The month of October 2021 has proven that drylines are not to be ignored in the fall months, although such features are most common and most intense in the spring and early summer seasons in the Southern Plains. Following the pattern of many recent severe weather outbreaks, the outbreak of October 26-27, 2021 featured a series of tornadoes alongside several damaging wind and hail reports. While not as rare as the event on October 12, 2021, this recent outbreak prompted the Storm Prediction Center (SPC) to issue categorial convective outlooks with an enhanced risk (level 3 of 5) for much of the Southern Plains and Louisiana across both days (Figure 1).
Figure 1: NOAA/NWS Storm Prediction Center (SPC) categorial outlooks and verification issued at 13Z for a) 26-27 October 2021, and b) 27-28 October 2021. Source: NOAA/NWS SPC Convective Outlooks.
On the evening of 26 October 2021, a weakening surface low moved eastward into northeastern Colorado, and eventually followed the Nebraska-Kansas border by the beginning of the overnight hours (Figure 2). In addition to the well-defined cold and warm fronts, this feature also has a rather sharp dryline, which separates moist air originating from the Gulf of Mexico from continental drier air. Unlike cold fronts and warm fronts, however, there is no significant change in temperature across the boundary; in fact, temperatures on the drier side are up to five degrees F warmer than on the moist side due to greater heat capacities found on the drier side. Like a cold front or warm front, there is a noticeable wind shift in which southerly to south-southeasterly winds east of the dryline are somewhat colliding with westerly winds west of the dryline. Since colliding air cannot all stay at the ground, increasing convergence will make for a sharper dewpoint contrast, which helps to lift air, and eventually will form thunderstorms along the boundary. This event was a classic example of this process, in which a quick collision of airmasses helped to spark thunderstorms during the evening and overnight hours across the Southern Plains (Figures 3 and 4).
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Figure 2: WPC surface analysis from 21Z 26 October 2021 to 06Z 27 October 2021 using three-hour intervals and selected station plots. Source: National Weather Service Weather Prediction Center (WPC) Surface Analysis Archive.
Figure 3: Loop of GFS forecasted surface dewpoint over the U.S. Southern Plains from 00Z 27 October 2021 to 06Z 27 October 2021 using three-hour intervals. Dewpoint (in F) is shaded using bottom scale, and 2m wind speed is denoted with black wind barbs. Source: Pivotal Weather.
Figure 4: Infrared satellite imagery centered over southern Oklahoma from 2241Z 26 October 2021 to 0831Z 27 October 2021. Colder (taller) cloud tops are marked with warmer shading using the top left scale. Source: GOES-R.
Once the initial convection fired off, other factors are needed for thunderstorms to become severe: moisture (found on the moist side of the dryline), instability (marked by higher CAPE), lift (along and ahead of the dryline), and wind shear. When there is strong wind shear present in a thunderstorm environment, the updraft will remain separate from the cooler downdraft of a thunderstorm, which helps to sustain and strengthen a thunderstorm. Between the surface and six (6) kilometers above the surface, wind shear speeds greater than 35 knots (40 mph) are usually associated with supercells and possible tornadoes. Figure 5 confirms strong vertical wind shear at the time storms began to fire along the dryline, albeit with higher shear values behind the thunderstorms, which allowed initial convection to become supercells – one of which produced an EF1 tornado close to the University of Oklahoma.
Figure 5: Loop of GFS forecasted surface to 500mb bulk shear over the U.S. Central Plains from 00Z 27 October 2021 to 12Z 27 October 2021 using three-hour intervals. Bulk shear (in knots) is shaded using bottom scale. Source: Pivotal Weather.
Figure 6: Loop of GFS forecasted 850 mb winds over the U.S. Central Plains from 00Z 27 October 2021 to 12Z 27 October 2021 using three-hour intervals. Wind speeds (in knots) is shaded using bottom scale. Using average surface winds from Figure 2, 0-1km wind shear can be approximated as 20 knots lower than forecasted 850 mb wind speeds. Source: Pivotal Weather.