On November 17, 2021, a surface low pressure system was forecast to be centered in western Ontario, Canada, with a cold front extending roughly along a line from Michigan southwest to northern Texas and New Mexico. This can be seen as the temperature gradient in the 850 mb temperature map below:
Figure 1: GFS 850mb temperature, height, and wind, initialized 12z 16 November 2021, valid 18z 17 November 2021. (Source: Pivotal Weather)
To analyze what will happen with this front, we can use a parameter called frontogenesis, which essentially determines how the temperature gradient across the front changes with time. The one-dimensional frontogenesis equation contains four terms: shearing, confluence, tilting, and diabatic. The math behind this is fairly messy, but essentially what this reduces down to is if the warm side of the front becomes warmer and/or the cold side becomes colder, frontogenesis occurs as the temperature gradient across the front strengthens. In the case of the front in the 850 mb map above, specifically the portion in northern Texas, winds to the south of the front are out of the west or southwest. This has the effect of advecting warmer air into the region from west Texas. However, to the north of the front, northerly winds are forecast to advect colder air from the northern plains into the region. Since the warm side is getting warmer and the cold side is getting colder, this is a frontogenesis situation. This can be confirmed with the map below, which shows areas of positive frontogenesis over northwestern Texas. Note that the tilting and diabatic terms are not included in this map, since these terms are difficult to calculate and usually are not as significant as the shearing and confluence terms.
Figure 2: GFS 850 mb frontogenesis and temperature, initialized 12z 16 November 2021, valid 18z 17 November 2021. (Source: Pivotal Weather)
To the east of the cold front, winds out of the south were expected to bring warm, moist air from the Gulf of Mexico northward into portions of eastern Texas, Arkansas, and Louisiana. Normally, one might expect showers and thunderstorms to develop in this type of environment. However, in this case, storms were not really forecast to occur due to a lack of instability. This is shown in the model skew-T diagram below:
Figure 3: GFS model sounding, initialized 12z 16 November 2021, valid 18z 17 November 2021. (Source: Tropical Tidbits)
Although temperatures are fairly warm at 79F, the moisture is not particularly impressive for this part of the country with a dewpoint of 57F at the surface. However, the warm temperatures at higher altitudes are the main reason for the stability of this environment. In particular, there is an isothermal or even inverted layer between 850 and 750 mb that acts as a cap to prevent storm activity. Surface based CAPE (instability) values are very small at less than 100 J/kg, with large CIN (inhibition) values of more than -200 J/kg. This suggests that thunderstorms are unlikely, and any that do manage to form should remain weak. The NAM 3km model agrees with this analysis, showing very little activity over this region aside from a narrow region of light to moderate precipitation near and along the cold front.
Figure 4: NAM 3km composite reflectivity, initialized 12z 16 November 2021, valid 00z 18 November 2021.