A warm front occurs when a cold air mass retreats and is slowly replaced by a warmer air mass. As the warmer and less dense air moves in to replace cooler and more dense air, a sloped transition zone between the air masses forms. The less dense warm air moves up and over the cooler air causing moisture in the warm air to condense as it rises. This condensation causes the formation of clouds along the warm front. As the warm front continues to approach an area, precipitation is likely. See the warm front diagram
for a visual representation of the transition zone.
The stability of the approaching warm front will determine the intensity of precipitation that will occur. If the air is relatively stable, a steady rain or snow, often with significant accumulation, is likely. If the air mass is unstable, heavier thunderstorms can occur.
The weather map symbol used to indicate a warm front is a red line with semi-circles pointing in the direction of movement of the frontal system as can be seen in this warm front symbol image.