The Influence of Dry Slots on Wildland Fire Growth during the 2011 Arizona Fire Season

During the May and June 2011 fire season, Arizona experienced several large wildland fires in various fuel types and terrains, many with erratic fire behavior and large fire growth. All those wildland fires were influenced by persistent dry slots and/or dry intrusions when compared with archived GOES satellite water vapor imagery (WVI). According to the literature on the subject, informed fire weather researchers and Operational Meteorologists conclude that these causal factors and signatures show up quite clearly as dark bands and/or dark tongues in the WVI. The literature indicates that they are responsible for rapid surface and near-surface drying in the form of reduced relative humidities and dewpoints, diminished fuel moistures, increased potent winds, and Haines Index 5 or 6. When these mechanisms combine and are in alignment, they can cause increased wildland fire behavior. This evidence held true for those 2011 Arizona fires and the effects were the same, with some of the fires exhibiting very erratic fire behavior and large fire growth during the recurrent dry slot episodes.

The Alignment of Forces principle established in the Campbell Prediction System (CPS)will be presented. The CPS method deems that when the forces of fuels, weather, and topography all line up, then the fire behavior will increase exponentially. Also addressed will be the effect of high nocturnal temperatures adversely influencing wildland fire weather and fire behavior. Th1s anomaly, as it applied to at least Arizona fires, was examined by [author] in 1962. The resultant increased fire behavior typically occurs on the day following the highest nighttime temperature. This was clearly an influential force on the Wallow Fire in June 2011. The author believes that high nocturnal temperature are much more of a factor influencing large fires nationally than researchers have credited to-date, and this is worthy of further research.

The author will review several of the relevant research papers regarding wildfires in the United States specifically influenced by dry slots to set the stage for the paper and the fires examined. Also examined will be: 'Fighting Fire by the Rules.' Wildland firefighters and fireline supervisors have specific 'rules of engagement' for fighting and/or managing fires, including prescribed burns. In fact, the very first Standard Firefighting Order deals precisely with weather -'Keep informed on fire weather conditions and forecasts.' In addition, two of the 18 Watch Out Situations deal specifically with temperature, dryness, and winds.

The paper will include several date-and-time stamped fire behavior photo images of the Wallow Fire in northern Arizona in May and June 2011. These are then compared with GOES satellite WVI of dry slots and dry intrusions at or close to the same time periods. The effects are dramatic, indicating the adverse impact of these mechanisms on the fire weather and hence the fire behavior.

These lower- to mid-level atmospheric phenomena can be significant safety issues for everyone on or near the active firelines. Therefore, accurate and timely recognition of these dry slot signatures and providing timely warnings clearly addresses the precepts of the Standard Firefighting Orders. This would allow for better recognition and mitigation of the 18 Watch Out Situations as well. Wildland fire weather forecasting in the United States is an important facet of mesoscale and synoptic operational meteorology. A case will be made for more consistent utilization of satellite WVI for dry slot recognition and forecasting in the United States resulting in more accurate operational fire weather meteorology.