A warm front is the leading edge of an advancing warmer air mass overriding cooler air at the surface, drawn on a surface map as a red line with rounded bumps pointing toward the cold side. As the warmer air moves in, it slides up over the denser cold air rather than pushing it back the way a cold front does.
How it forms
Warm fronts slope much more gently than cold fronts, sometimes only a few hundred meters of rise per hundred kilometers of horizontal distance. Warmer air rides up that shallow ramp, cools as it rises, and produces widespread stratiform cloud and rain well ahead of the surface boundary.
Because the warmer air is lifted through a deep, gentle layer, the resulting precipitation is usually broader and steadier than what a cold front produces. Thunderstorms are less common than along a cold front, but they can develop where the warm sector air is unstable and where the frontal surface reaches the level of free convection.
How forecasters use it
A warm front is the northern boundary of the warm sector where surface-based instability lives on a typical severe weather day. Its position matters because storms that develop north of it usually feed on air lifted above the surface cold pool, producing elevated convection with different hazards than surface-based storms.
The warm front is also where low-level shear is often strongest on a severe weather day. Cells that ride along or just north of the boundary can sample enhanced streamwise vorticity and stronger directional turning through the lowest kilometer, both of which can support low-level rotation.
Important limits
Warm front position is often uncertain, because the temperature contrast can be small early in the day and because rain and cloud cover can hold the cold side in place longer than models expect. A front that fails to push north as forecast keeps the warm sector smaller and shifts the storm threat area south of what the map suggested.
Elevated storms north of the warm front should not be dismissed. They can produce large hail and locally damaging wind, and the same air will sometimes become surface-based later in the day as the boundary lifts. We'll be watching for changes.