5.1 Large Wildfire Growth influenced by Tropospheric and Stratospheric Dry Slots in the United States

Wednesday, 19 June 2013: 8:30 AM
Viking Salons DE (The Hotel Viking)
Fred J. Schoeffler, US Forest Service, Flagstaff, AZ
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Dry slots are responsible for rapid surface drying and increased gusty winds causing increased wildland fire behavior in the United States (U.S.). There is a plethora of research papers dealing with fire weather and wildland fires in the U.S. However, few of them discuss the influence of dry slots. A re-examination and re-evaluation of the previously published research regarding wildfires specifically influenced by dry slots is approached from the aspect of utilizing more water vapor satellite imagery for operational wildland fire weather meteorology in the future in the U.S. Wildland fire weather forecasting in the U.S. is an important facet of mesoscale and synoptic meteorology. Indeed, the very first Standard Firefighting Order deals precisely with weather - 'Keep informed on fire weather conditions and forecasts.' Moreover, two of the 18 Watch Out Situations deal specifically with dryness and wind. It is well documented in the available literature that surfacing lower-stratospheric and mid- to upper-tropospheric weather are responsible for dry air intrusions and descending dry air. Most times, these dry intrusions manifest themselves as clearly visible dark bands in the satellite water vapor imagery, referred to as dry slots. These dry slots usually result in abrupt surface drying and strong, gusty winds often radically influencing wildland fire behavior and hence fire growth. These phenomena can be significant safety issues for fire managers, especially for those fireline supervisors and firefighters on or near active firelines. The same issues very well apply to active prescribed and/or controlled burns. Therefore, accurate and timely dry slot recognition and warnings clearly address the rules in the Standard Firefighting Orders and would allow for better recognition and mitigation of the 18 Watch Out Situations. Dry slots are particularly well documented for Australian wildfires thanks to the work of Dr. Graham Mills and others. Due in large part to their research and efforts, the Australian Bureau of Meteorology regularly utilizes dry slot forecasting and nowcasting on a regular basis during their bushfire seasons. In fact, they even utilize a 'dry slot poster' to better educate and inform their fire managers to 'beware the dry slot.' However, little is known or documented for such dry slot occurrences in the U.S. In the cases examined, taken from the existing literature, it will be shown that several wildland fires in the U.S. have in fact been influenced by these dry slots resulting in extreme and unusual fire behavior and large fire growth. One of the issues addressed by the author is the fact that the 'dry slot' term is not very well utilized in the U.S. and there as many as thirty synonymous expressions used in the relevant literature.
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