Coastal fronts are shallow (less than 1 km deep) boundaries that separate colder, continental air masses from much warmer and moisture laden maritime air masses. Temperature differences across the front can be as much as 30-50oF (17–28oC). Typically, coastal fronts develop in coastal waters or within 100–200 km of the coast during the cooler half of the year when the land is cold relative to the ocean. In the United States, coastal fronts are most frequent in New England, the Middle Atlantic states, the Carolinas, and Texas.
The typical coastal front is oriented quasi-parallel to the coast and may extend for several hundred miles. At times with a strong onshore flow, it can move well inland. During the winter, the coastal front may mark the boundary between frozen and nonfrozen precipitation. Given that coastal front development usually precedes synoptic-scale cyclogenesis and marks an axis of enhanced thermal contrast and a maximum in cyclonic vorticity and convergence, the coastal front often serves as a boundary along which intensifying synoptic- scale cyclones move poleward.
Surface coastal front development typically occurs beneath the forward side of advancing troughs following the passage of the ridge axis aloft. Coastal fronts most frequently form equatorward of cold anticyclones where a warmer onshore flow encounters a colder continental air stream. Damming of cold air on coastal orographic barriers such as the Appalachian mountains (Eastern United States) often appears to play an important role in coastal front development.
Coastal thermal contrasts are augmented by differential diabatic heating where the onshore flow has passed over oceanic thermal boundaries such as the Gulf Stream and the adjacent continental airstream has passed over snow-covered land.
Coastal fronts may form independently of cold anticyclones and associated cold air damming. In situ coastal front developments can occur near mountain barriers where upslope flow results in differential airmass cooling and stabilization and where offshore troughs form due to differential heating across oceanic thermal boundaries. Coastal front dissipation typically occurs with the cessation of onshore flow following cyclone passage.