fire Article Verification of Red Flag Warnings across the Northwestern U.S. as Forecasts of Large Fire Occurrence Joshua Clark 1,*, John T. Abatzoglou 2, Nicholas J. Nauslar 3 and Alistair M.S. Smith 4 1 Wildfire Division, Washington State Department of Natural Resources, 1111 Washington Street SE, Olympia, WA 98502, USA 2 Management of Complex Systems, College of Engineering, University of California, Merced, 5200 North Lake Road, Merced, CA 95343, USA;
[email protected] 3 Predictive Services, National Interagency Fire Center, 3833 Development Avenue, Boise, ID 83705, USA;
[email protected] 4 Department of Forest, Rangeland, and Fire Sciences, College of Natural Resources, University of Idaho, 875 Perimeter Drive, Moscow, ID 83844, USA;
[email protected] * Correspondence:
[email protected] Received: 4 August 2020; Accepted: 29 September 2020; Published: 20 October 2020 Abstract: Red Flag Warnings (RFWs) issued by the National Weather Service in the United States (U.S.) are an important early warning system for fire potential based on forecasts of critical fire weather that promote increased fire activity, including the occurrence of large fires. However, verification of RFWs as they relate to fire activity is lacking, thereby limiting means to improve forecasts as well as increase value for end users. We evaluated the efficacy of RFWs as forecasts of large fire occurrence for the Northwestern U.S.—RFWs were shown to have widespread significant skill and yielded an overall 124% relative improvement in forecasting large fire occurrences than a reference forecast. We further demonstrate that the skill of RFWs is significantly higher for lightning-ignited large fires than for human-ignited fires and for forecasts issued during periods of high fuel dryness than those issued in the absence of high fuel dryness.