The fire season in both states usually ends in late April. Normally by this time, vegetation is turning green. Fire control agencies are shifting to other forestry operations, and seasonal fire control crews are leaving. But April 1970 was unusual.
Rain fell in above-normal amounts during the early spring months. Periods of rain were so spaced that all fuels, except the fine ones, remained wet. Temperatures remained well below the seasonal normal keeping the vegetation in the cured stage. Except for a few border stations, fire danger stations did not go into the transition stage until mid-April. Rainfall, that had been coming in substantial amounts, dropped off in late March to almost nothing. This dry spell continued into mid-April and temperatures started rising to more normal levels. This was just the type of weather the people were waiting for: to begin field clearing by burning, brush pile burning, and garden and household debris burning. During this period, a great number of fires roared out of control.
Synoptic Situation and the Black
Wednesday Forecast The dry spell, begun in late March, stretched into April as dry, high pressure spread over Oklahoma and Arkansas. It blocked frontal systems from the area. By April 7, high pressure extended upward to 20,000 feet (6,100 m), but the surface high center had moved to the lower Mississippi Valley. Moderate-to-strong, southwesterly, low-level winds pumped even drier air over Arkansas and Oklahoma. Afternoon relative humidities dropped to the 20-percent level, and some places had humidity readings down in the teens. With fuels already bone-dry, an extremely dangerous fire situation was in the making. Fires by the hundreds were being reported in Arkansas and Oklahoma. But most of them were not too difficult to control.
Wednesday morning, April 8, another dangerous weather feature entered the weather picture. The 6 a.m. radiosonde observations at Oklahoma City and Little Rock showed the air to be conditionally unstable to about 15,000 feet (4,800 m). It would become absolutely unstable from the surface up to 4,000 feet (1,200 m) by the middle of the afternoon. Widespread surface whirlwinds or dust devils resulted from the great instability in the lower 1,500 feet (460 m). Warnings were called to the State Fire Control Chiefs, as well as to the Ozark and Ouachita National Forests. The warnings were for potential blow-up conditions. Hard-to-control fire behavior such as rapid crowning, long-distance spotting, and large convection columns was expected.
All conditions were favorable for fires in Oklahoma and Arkansas. There was a significant deficiency in rainfall during the last half of March and the first half of April. There had been an extended period of extremely low relative humidities. When these conditions combined with an unstable atmosphere, all conditions were “go” for blow-up fires. And blow-up fires did occur.
At 9 p.m. that Black Wednesday evening the Ouachita National Forest called to report one of their worst fires in 3 years had been burning out of control. Aerial tankers, as well as hand crews, had been ineffective against this fire. The Oklahoma Division of Forestry reported a total of 35 fires that burned 7,669 acres (3,103 ha), while one fire roared over 2,080 acres (841 ha). Arkansas (State and National Forests) had a total of 142 fires which burned 12,559 acres (5,082 ha).
Air Stability the Key
When fire weather conditions are conducive to many fires (i.e. large precipitation deficiency, and low relative humidities) the fire weather meteorologist gives special attention to the stability of the atmosphere. The key to identifying this situation is interpretation of the early morning radiosonde observation, including temperature, humidity, and wind from the ground upward, thousands of feet. The fire control agency, informed of dangerously unstable atmospheric conditions by the fire weather meteorologist, is warned to expect erratic fire behavior.
This article originally appeared in Fire Management Today, vol. 63, no. 4, and was reprinted in This Land: Winter 2015.