Assessing the Predictive Skill of WRF Lightning Threat Proxies for the California Dry Lightning Outbreak of June 21, 2008

Thunderstorms are rare in coastal California during the summer months. When they do occur, they can be particularly dangerous. Synoptic situations that encourage these summertime thunderstorms typically involve elevated instability that promotes dry lightning thunderstorms, where less than 0.1 in of precipitation reaches the ground. On June 21, 2008, a poorly-forecasted dry lightning outbreak was responsible for starting hundreds of wildfires in Central and Northern California. Model guidance and NWS AFDs from the previous day contained little mention of instability. Red flag warnings for the San Francisco Bay Area and Central California were only issued the morning of the outbreak.

In this project we are using the NAM-initialized WRF-ARW model to simulate the event, and determine the extent to which it could have been better forecast the day before (e.g., with different initializations, choice of physics etc.) Simulated maximum reflectivity (mdbZ) is compared with lightning strike observations for verification. We also explore the usefulness of lightning forecast proxies from model variables in dry lightning events. These proxies include graupel flux across certain isotherms, vertically-integrated ice, and elevated CAPE.