This week had a significant weather event in the southeast region. Over the span of a couple of days, a low-pressure system, shown by the counterclockwise flow, strengthened, and led to a lot of precipitation coming across Alabama and Mississippi before moving north towards Kentucky and Tennessee. As seen in Figure 1, the precipitation the night of March 17 was severe and even resulted in a few tornadoes and as the low-pressure system continued moving, the precipitation lightened slightly but was still very present in the 12Z surface map.
Figure 1: Surface map showing pressure contours and precipitation type on 2021- 03-17- 21Z (right) and on 2021-03-18-12Z (left) to show how the low pressure system moves and strengthens as it moves towards the eastern ridge.
The surface analysis map below, shown as figure 2, shows the cold front that came across and the warm sector ahead of it. The warm front present at 21 Z Wed March 17 goes away nearing 12Z Thur March 18 but there is still a slight east to west wind and a turning of wind by the South Carolina border. The air north of the warm front, it is cooler and has lower dew points. In the pocket of the warm front, there is moisture advection as the warm moist air is pulled from the Gulf of Mexico which strengthened this low pressure system. Looking at the surface analysis at 12Z there is possibly a weak wedge event but it did not do anything to stop the severe weather present.
Figure 2: Surface analysis map showing the different temperatures, dew points and wind speed across the United States. The different fronts can also be seen, speficially the cold front moving across the south eastern U.S. form 21Z Wed Mar 17 to 12Z Thur Mar 18
Analyzing the upper levels, there is not a present jet streak occurring in the 300 mb level but there is convection and strong wind speeds northeast of the low-pressure system. The trough ridge flows at this level are what contribute more to the dynamics that strengthened this low pressure system. The 500 mb did show a maximum vorticity over the low shown by the deeper red colors in figure 3 but no positive vorticity advection at the surface since the low is not in the optimal location at 500 mb between the trough and the ridge. The 500 mb height anomaly shows the low pressure system moving and how it lines up with the vorticity map. This low is positioned between a somewhat western trough and an eastern ridge so other upper level dynamics present are ageostrophic divergence and moisture advection from the gulf.
Figure 3: (Left) 500 mb map showing relative vorticity
(Right) 500 mb height anomaly map showing the extreme low pressure system in the south east.