Southeastern States

The vegetation in the Southeastern United States consists mainly of pines along the coastal plains, hardwoods in bays and bottomlands along stream courses, and mixed conifers and hardwoods in the uplands. Flash fuels, flammable even very shortly after rain, predominate in this region. The topography along the Gulf and Atlantic is low and flat. Inland from the Atlantic Coast it merges with an intermediate Piedmont area. The southern Appalachians are included in this region, and the central portion includes the lower Mississippi Valley.

Summers are warm and generally humid, because the region is almost continuously under the influence of an mT air mass. Winters have fluctuating temperatures. When mT air moves over the region, high temperatures prevail. Following the passage of a cold front, cP air may bring very cold temperatures--well below freezing--throughout the Southern States.

Annual precipitation varies from 40 to 60 inches over most of the region, except for about 70 inches in the southern Appalachians and over 60 inches in the Mississippi Delta area, and falls mostly as rain. The influence of the moist mT air from the Gulf of Mexico causes abundant rainfall in all seasons, with slightly higher amounts in August and September due to the presence of hurricanes in some years. Spring and fall have less precipitation than summer or winter, with spring being wetter than fall. Winter precipitation is usually associated with frontal lifting or with Lows that develop over the Southern States or the Gulf of Mexico and move through the region. Summertime precipitation is mostly in the form of showers and thunderstorms. During the colder months, much fog and low stratus are formed by the cooling of mT air as it moves northward.

The fire season in the Southern States is mainly spring and fall, although fires may occur during any month.

The four synoptic types that bring high fire danger to the other regions east of the Rockies also bring high fire danger to the Southern States. The Hudson Bay High and Northwest Canadian High types affect this region less often than the regions to the north. The airflow pattern aloft must have considerable amplitude for Highs from Canada to reach the Southern States.

The Pacific High type causes more days of high fire danger than any other type. Pacific Highs may reach this region with either meridional or zonal flow aloft. Very often, the most critical fire weather occurs with the passage of a dry cold front. The air mass to the rear may be mP or cP. The strong, gusty, shifting winds with the cold front and dry unstable air to the rear set the stage for erratic fire behavior.

The Bermuda High type is second to the Pacific High in causing high fire danger in this region. This type is rather stagnant and persists over the region for long periods of time, mostly in spring, summer, and fall. The cutting off of Gulf moisture by the Bermuda High, when it extends westward across the Southern States to Texas, is the typical drought pattern for this region. Aloft, a long-wave ridge is located over the central part of the continent and the belt of westerlies is far to the north, near the Canadian border.

Subsidence and clear skies produce low humidities and high temperatures. These factors, plus the extended drought, set the stage for high fire danger. Peaks in fire danger occur as winds increase with short-wave trough passages and their associated surface cold fronts on the north side of the Bermuda High. Lightning accounts for only a minor number of fires.