The warm sector is the wedge of warm, often moist air inside a midlatitude cyclone, bounded by the cold front to the west and the warm front to the north, with the surface low anchoring the northwest corner. In the Norwegian cyclone model this is the classic triangle of warm, moist air that a mature cyclone wraps around itself before occluding.
How the sector fits inside a cyclone
The AMS Glossary defines the warm sector as the region within a wave cyclone that contains the warm air, bounded by the cold and warm fronts. As the cyclone matures, the cold front rotates around the low faster than the warm front does, so the sector narrows over time.
When the cold front eventually catches the warm front, the warm sector lifts off the surface entirely, and the cyclone is called occluded. Everything a storm chaser or severe-weather forecaster cares about happens before that point.
How forecasters use it
The WFO Louisville severe weather forecasting guide places the corridor of highest tornado risk about 250 miles south and east of the surface low, well inside the warm sector. That is where the low-level jet is transporting Gulf moisture northward, where surface dewpoints are usually highest, and where the flow beneath the mid-level jet produces the strongest hodographs.
The dryline in the southern Plains typically forms the west edge of the warm sector on high-risk days. The cold front trails southwestward from the low along its northwest boundary, and the triple point sits where those two boundaries meet the warm front. Chase and warning strategy usually starts from those boundaries and works inward.
Where it misleads
The warm sector's usefulness ends at the depth of its moisture. A wide sector with 60-degree dewpoints capped near 900 mb can be almost entirely capped off during peak heating, and the day passes with only a shower or two. A narrower sector with rich moisture that mixes deep can produce a full outbreak.
Sector geometry also shifts fast. The cold front can outrun the warm front by dinnertime, sweeping storms into a line or pushing initiation east of the target area. Read the sector in motion, using surface observations and short-range models, rather than as a fixed feature on one map.