A mesoscale meteorological and fire danger analysis of the April 9, 2009 severe wildfire outbreaks in Oklahoma

On April 9, 2009 all the meteorological ingredients came together across western and central Oklahoma to create a “perfect storm” for severe wildfire outbreaks. On that day at least 14 wildfires were reported, burning a total of 117,000 acres and injuring 62 people. Some of these fires continued to burn into the next day. Portions of major state highways and interstates were shut down for periods of time. Of the 269 structures damaged by these wildfires, 228 were destroyed, including over 160 homes (70 of these were in the Oklahoma City area alone). The largest fire was reported in Stephens County in south central Oklahoma; it consumed 56,688 acres and at that point in time was the largest single wildfire across the United States reported to the National Interagency Fire Center since the start of 2009. Losses from all the fires are estimated at over $30 million.

As measured by the Oklahoma Mesonet, the state's 120-station automated weather station network, 10-m winds during the afternoon of April 9 across western and central Oklahoma were in most areas sustained at 30-40 mph with gusts as high as 74 mph reported. A strong low pressure system (sea-level pressure of 29.24 inches) was situated across northern Oklahoma. A strong dry line developed by early afternoon and was situated in a north-south orientation along Interstate 35 (which divides eastern Oklahoma from western). Behind (to the west of) this dry line, strong west to southwesterly winds were advecting hot and dry air into the region from Texas. In addition, a slow-moving cold front was entering northwest Oklahoma, resulting in a wedge-shaped area of hot, windy, dry air between the dry line and cold front. It was in this area that the wildfires occurred. Temperatures in this region approached 90F in a number of locations, and relative humidity fell as low as 6%.

The Oklahoma Fire Danger Model, which is an implementation of the National Fire Danger Rating System (NFDRS) to the Oklahoma Mesonet, indicated extremely high fire danger conditions in this wedge area. Based on the default NFDRS fuel models that are assigned to each Mesonet location, burning index (BI) values over 80 were commonplace, with values rising as high as 165 in southwest Oklahoma. Ignition component (IC) values over 50% were prevalent, with values rising as high as 84% in south central Oklahoma. The Nelson dead fuel moisture model, which is run operationally, indicated 1-hour dead fuel moisture as low as 2% in south central Oklahoma, with values under 5% widespread in the wedge area.

This presentation will provide a mesoscale analysis of the weather and fire danger conditions of April 9, 2009, which led to the widespread severe wildfire outbreaks in Oklahoma that afternoon. Using the Oklahoma Mesonet and the Oklahoma Fire Danger Model, animated maps and site-specific charts will be presented detailing the extreme fire danger conditions, the most severe of which occurred in the areas reporting the worst wildfires.