Several disturbances in the mid-levels were present over the 48hr time period from 18z Thursday the 8th-18z Saturday the 10th that caused widespread showers and thunderstorms (including severe thunderstorms) across the Midwest and South. Both disturbances resulted from moderate to strong positive vorticity anomalies due to colder and more stable stratospheric air penetrating into the troposphere. This brings higher values of potential vorticity into the troposphere that can translate to regions of positive vorticity advection in the mid-levels and strengthening surface cyclones. Positive vorticity anomalies in addition to positive vorticity advection in the mid-levels and temperature advection at the lower levels (looking at QG-Theory) will lead to the formation of convection.
Fig. 1 below shows a 500hPa relative vorticity map from Alisha Bentley’s website at 00z Friday (~8PM EDT Thursday). A mid-level cyclone is present over the Midwest and the warm-color fill pattern indicates higher vorticity values. These values of higher vorticity are likely caused by a positive vorticity anomaly in the tropopause that’s translating to enhanced vorticity at the mid-levels. In order to deduce where any upward vertical motions are occurring we can
Figure 1: 00z Friday, April 9th 500hPa Relative Vorticity (Alisha Bentley)
Geopotential Heights (black contours every 60m)
Cyclonic, relative vorticity (red, orange, yellow fill pattern, 1/s)
Ascent (blue contours, hPa/s)
use QG theory to perform sign analysis on the QG-omega equation. Term A of QG-omega deals with positive differential vorticity advection by the geostrophic wind. What you’re looking for on the map for Fig. 1 is are higher values of vorticity being advected into regions of lower values of vorticity. Following geopotential height contours, we can see that the geostrophic wind is advecting higher values of vorticity from the mid-level cyclone eastward into the Midwest and parts of the South. This is where you can see the blue contours representing upward vertical motions occurring just downstream of the cyclone. I would expect a positive Term A in this scenario. If we move on Term B, this deals with whether warm air advection is present over
Figure 2: 00z Friday, April 9th 850mb Temp. Adv. (Meso. Analysis Archive)
Geopotential Heights (black contours every 30m)
Temp. Advection (fill pattern)
Wind barbs (knots)
a region. Fig. 2 shows 850mb temperature advection at 00z Friday and the red colors indicate areas of warm air advection. You can see this is occurring over the same region as the positive vorticity advection and while not very strong, this will still result in a positive Term B. This makes righthand side of the QG-omega equation positive, implying a positive left-hand side as well. This indicates a minimized omega term which implies upward vertical motions and possible convection.
Taking a look at the second disturbance that made its way across the same region Friday night and Saturday, Fig. 3 shows the same relative vorticity map from Alisha Bentley’s website but at 12z Saturday (8AM EDT). Again, there is noticeable high values of relative vorticity being advected into regions of lower values of relative vorticity and resulting rising motions so if
Figure 3: 12z Saturday, April 10th 500hPa Relative Vorticity (Alisha Bentley)
Geopotential Heights (black contours every 60m)
Cyclonic, relative vorticity (red, orange, yellow fill pattern, 1/s)
Ascent (blue contours, hPa/s)
looking at the QG-omega equation and Term A again, a positive term is expected. One thing that is different with the second disturbance however is a more well-defined vorticity anomaly present over the Midwest. Fig. 4 is a 2PVU surface map from Pivotal Weather. PVU stands
Figure 4: 12z Saturday, April 10th 2PVU Surface Theta Map (Pivotal Weather
Theta (color fill pattern, Kelvin)
Wind barbs (knots)
for “positive vorticity unit” and is a way to quantify vorticity in the troposphere and stratosphere. Specifically, 2 PVU is representative of the tropopause, separating higher PVU air in the stratosphere above and lower values of PVU in the troposphere below. This map specifically uses values of theta (fill pattern) to indicate where higher and lower values of PVU are because regions of higher vorticity have lower air columns which means colder temperatures relative to the surrounding air columns and vice-versa with regions of lower vorticity. While this doesn’t point out anomalies, it gives an idea of where higher and lower values of vorticity may be. There is a well-defined region of lower theta values (meaning higher PVU air) right where the mid-level cyclone is located. Mid-latitude cyclones are what are known as cold core systems meaning the air right below them is colder relative to the surrounding air. This is due to positive vorticity anomalies that are resulting in positive vorticity advection occurring at the mid-levels as depicted by Fig. 3.
Getting back to QG theory, we already determined a positive Term A but now let’s look at Term B again and see if any warm air advection is occurring in the region of the cyclone.
Figure 5: 12z Saturday, April 10th 850mb Height, Wind, Temperature (Pivotal Weather)
Geopotential Heights (black contours every 30m)
Temperature (color fill pattern, Celsius)
Wind barbs (knots)
Fig. 5 shows 850mb temperatures from Pivotal Weather. Using the wind barbs provided with this map you can see where any warm air is heading towards. South/Southwesterly winds are accompanying warmer temperatures at 850mb from the Gulf Coast up into the Midwest and South. This hints at a positive Term B within the QG-omega equation which implies an overall positive left-hand side. This implies a positive righthand side and minimized omega, thus resulting in upward vertical motions. Not only are upward vertical motions present over the Midwest and South but upper-level vorticity anomalies can translate down to the mid-levels (as shown earlier with the Alisha Bentley maps) and to the lower-levels by strengthening surface cyclones. A sub-1000mb low did indeed form over East Texas and moved east towards the Southeast which made the overall storm event more potent.