Synoptic weather regimes associated with wildfire outbreaks in the southern Great Plains

The southern Great Plains, which includes large portions of the states of Texas, Oklahoma, and Kansas, is subject to frequent wildfires in grasslands as well as in other fuel complexes. A large portion of these states consist of wildlands, and their presence, combined with certain synoptic weather patterns, result in high fire danger periods, especially during late fall through early spring when native grasses and deciduous woody fuels are dormant. Drought, when combined with these weather conditions, not only intensifies the fire danger potential in mixed fuel complexes but also extends it in all fuel complexes if the normal spring rains do not occur.

This presentation will focus on three synoptic weather regimes that are associated with wildfire outbreaks in the southern Great Plains during the dormant season. The year 2011 has thus far seen a large number of wildfires in this region, especially in Texas where, as of the time of this writing, almost 10,000 fires have burned over 2.5 million acres since November 15, 2010 (Texas Forest Service). In addition, much of Texas, Oklahoma, and western Kansas have seen severe (D2) to exceptional (D4) drought during this period according to the U.S. Drought Monitor.

Using selected wildfire outbreaks identified during January through April 2011, the presentation will illustrate the synoptic regimes using Oklahoma weather and fire danger conditions as monitored by the Oklahoma Mesonet of 120 automated weather stations and as calculated by the Oklahoma Fire Danger Model, a model which is integral to the OK-FIRE system used in Oklahoma for operational wildland fire management. Radar and satellite imagery, available through OK-FIRE, will also be used to show the location of the wildfires in the scenarios discussed.

The three synoptic weather regimes to be discussed are as follows: (1) pre-trough, where the region is on the back (west) side of a high pressure region with moderate to strong S / SW winds and the air mass is dry (moisture return is limited or blocked out); (2) post-frontal, in which a dry air mass enters the region with moderate to strong NW / N winds behind a cold front; and (3) dry line, with strong W / SW winds and extremely dry air behind (to the west of) the dry line. In situations (1) and (2) a large portion of the geographical area experiences low relative humidity (RH), while in (3) there is a strong RH gradient across the dry line, with very low RH to the west and much higher RH to the east. Often a cold front to the northwest intersects the dry line, resulting in a “wedge” of extremely high fire danger between the dry line and the cold front until the cold front overtakes the dry line.

The presentation will utilize some striking examples of wildfire outbreaks occurring under each of the three synoptic weather regimes discussed above. Examples of wildfire outbreaks in the pre-trough regime (1) include wildfires during January 29, March 11, April 2, April 5, April 17, and April 29. The March 11 outbreak saw numerous large fires in Oklahoma with many structures damaged. Statewide RH was in the 6-20% range with S/SW winds of 20-30 mph. Post-frontal regime (2) wildfire outbreaks are represented by fires occurring on March 23 and April 15. On April 15 western Oklahoma had RH values in the 8-20% range with NW winds of 25-50 mph. Finally, examples from the dry line regime (3) include wildfire outbreaks on February 27, April 3, April 9, and April 14. The April 9 outbreak featured RH values of 8-15% behind the dry line with SSW winds of 25-40 mph.