On the night of October 12, 2021, a severe weather outbreak occurred across portions of the southern Great Plains. Several reports of tornadoes, large hail, and damaging winds were collected by the Storm Prediction Center over the course of that day, primarily over southern Kansas and the panhandles of Oklahoma and Texas. A map of these storm reports can be seen below, along with the SPC’s convective outlook from that afternoon which featured a rare Moderate Risk area:
Figure 1: SPC 20Z convective outlook and storm reports from 12 October 2021.
On the afternoon of October 12th, a surface low pressure system was centered over eastern Colorado, with associated warm and cold fronts extending to the east and southwest respectively. However, the more noteworthy feature with respect to this event was the dryline analyzed extending south across western Texas almost all the way to Mexico. Drylines separate regions of moist air from regions of dry air. This can be seen in the dewpoint data from the surface observations in Figure 2: dewpoints range from 30s-40s west of the dryline to 50s-60s east of it. A wind shift is also visible at the dryline: winds east of the dryline are coming from the south to southeast, while they are out of the southwest on the west side. This makes sense, as a southeast wind would bring in moist air from the Gulf of Mexico, while a southwest wind would bring in dry air from the deserts of Mexico and the southwestern US. However, despite the dewpoint change and wind shift, there is not usually much of a temperature change across a dryline, and in this case temperatures are in the 80s on both sides. In fact, temperatures on the west side of the dryline are generally a couple degrees warmer than on the east side. This is because dry air has a lower heat capacity than moist air, so with daytime heating the western, dry side will typically heat up slightly more than the eastern, moist side. The surface map below from the afternoon of October 12th shows the dryline as an orange line with open semicircles on the moist side:
Figure 2: WPC surface analysis, 21Z 12 October 2021.
However, even east of the surface dryline, the effects can continue through what is known as an elevated mixed layer, or EML. These initially form as surface layers in the dry air over the higher terrain of the western United States and Mexico. However, when they are advected eastward towards the central plains, they maintain their original higher elevation and become elevated above moist air moving northward from the Gulf of Mexico. EMLs can be identified on a skew-T plot as layers above the surface where potential temperature is constant (temperature line parallel to dry adiabats) and mixing ratio is constant (dewpoint line parallel to lines of constant mixing ratio). An EML can be seen in the sounding from 00Z October 13 (Fig. 3) in roughly the 800 to 600 mb layer. This is a loaded gun sounding – if rising air parcels can break through the cap, strong thunderstorms are likely.
Figure 3: 00Z 13 October 2020 sounding from Dodge City, KS (KDDC) with skew-t (left) and hodograph (top right) plots.
That sounding also shows the significant amount of wind shear present, another factor important for strong thunderstorms and tornadoes. Near the surface, winds are out of the southeast, but then quickly shift with height to southwesterly. In addition, wind speeds are almost continuously increasing with height. The hodograph (diagram at top right of Fig. 3) shows this wind profile, with strong directional shear at low levels and speed shear throughout. This type of wind shear environment is favorable for severe weather such as supercells, squall lines, and tornadoes. Ultimately, multiple severe thunderstorms occurred in this highly unstable airmass. A couple individual supercells developed in western Oklahoma early in the evening due to a low-level jet, spawning a couple tornadoes, but most of the severe reports that day were from a squall line that initially formed in the late afternoon over eastern Colorado and New Mexico. This line tracked east through Kansas, Oklahoma, and northern Texas through the night, producing several tornadoes and reports of wind damage. Figure 4 below shows radar imagery of the two Oklahoma supercells and the developing squall line over western Kansas and the Oklahoma panhandle.
Figure 4: NEXRAD radar reflectivity, 0125Z 13 October 2021. (Source: UCAR Archive)