As of 3Z on Tuesday November 2nd, a 1032 mb high was dominating the Midwest and great plains. As a result of this high pressure, there is anticyclonic motion around the feature, which was causing a cold front that was stagnant over Texas to progress southeastward. A cold front is a weather feature where a mass of cold air pushes a warm air mass preceding it up and over itself and cools down the warmer region ahead.
A dryline is a surface weather phenomenon that occurs when warm, moist air from the Gulf of Mexico is advected, or transported, into a region and is met with cooler, drier air that is advected from the Rocky Mountains or the Desert Southwest. The dryline in Texas that appears on the surface weather map (Figure 1) at 03Z issued by the Weather Prediction Center was likely a result of the natural dewpoint gradient between Texas and New Mexico and the dry, cool air advected by the cold front that propagated southward because of the high-pressure system.
Figure 1: Weather Prediction Center surface analysis map issued at 03Z. The dryline is located over the panhandle of Texas and is represented by the orange lines with the empty semicircles branching off the solid orange line.
Ahead of the dryline in Del Rio, Texas (Figure 2) is an example of an elevated mixed layer on a sounding, skew-t plot. An elevated mixed layer separates a moist layer of air at the surface from a layer of dry air aloft. There is also an area separating the moist layer from the dry air aloft called a capping inversion which is an area in the atmosphere where the air is extremely stable, and the temperature increases with height. The elevated mixed layer occurs before the dry line passes because of the transportation of dry air eastward.
Figure 2: Skew-T Plot (Sounding) from Del Rio, Texas at 12Z on November 2nd. The components of the elevated mixed layer (EML) are highlighted with different colored boxes. The orange box is moist layer, the red box is the capping inversion, and the blue box is the dry air aloft – the environmental temperature is parallel to the adiabat (blue line).
At 15Z on November 2nd, 2021, I issued a forecast on the dryline event. Based on the GFS dewpoint forecast from 12Z on November 2nd, the dryline was not expected to exist further than 6 hours into the future (18Z on November 2nd). Between 12Z and 15Z, the dryline disappeared, as on the 15Z Weather Prediction Center surface analysis, the dryline was no longer plotted (Figure 3). As the cold front continued to move through Texas, based on the forecast from Tuesday November 2nd, there was potential for pop up thunderstorms. The cold front would provide the necessary lifting mechanism for the air parcels, allowing for the formation of thunderstorms. Additionally, as shown in Figure 2, there is also sufficient vertical wind shear, the change in both wind direction and speed with height. A shear profile, like the one in Figure 2, promotes the formation of thunderstorms in combination with a lifting mechanism – in this case, the cold front.
Figure 3: Weather Prediction Center surface analysis map issued at 15Z on Tuesday November 2nd in 2021. The dryline over Texas is no longer plotted.