A Forecast Procedure for Dry Thunderstorms
AdvisorBrown, Timothy J
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Dry thunderstorm (traditionally less than 2.5 mm or 0.1" of rainfall) forecasting has long been a forecast problem for the western United States. Dry thunderstorms are responsible for starting thousands of wildland fires every year. In the largest lightning outbreaks (or busts in the wildland fire-meteorological community), hundreds of fires may be started in a 24- to 36-hour period. These extreme events put a huge strain on fire suppression efforts. Many of these fires may go unstaffed due to the lack of available fire personnel simply because of the large number of fire starts. Forecasting these events in advance, even just 24-48 hours, could help fire agencies plan resources in preparation of a large outbreak. Fires are much more likely to be controlled during the early stages, and therefore cost much less to suppress.Due to the seemingly innocuous conditions preceding dry thunderstorm development across the western United States (west of the Rocky Mountains), forecasting dry thunderstorm events can prove challenging and inconsistent. To improve dry thunderstorm forecasting, the National Weather Service (NWS) Reno Weather Forecast Office (WFO) developed WA04 (Wallmann 2004, 2010), a conceptual model of dry thunderstorms that includes the pressure of the dynamic tropopause, jet streak dynamics, equivalent potential temperature, and upper level lapse rates in conjunction with the High Level Total Totals. This thesis supplements WA04 by adding moist isentropic analysis and enhancing the jet streak analysis to help a Dry Thunderstorm Procedure (DTP). Moist isentropic analysis resolves moisture and instability better than analyzing constant pressure maps, thus making it ideal to find the pockets of instability and plumes of moisture that spawn dry thunderstorms. The enhanced jet streak analysis in DTP more completely resolves upward motion and divergence aloft that might not be distinguished using constant pressure maps or traditional quasi-geostrophic theory. The goal of this research is to build upon previous work by examining six case studies using WA04 while enhancing the procedure by adding moist isentropic analysis and augmenting the jet streak analysis. By examining the procedure and investigating the case studies, an operational checklist is developed to accurately depict the potential for dry thunderstorms across the western United Sates. This procedure and checklist is designed to be applied operationally in NWS WFO's and Geographical Area Coordination Center's (GACC) Predictive Services offices throughout the western United States to ascertain daily potential for dry thunderstorms. DTP was applied to six dry lightning events within the past five years across all regions of the western United States. Dates of the investigated events included 25-27 June 2006, 20-21 August 2006, 16-18 July 2007, 20-21 June 2008, 1 August 2009, and 20-21 August 2009. These events show the diversity of dry thunderstorms in terms of development, location, and timing. The procedure proved useful in determining the potential for dry thunderstorm development 36-48 hours before initiation of the event. This thesis summarizes these case studies, describes the dry lightning forecast procedure, and depicts a goldilocks environment of moisture and instability conducive for dry thunderstorms.