led to this convective activity was the cold front. While certain factors regarding the mesoscale may have had a greater effect on tornado development and localized severe weather, this blog entry will focus primarily on the synoptic scale environment and how it strengthened this extratropical cyclone.
Figure 1: WPC Surface Analysis map from 00Z 08 April 2021. Image courtesy of the WPC.
Identifying troughs and ridges at the 300 mb level along with jet streaks can provide valuable information regarding vertical motions in the mid-troposphere. Around 12Z 07 April 2021, a trough was present over Kansas and extended south into Texas. The surface low was located downstream of the trough and upstream of the ridge feature to the east (Figure 2A). Ageostrophic divergence was occurring aloft in this region which induced mid-tropospheric upward vertical motions via mass continuity. These upward vertical motions helped strengthen and maintain the surface low. There was a jet streak feature located over the 4 corners region of the US, but the upward vertical motions associated with the left jet exit region were a bit too far away to have an impact on the surface low. Moving forward in time to 18Z 07 April 2021, the trough has now propagated slightly further east. As a result, the upward vertical motions from ageostrophic divergence were still present downstream from the trough. However, the jet streak that was present was now positioned further east at this time (Figure 2B). Due to this, the ageostrophic divergence aloft in the left jet exit region led to upward vertical motions and strengthened the surface low. Evidence of this strengthening was seen as the central pressure of the system was 1002 mb at 12Z 07 April 2021 and it dropped to 998 mb around 00Z 08 April 2021.
Figure 2A (left) displays 300 mb height (black contours, mb), wind speed (fill pattern, knots), and divergence (contours, purple) from 12Z 07 April 2021. Figure 2B (right) displays 300 mb height (black contours, mb), wind speed (fill pattern, knots), and divergence (contours, purple) from 18Z 07 April 2021. The center of the surface low is labeled as a red “L”.
The tropopause can also provide valuable information on vertical motions in the atmosphere. Taking a look at potential vorticity can provide a view of what the tropopause looked like. A positive PV anomaly was present over the surface cyclone (Figure 3A). This indicates the height of the tropopause was lower over this region. Cross sectional data of this anomaly will allow for the analysis of vertical motions in the mid-troposphere. The cross-sectional data clearly depicts a positive PV anomaly, as the stratosphere dips down into the troposphere which lowers the tropopause height (Figure 3B). A positive PV anomaly is a max in vorticity, and due to the thermal wind flowing to the east, it advected higher values of vorticity east of the anomaly. As a result, upward vertical motions occurred east of this anomaly, and strengthened the surface low.
Figure 3A (left) displays PVU Pressure (hPa, fill pattern), wind (barbs), and low-pressure centers. Figure 3B (right) displays a cross section of the +PV anomaly. Images courtesy of Tropical Tidbits.
Another way the cyclone strengthened was due to condensational heat release from precipitation along the cold frontal boundary (Figure 4). The precipitation along the cold front causes latent heat release. This diabatic heating maximum strengthened the positive PV anomaly at the surface and weakened it in the upper troposphere. This in turn raised the tropopause height through the creation of negative PV and this led to ridge building downstream of the trough. This enhanced the trough-ridge dynamics that were previously mentioned and strengthened the surface cyclone. Overall, the synoptic scale environment had a large impact on the strengthening of this extratropical cyclone.
Figure 4: Radar reflectivity data from 07 April 2021 at 23Z. The fill pattern (dBz) represents precipitation and its intensity.