Forecasting dry thunderstorms as a part of the fire weather outlooks at the Storm Prediction Center

Beginning in mid 1998, the Storm Prediction Center (SPC) began issuing nationwide fire weather forecasts for days one and two. The outlooks are available on the Internet at: www.spc.noaa.gov/fire. These are similar to the SPC one and two day convective outlooks. The fire weather forecasts cover surface and upper-air conditions related to fire weather conditions (temperature, humidity, wind, and thunderstorms (wet and dry)). In the convective outlooks, the general thunderstorm forecasts are for a 10 percent chance or greater of thunderstorms. The types of thunderstorms normally associated with the fire starts in the west represented a unique forecasting challenge. They generally produce very little precipitation and except for possible wind gusts and erratic surface winds, they are not associated with severe weather. They may actually be relatively isolated (less than a 10 percent chance of occurrence), high-based thunderstorms producing only a few flashes of lightning. However, these flashes may trigger very costly and deadly forest fires. These storms generally represent a class of storms at the opposite end of the spectrum of storm types that the SPC constantly monitors (i.e., severe storms with wind, hail and tornadoes).

With the goal of improving the ability to forecast these types of storms, attempts are made 1) to monitor and verify when lightning does start fires and 2) to evaluate the instability, sub-cloud humidity and general weather conditions at the surface and aloft at the time of the lightning. In May 1999, the SPC also began a new experimental (model based) product designed to highlight possible areas in the western United States that may experience thunderstorms. This product, available on the Internet as part of the SPC Fire Weather Outlook Web page is produced entirely from the Rapid Update Cycle version 2 (RUC 2) computer model. The manually produced forecast including the possibility of dry thunderstorms is issued once per day at 4 AM (CST/CDT), but is not updated due to limited staffing. The new experimental product based on the RUC 2 provides additional quidance and produces updated three, six, nine and twelve hour forecasts every three hours from 12 UTC to 00 UTC (effectively covering the first full 24 hours of the day 1 fire weather forecast). Examples from the 1999 fire weather season will be presented and discussed at the conference.