A low-precipitation (LP) supercell is a supercell that produces relatively little visible rain or hail near its updraft. The storm has a sculpted, striated appearance, and its small precipitation core is often displaced well downstream of the main rotation.
The environment that produces LP structure
LP supercells form in environments with very dry mid-levels, high cloud bases, and a strong cap that focuses convection into a small number of discrete cells. These conditions are common on the high plains and along the dryline, where mixed-layer parcels are hot and dry and moisture is thin.
The dry mid-levels rob the storm of the mid-level entrainment that produces most of the visible precipitation shroud in wetter supercells. The updraft can still be strong, but much of the condensate is either lofted high or evaporated before reaching the ground near the base.
How to recognize it
On radar the LP storm often looks small. Reflectivity cores are compact, sometimes less than 40 dBZ, and the hook echo may be faint or absent even when a mesocyclone is clearly present in the velocity data. Vertically the storm is often taller than it is wide, with strong reflectivity aloft and a weak echo region.
Visually the LP mode is dramatic. A high, bell-shaped or barber-pole updraft, deep striations, an exposed inflow band, and a very small rain foot are the signature look. Wall clouds are less common, and rotation is often seen as slow, sculpted turning of the whole updraft.
How forecasters interpret it
LP supercells produce fewer tornadoes than the classic or HP modes, because the environments that favor them are usually too dry at low levels and have LCLs that are too high for surface tornadogenesis. When they do produce, tornadoes tend to be brief.
The bigger operational concern is giant hail. The updraft in an LP storm can loft ice for a long time before it falls, and stones of very large diameter are common. On radar, high mid-level reflectivity and a persistent bounded weak-echo region are the cues, even when the surface reflectivity looks unremarkable.