On November 2nd, 2021, no severe weather was present in the southwestern United States, but there were some features that lead to an environment suitable for possible severe weather conditions. Figure 1 shows a strong cold front coming from New Mexico eastward across Texas as well as the formation of a dryline. Drylines separate regions of cold and warm air. This dryline is not very prominent, however there is still a significant temperature change at 09Z between the cold, dry air in the high 40s heading eastward from the Rockies and the warm, moist air in the mid-50s coming from the south off the Gulf of Mexico. Figure 1a shows that as the cold front moves east, the dry line shrinks. At this same time there is the formation of a low-pressure system near Odessa, Texas.
Figure 1: a. WPC surface analysis 09Z 02 November 2021 b. WPC surface analysis 12Z 02 November 2021
Typically, when a low-pressure system is present in a region it leads to severe weather features such as high winds and often precipitation. Figure 2 shows a sounding from the MAF station, located in between Odessa and Midland, Texas, that shows there is also a lot of convective available potential energy or CAPE in this region. CAPE describes the instability of the atmosphere and how much energy is available for a developing thunderstorm. Since there is CAPE present, the environment is unstable and suitable for thunderstorms and other severe weather. The dew point temperature (51) is also very similar to the environment temperature (55) so the air is moist and suitable for severe weather to occur since severe thunderstorms are more likely when surface dewpoint is 55 degrees F or higher. However, right below the 850 mb layer there is an inversion seen in the sounding which is what has led to the lack of precipitation or storms in the Texas region. This capping inversion is where temperature increases with height instead of the typical situation where temperature decreases with height. So even though there is a lot of CAPE at higher altitudes in this area, the inversion layer stops it from affecting the surface.
Figure 2: 12Z November 2021 sounding from the MAF station located in between Odessa and Midland, Texas. The green line represents the dew point temperature, the red line represents the environment temperature, and the red dotted line is the parcel path showing the CAPE.
The higher dewpoint values can also be seen at the 850 mb level. In Figure 3 the regions of green show higher dewpoint values. Looking between 09Z and 12Z there is an increase in dewpoint temperatures which corresponds to the low-pressure system and an increase in instability in the area. Although there is no severe weather in the area currently due to the inversion layer, if there is daytime heating or some other factor that would increase the surface temperature, then severe weather has all the necessary components to occur.
Figure 3: 12Z November 2021 850 mb map showing the dewpoint values in the green areas
An inversion layer is also associated with a shallow cold front. As seen earlier in Figure 1, there is a strong cold front that moved east across Texas, contributing to instability of the environment by allowing upward vertical motion. Low-pressure systems form through upward vertical motion and divergence aloft. Often times these processes come from the jet stream at 300 mb. However, when looking at the 300 mb map in Figure 4 the jet streak, which is the filled light blue area, is up near the northeast region and therefore has no real impact on the Texas and southwestern area. The main contributing factor to the low-pressure system forming then in this scenario is the upward vertical motion associated with the strong cold front that has upward motions along it due to the colder air forcing the warmer air upward.
Figure 4: 12Z November 2021 300 mb map showing wind speed(knots) in the fill pattern and divergence in the purple contours