A precipitation core is the region of a thunderstorm containing the heaviest concentration of rain, hail, or both. It is the storm's densest reflectivity area on radar and often appears as a dark, opaque column or curtain in the field.
How it forms and where it sits
Precipitation is generated within a storm's updraft, then carried by upper-level flow and by the storm's own tilt away from the updraft column. In a well-sheared supercell, the updraft is tilted enough that condensate can fall out downshear rather than raining back into the updraft. The result is a distinct precipitation core sitting off to one side of the rain-free base.
In multicell and pulse storms with weaker shear, the precipitation falls closer to the updraft and can suffocate it, which is one reason those storms have shorter lifespans than supercells.
How meteorologists use it
The size, height, and position of the core carry a lot of forecast information. A tall, high-reflectivity core with a strong overhang above the low-level inflow suggests large hail is being suspended aloft. A core positioned behind a rain-free base with a hook on its trailing edge is the classic supercell radar signature.
Spotters use the core's location relative to the wall cloud to orient themselves. In a classic supercell, the core is downshear of the updraft and the wall cloud sits on the inflow side. A storm that has its precipitation wrapping all the way around the base is behaving as a high-precipitation supercell and calls for a different, more cautious viewing position.
Where it misleads
Radar reflectivity in the core does not cleanly separate rain from hail. A very bright core may be a small volume of large hail or a large volume of very heavy rain, and the visible appearance is often not enough to tell them apart from a distance.
The core also does not mark the storm's most dangerous feature by itself. In supercells, tornadoes are usually near the updraft, not in the heart of the heaviest rain, so treating the core as the entire storm risks missing what is happening on the storm's back side.