A dryline is a narrow boundary that separates moist, low-level air from much drier air, usually with only a small temperature difference across it but a sharp drop in dewpoint. In the classic Plains configuration, humid air moving up from the Gulf meets drier air off the higher terrain of the desert Southwest, and the two air masses meet somewhere across west Texas, western Oklahoma, and the Panhandles.
How the dryline forms and moves
The dryline is really a horizontal moisture gradient made steep by daytime mixing. During the day, mixing on the dry side reaches deep into the boundary layer and pulls dry air down to the surface. On the moist side, a shallow layer of low-level moisture is protected under a cap, so surface dewpoints stay high. The contrast sharpens where those two behaviors meet.
As mixing deepens, the dryline advances east through the day because the moist layer on its eastern side is progressively eroded from above. After sunset, mixing shuts down, and low-level moisture returns westward under a southerly boundary layer flow. The dryline retreats overnight and reforms the next afternoon further east or west depending on the synoptic pattern.
How forecasters use it
The dryline focuses convergence in a narrow band, and the moist air on its eastern side often has the deepest CAPE anywhere in the region. Where the boundary bulges east into moist air, low-level convergence and shear locally increase, and that bulge is often where storms initiate first.
Storms that fire on a dryline usually root in the moist air just east of the line, then move northeast into the richer inflow. Cells that stay tied to the boundary can be reinforced by the sustained lift, while cells that pull away from it depend entirely on the environment they carry with them.
Important limits
Cap strength above the moist layer decides whether the dryline actually produces. On days with a strong cap and no larger-scale forcing, the boundary can sit sharp and quiet all afternoon while the environment east of it is technically explosive. The saying that a capped dryline is the loudest quiet feature in the Plains is not an exaggeration.
Dryline position on models is sensitive to boundary layer schemes, terrain resolution, and how each model handles moisture return the night before. Different models can place the boundary tens of kilometers apart, and that difference changes which counties see initiation. We'll be watching for changes as the day evolves.