The seasonal and interannual variability of the structure, evolution, and propagation of midlatitude cyclones in the southeast United States are studied using a composite analysis. In the upper levels, the composites show that the axis of the wintertime upper-level trough remains north-south oriented and propagates eastward along 408N, while the summertime upper-level trough has a much slower propagation at a farther north latitude and an axis that is tilted in the northeast-southwest direction. Upper-level circulation changes are consistent with a shift from wintertime ''cyclonic behavior'' to summertime ''anticyclonic behavior'' midlatitude cyclones. Significant changes in the low-level structure and precipitation patterns of midlatitude cyclones ensue from these upper-level changes. While the winter composite is characterized by eastwardpropagating midlatitude cyclones that extend deep into the subtropics, the summer composite is characterized by semistationary midlatitude troughs that only briefly skirt the subtropics. Wintertime precipitation occurs only in and ahead of the surface low pressure center, whereas summertime precipitation occurs in all days of the composite. As a result, over 70% (30%) of wintertime (summertime) precipitation in the Carolinas occurs on days when midlatitude cyclones are present. The wintertime composites also show that midlatitude cyclones produce more precipitation on the windward side of the Appalachians than over the Carolinas, suggesting a rain shadow effect of the mountains.The ENSO-related variability of the structure, evolution, and propagation of midlatitude cyclones shows the presence of a more intense and southward-displaced upper-level jet, stronger midlatitude cyclones, and more intense precipitation over a larger area during El Niño than La Niña or normal years.

Midlatitude cyclones that affected the Southeast (SE) United States between 1998 and 2010 were classified into five types according to their region of origin: (1) Continental United States, (2) Canadian, (3) Gulf Low, (4) Hatteras Low, or (5) Stationary. A composite analysis was used to examine differences in the structure, evolution, and propagation of these cyclones during boreal winter, when the largest number of cyclones affect the SE United States. Most of the midlatitude cyclones that affect the SE in the wintertime formed within the continental United States (∼11 events/winter), followed by Gulf Lows (∼4 events/winter) and Canadian cyclones (∼4 events/winter). Hatteras Lows were relatively rare (∼1 events/winter), and Stationary events did not occur during winter. While Canadian and Continental US cyclones occurred year‐round the frequency of Gulf Low events peaked in the boreal winter when the upper‐level jet was strongest and located further south. Hatteras Lows peaked in the boreal fall and winter, and Stationary events occurred almost exclusively during the boreal summer. Overall, Gulf Low and Continental US cyclones were the only cyclones that brought wintertime precipitation to the SE United States. When integrated over the season, Continental US cyclones made the largest contribution to the wintertime precipitation in the SE United States. On a per‐event basis, however, Gulf Lows were the biggest wintertime precipitation makers in the SE United States while Canadian cyclones and Hatteras Lows did not bring much precipitation to the SE United States. Gulf Lows were more common during El Niño than La Niña years and tended to occur back‐to‐back with either another Gulf Low or with a Hatteras Low.