A surface low is a local minimum in sea-level pressure with air spiraling inward and rotating counterclockwise in the Northern Hemisphere. On a surface analysis, it appears as a bull's-eye of closed isobars, often labeled with a red L.
How a surface low forms
Surface lows most often develop when an upper-level disturbance in the jet stream moves over a pre-existing temperature boundary at the surface. The Norwegian cyclone model describes this as a wave growing on a stationary front, then organizing into a mature cyclone with distinct warm and cold fronts wrapping around a central low.
As the low deepens, air converges toward the pressure minimum near the ground and rises above it. The rotation comes from the Coriolis force acting on that inflow. The result is a rotating column of ascent that transports warm moist air poleward on the east side and cold dry air equatorward on the west side.
How forecasters use it
Forecasters treat the surface low as the anchor for the surface pattern. The warm sector sits south and east of the low, between the warm front and the cold front, and that is where surface-based storms usually feed from the deepest moisture. The dryline often extends south from the low across the southern Plains.
The triple point, where the warm front, cold front, and dryline meet near the low, is a preferred spot for discrete supercells because three boundaries and a corridor of enhanced low-level shear come together. Trajectory of the low over the next 6 to 12 hours often frames the day's target.
What the low does not tell you
Central pressure and pressure tendency describe the strength of the cyclone, not the severity of any storm that fires within it. A rapidly deepening low can produce a wind-dominant, less tornadic day if moisture return is poor, and a modest low over a very moist warm sector can support a significant tornado outbreak.
The low is one ingredient in a larger setup. It has to be read together with 500 mb flow, low-level moisture, and the placement of boundaries before it says anything about the storm mode that afternoon.