Demystifying the 6 o'clock Magic Phenomenon: Environmental Changes During the Southern Great Plains Afternoon to Evening Transition

Over the past few decades, various investigators have termed the intensification or development of deep, moist convection during the Southern Great Plains afternoon-to-evening transition (SGP AET) as the “6 o’clock magic.” Hypotheses for why convection seems to intensify during this time period tend to be primarily focused around the development of the nocturnal low-level jet stream (NLLJ) or the development of a maximum in surface instability caused by surface temperatures. Regardless, changes to convective ingredients (e.g. moisture, shear, instability, and lift) during the AET can be quite significant and the scientific consensus focuses typically only on shear and static stability. While increases in low-level static stability due to the loss of surface heating during the AET may act to destroy deep convection, accompanied increases in low-level shear may promote the development and maintenance of updrafts. Consideration of only these two ingredients offers an incomplete and conflicting conceptual model of how AET environmental changes may impact the behavior of ongoing deep convection.

Limited studies in other portions of the world have demonstrated that during the AET time period, an increase in low-level moisture (roughly 1 g/kg) may occur as convectively-generated turbulence decays. This work confirms that such increases also occur in the SGP AET and additionally demonstrates that such increases may serve to rapidly increase the instability in the atmosphere prior to the development of the NLLJ. Additionally, results currently suggest that the Great Plains Winter Wheat Belt may play a key role in modulating the SGP AET and focuses when and where rapid increases in convective ingredients may occur. Within this presentation, observations from the ARM SGP site in Lamont, Oklahoma, the Oklahoma Mesonet, and the PECAN project will be shown to illustrate how various convective ingredients may become maximized during the AET.