The net result of warm advection is to make a region warmer. The animation below shows (in a very general sense) how warm advection can produce upward motion. Warm advection is occurring in Figure A while Figure B shows a vertical cross section through the region of warm advection. It is important to realize that Figure A is along the ground and that Figure B is from the ground up to a higher level in the atmosphere, directly over the region of warm advection.
Animation by: Van Dorn
With the onset of warm advection (Figure A), the isobar in Figure B starts to bend upward since warmer air is less dense and occupies more space than colder air. The bending of the isobar due to warm advection creates a localized area of high pressure ("H" in Figure B), thus altering the pressure gradient force. As air moves from the local region of high pressure to the regions of lower pressure ("L" in Figure B), air is drawn upward from below, which is the rising motion produced by warm advection.