Dry thunderstorm forecast procedure

“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. These dry thunderstorms are responsible for starting thousands of wildland fires every year. In the largest lightning outbreaks (or “busts” in the wildland fire 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 fight.

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. NWS Reno WFO developed WA04, 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 (Milne 2004). Isentropic analysis was added to WA04.

This procedure was applied to several studies from past five years including the dry lightning events across all regions of the western United States. Dates of the investigated events are June 25-27, 2006, August 16-18, 2006, July 16-18, 2007, June 20-21, 2008, August 1, 2009, and August 20-21, 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 in the preceding days and hours to the initiation of the event and if the event lasted more than one day. This paper summarizes these case studies, and describes the dry lightning forecast procedure.