Firefighters in Horry County, S.C., believe they have one of the worst interface-fire situations in the country. Just inland, the pocosin/bay regions are comprised of mainly pine forests, often with a dense understory of shrubs. All of this is very flammable when dry and is capable of supporting very hot fires. The underlying organic soils can make firefighting very difficult with equipment often getting bogged down. This area has a long fire history, including the 30,000-acre Clear Pond Fire in 1976 — the largest fire in the state's recorded history.

Right along the coast is the sprawling resort area of Myrtle Beach. Affluent residential areas coexist with the normal attractions and amenities common of a major coastal resort. The worst fire conditions occur when strong westerly winds bring in dry, continental air. These winds often drive fires to the east, out of the forests and into the urban and suburban areas. The potential for major damage and the threat of casualties is high.

And that potential became reality earlier this year when the Highway 31 Fire started on April 22, just west of Myrtle Beach. In little more than 24 hours, this fire would become the most destructive wildfire in the history of South Carolina — burning more than 19,000 acres, destroying 75 homes and damaging 100 more, and producing more than $40 million in losses.

Weather Conditions

Precipitation had been below normal in the coastal plain for the winter, but conditions weren't excessively dry. A traditionally dry month here, April only recorded 1.26 inches of rain at the official North Myrtle Beach rain-gage site. All of this fell in the first half of the month. April 15 marked the last measurable precipitation at this site and the last significant rain occurred on April 2. It should be noted, however, that the official U.S. Drought Monitor did not indicate any drought conditions in the region as of April 21, one day before the fire began.

Temperatures in the weeks preceding the fire were somewhat below normal. A series of cold fronts had passed through the region, each one accompanied by limited amounts of precipitation. Following these frontal passages, dry Canadian air masses dominated. With such an active pattern, average wind speeds also were up, aiding the drying process.

At the upper levels of the atmosphere, a long wave trough had established itself over the eastern United States early in April. In this pattern, the jet stream takes a big dip toward the south, sometimes making it as far as the Gulf of Mexico. The northwest flow on the back side of this trough allows cold, dry Canadian air masses to penetrate well south. This northwest flow precludes Gulf or Atlantic moisture from being drawn into the frontal systems, and they pass through with little if any precipitation.

Long-term drought conditions are not essential for a major fire occurrence. Fuels in this region can dry out quickly. Warm temperatures also are not necessary for significant drying to occur. It really seems the stage was set for this fire in just the week prior to ignition. A lack of rain and high winds for seven days were the catalysts.

A cold front passed through the region and off the coast Monday evening, April 20. It produced strong winds over the area but no precipitation. Another, secondary cold front moved through on the following evening. Once again, no rainfall accompanied the frontal passage. A low-pressure area developed on this front just off eastern North Carolina. The counterclockwise flow and tightening pressure gradient of this system generated strong west-to-northwest winds over the region.

My original research on extreme fire behavior in the eastern United States showed that dry, cold frontal passages were associated with many of the larger fires here. The strong winds and dry air masses following frontal passage produced dangerous weather conditions favorable for rapid fire spread and extreme fire behavior. The lack of precipitation ahead of the front is unusual and, obviously, exacerbates the problems.

The morning of April 22 was characterized by fair skies but with strengthening northwest winds. Wind speeds increased from 10 to 20 mph as the morning went on. Temperatures rose from the high 50s into the mid 60s by noon. Dewpoints were low, in the 30s, indicating the dryness of the air.

The 8 a.m. sounding from nearby Charleston indicated fairly strong winds, especially for this time of year, throughout the vertical profile. Winds exceeding 30 mph were within 1,500 feet of the surface. Gust potential at the surface would be at least this strong. The relative humidities were fairly low, even this early in the morning.

Although weather conditions were forecast to be at or above Red Flag criteria, the actual threat from wildfires was thought to be low. It was assumed that the fuel moisture was high enough to impede ignition. And, in fact, wildfires had not been occurring in the days preceding April 22.

The Start of the Fire

The fire started around noon on April 22, just west of Myrtle Beach, about halfway between the town of Conway and Myrtle Beach itself. Official Forest Service records report the cause as a “escaped debris burn” that a homeowner had done on April 18. The fire was said to have rekindled on April 22. Although there were conflicting stories of exactly what happened, the homeowner was issued two tickets. The first fire units arrived at the site at 12:24 p.m.

Weather conditions were extreme as the fire started. Winds were from the west to northwest at 15 to 20 mph, with gusts over 30 mph. The temperatures were in the mid-60s with a dewpoint near 30°. Relative humidity dropped under 30%. Under these conditions, the fire was basically uncontrollable. It made a 6-mile run primarily to the east during the afternoon.

The 8 p.m. sounding for Charleston was indicative of the extreme weather conditions over the fire area. Although wind speeds at the surface were decreasing at this time, winds aloft were still strong. Relative humidity was very low, under 30% from the surface to 2,000 feet. Lapse rates in the lowest levels were very steep. From the surface to around 8,000 feet, the lapse rate was 5.1° F per 1,000 feet. This indicated extreme instability at low levels. The Haines Index, a widely used parameter derived from vertical lapse rates and humidity to indicate possible fire severity, was at a maximum value of 6.

Atmospheric instability can affect fire in several ways. First, it means the atmosphere is well-mixed vertically. This would allow stronger winds aloft to be easily transported down to the surface. Instability also aids the convection over the fire. This can help draw air into the convective column over the fire, further fueling it. Instability also is associated with extreme or erratic fire behavior.

The Second Major Run

By that evening, weather conditions seemed to be improving. Winds lessened and the relative humidity increased. It seemed that a typical nighttime inversion was setting up. In these cases, the surface layer of air “decouples” from the atmosphere above it. A cooler, moister and calmer layer of air near the surface is separated from the warmer, drier, and windier conditions aloft. The forecast called for light winds to continue and, although a shift in wind direction to the southwest was predicted, fire-control problems were not anticipated. In fact, the fire had stopped short of the highly developed resort area. It seemed that the worst was over and a major disaster had been averted.

But before midnight, the winds started to pick up again. The fire quickly trapped five firefighters who were engaged in a direct assault on the head of the fire. Fortunately, all escaped without injury, including two who had to deploy fire shelters as the fire burned over them. Weather conditions worsened. Winds gusted over 40 mph. Relative humidity fell again, which is very unusual for the overnight hours. Embers were being blown a half mile ahead of the main fire, starting spot fires. Just before 2 a.m., the main fire merged with one of these spots fires. As the fire approached Highway 22, a massive crown fire sent flames an estimated 270 feet into the air. Sustained crown fires are very rare in this part of the country. The fire continued its run to the northeast, crossing the six-lane Highway 22 and moving into the Barefoot Resort area. Most of the houses lost occurred with this run. An evacuation of 2,500 people prevented any casualties.

The shift in wind direction over the fire area was the result of the low-pressure area off the North Carolina coast continuing to move away to the northeast. The region then came under the influence of high pressure to the south. The clockwise flow around the high changed the wind direction to the southwest.

The cause of the strong winds at the fire site is not as clear. Nearby weather stations reported much calmer conditions, a more typical situation with high pressure building in. It would appear that this was a classic case of a fire producing its own environment. For this to happen, a significant convective column must develop over the fire. Reports from the fire site and observations from the nearby National Weather Service radar unit in Wilmington, N.C., indicated that this did indeed happen. Super-heated air above the fire was rising to heights of 10,000 feet or greater. Downdrafts that were induced would hit the ground and spread out, producing the strong surface winds.

Conditions which would favor the development of a major convective column over a fire include steep lapse rates, light winds and a hot fire. All of these were present in this case.

In these situations, the convective column of a fire is similar to that of a thunderstorm. As we know, thunderstorms can produce strong winds at the surface. Although the convective column itself is comprised of rising air, downdrafts can be induced as the atmosphere attempts to balance things out. The sinking air could transport stronger winds from aloft to the surface. This air also would be much drier in this case.

A tall convective column easily could break through the nocturnal inversion layer. Often at the top of the nocturnal inversion, winds are stronger. Sometimes a low-level even jet can be present. Downdrafts could transport stronger winds from aloft down to the surface.

The next morning, weather conditions began to improve. Winds lessened and the relative humidity rose. The atmosphere also became much more stable. Fire-control efforts took effect. The fire was fairly quickly contained after this.

The Highway 31 Fire was a textbook example of critical fire-weather situations. The first major run in the afternoon was produced by strong winds over the entire region following the passage of a dry cold front. The second major run in the overnight hours seemed to be induced by the fire itself. Strong, dry winds were pulled down from aloft by the convection associated with the fire. Instability in the atmosphere was a key factor. Under these extreme weather conditions, fire control is difficult or even impossible at times.

---

Ed Brotak is a longtime fire weather researcher. His analysis of weather conditions associated with major wildland fires lead to the development of the Brotak Wind Profile, the Haines Index, and classification of critical fire situations in relation to synoptic weather patterns.

Related Stories

• Wild in the City