Southeast US High Fire Danger Weather Patterns
A number of generalizations can be made about the fire season and critical fire weather patterns in the Southeast. The "traditional" fire season in the Southeast is Spring and Autumn. Wildfires do occur at other times of the year.
The fire season in the Spring occurs in the weeks before "greenup" (period of budding of grasses and trees). This is usually during the month of March for Mississippi. The Fall fire season in Mississippi occurs in October and November, normally after the first frost, and fires are typically "carried" by the buildup of leaf litter on the forest floor.
Wildfires that emerge during the Winter months burn relatively few acres due to the short burning period during the day and are usually restricted to grassy or light fuel areas. Wildfires that emerge during a summer drought are also normally restricted to a grassy or brushy areas.
Deficient rainfall plays an important role in the severity of a fire season, for that matter it is an important factor throughout the year.
This deficit can occur in two ways: A long interval of several months of below normal rainfall,
Almost a total lack of rainfall for several weeks.
Few fires in the Southeast are ignited by lightning, most are started by the activities of people, either intentionally or by accident.
The traditional Severe Fire Season for Mississippi may last for one week at best...or more on rare occasions. While fire danger may gradually build up to a point that fire starts are numerous, extreme fire behavior and large acreage loss is a climax of one or two days that is abruptly halted when the next weather system arrives.
The Gulf of Mexico and at times the Atlantic are ever present sources of moisture and it is an abnormal weather pattern that keeps these key players out of the picture for more than two or three weeks. It is these abnormal patterns that define a critical fire weather pattern for Mississippi.
Surface Weather Features
In the Lower Mississippi Valley, one will hear that "conventional wisdom" has it, that most active fire periods occur in the warm sector of a surface low. It is implied that there is a strong southwesterly gradient, and that humidity is lower than would be the case with a southerly flow and An "open" Gulf. In those cases where high fire activity is associated with a surface low, the track of the low pressure system is to the north of the region.
However, it is more frequently the case, that high fire activity is associated with high pressure systems at the surface, and an unchanging upper level flow. High pressure systems over the region are effective in keeping moisture from returning from the Gulf of Mexico.
Days with maximum sunshine and low humidity are more likely with high pressure in place across the state. This results in a large diurnal temperature range and high evaporation rates. The temperature range on the day of the Brasstown fire in South Carolina (March 1953) ranged from a morning low of 33 degrees to 80 during the afternoon. Daily evaporation rates can be as high as .45 of an inch.
Normally, surface patterns are highly transient in the Southeast unless one of three upper level patterns becomes established. For that reason it is difficult to discuss critical fire weather patterns without linking the surface features to the upper level flow.
Upper Air Patterns
Three upper air patterns are effective in keeping the State under the influence of high pressure at the surface. If the antecedent conditions of below normal rainfall are in place, and if one of the three upper patterns has been in place for over a week, then a critical fire weather pattern emerges.
Strong Zonal Flow Aloft
A Spring or Autumn phenomenon. Strong westerlies aloft result in a rapid succession of Pacific fronts traversing the Lower Mississippi Valley. Little if any moisture from the Gulf of Mexico is able to return to the region in advance of the cold front. Rainfall with the front is sparse and light. Exceptionally low humidity may occur the next day after frontal passage, and little recovery can be expected before the next front arrives. Strong and gusty winds are a distinct possibility.
Northwesterly Flow Aloft
Important mainly during the Spring and Autumn. Dry air associated with high pressure systems from Canada spreads across the region. The initial Canadian cold front move through the state and remains stationary far south of the region until the upper level pattern changes. A large and stagnant high pressure settles over the region. Weak fronts from the north may reinforce the dry airmass now lying over the Lower Mississippi Valley. Humidity will not be quite as low as with Pacific fronts and better humidity recovery at night can be expected. Surface winds are normally light or moderate.
Blocking Ridge Aloft
May be persistent or reoccurring for long periods. Axis of upper level ridge or center of high near the Atlantic coast. The subtropical high or Bermuda high is associated with this upper level pattern. This pattern is also effective in blocking systems from the west or north from moving through the region. Surface lows and associated fronts stall before moving very far into the region, and upper level dynamics associated with surface systems are sheared, or lifted out to the north of the region. Little rainfall is produced during the period that the upper level ridge is in place.
The position of the Bermuda high is important in determining the low level flow of air across the region. Lowest humidity may drop below 40 percent, but humidity recovery at night is generally good. Surface winds are usually light, but may increase ahead of an approaching cold front that subsequently stalls before reaching the area.
In order to anticipate an emerging critical fire weather situation in the State, rainfall should be tracked. Coordinate with Federal and State Agencies. Consider the indicators of drought at the Forestry Section Links. When 10 hour fuel moisture is below 10-12, look for the three upper patterns, and more importantly, decide if one of the three patterns may persist for a week or more.
One of the best forecast tools for forecasting abnormally low humidity in the Southeast is the uses of NCEP's forecast boundary layer moisture progs from the ETA model and MESO ETA model.
With fires in progress, or with high fire danger, carefully examine the wind profile of the lower levels of a nearby rawinsonde sounding to assess the possibility of a low level jet. Past Research has shown a Strong Connection between Low Level Jets and "BLOW-UP" Fires in the Southeast.
So if you see these patterns developing during a dry period of below normal rainfall and the fire danger indexes are high...dont hesitate to call the state and federal forestry agencies to coordinate on any possible rare action to be taken such as Fire Danger Statements...Fire Weather Watch or Red Flag Warnings.