A forecast procedure for dry thunderstorms

Abstract

Dry thunderstorms (traditionally <2.5 mm or 0.1 in of rainfall) have long challenged the weather forecasting community in the western United States (Pacific coast to the Rocky Mountains). Dry thunderstorms are responsible for starting thousands of wildland fires every year, including hundreds of fires in 24 h during larger events. The National Weather Service (NWS) Weather Forecast Office (WFO) in Reno, Nevada, developed a conceptual model for dry lightning called the dry lightning procedure (DLP); this includes the pressure of the dynamic tropopause (DT) on the 1.5 isentropic potential vorticity (IPVU) surface, various vertical motion structures based on straight and curved jet configurations, 850–700-hPa layer-average equivalent potential temperature (θe), high-level total totals (HLTT), and upper-tropospheric lapse rates (UTLR). The research described herein substitutes cross sections of θe, mixing ratio, and relative humidity for the 850–700-hPa layer-average θe in a modified version of the DLP. It also adds 250-hPa divergence plots to better ascertain vertical motions associated with straight and curved jets. These two additions, along with the rest of the DLP (DT on 1.5 IPVU surface, HLTT, UTLR) form the dry thunderstorm procedure (DTP). This paper examines the DTP for three case studies, including an overforecast event. The DTP adequately exhibited the ability to depict the potential for dry thunderstorms based on these case studies. The Reno WFO, and two interagency Geographical Area Coordination Center’s Predictive Services offices, have implemented the DTP operationally to ascertain the potential for dry thunderstorms.