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Verification of Red Flag Warnings Across the Northwestern U.S. As

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Verification of Red Flag Warnings Across the Northwestern U.S. As 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. The results of this first verification study of RFWs related to actualized fire activity lay the groundwork for future efforts towards improving the relevance and usefulness of RFWs and other fire early warning systems to better serve the fire community and public. Keywords: fire weather; forecast verification; fire danger forecasting 1. Introduction Wildland fire plays an important role as a natural and increasingly anthropogenic disturbance found in most vegetated ecosystems globally [1,2] and generally serves to promote healthy, resilient landscapes [3]. However, when fire threatens to directly impact human life and property, it may be considered hazardous [4], prompting land management action to mitigate impacts on ecosystems and communities [5,6]. In recent decades, longer periods of critical fire weather [7] coupled with human-caused fire activity [8] and increased fuel loads as a result of fire suppression activities [9] have expanded the threat of hazardous fires in the United States (U.S.), making land management objectives more difficult and costlier to achieve, while also placing the safety of fire suppression personnel and the public at greater risk. These effects have been demonstrated by recent fire events in the U.S. [10–12] and are expected to continue due to anthropogenic climate change [4,13] and the growth of wildland–urban interfaces [14]. Many early warning systems have been developed for fire hazards, including the development of numerous fire danger indices [15]. Comprehensive fire danger systems integrate weather, fuels, Fire 2020, 3, 60; doi:10.3390/fire3040060 www.mdpi.com/journal/fire Fire 2020, 3, x FOR PEER REVIEW 2 of 18 Fire 2020Many, 3, 60 early warning systems have been developed for fire hazards, including the development2 of 17 of numerous fire danger indices [15]. Comprehensive fire danger systems integrate weather, fuels, and climate information to generate daily predictions regarding the fire environment and fire behaviorand climate characteristics information [16 to–18 generate]. These outputs daily predictions are generally regarding considered the in fire fire environment management andplans fire as actionablebehavior characteristics criteria that prompt [16–18]. some These prevention, outputs are preparedness, generally considered or resource in fire allocation management decision plans [15 as] andactionable have seen criteria widespread that prompt adoption some prevention, in wildland preparedness, fire management or resource communit allocationies in decisionNorth Ameri [15] andcan andhave Australasia seen widespread [16,17 adoption,19,20], with in wildland other countries fire management like the communitiesUnited Kingdom in North in the American process and of Australasiadeveloping [similar16,17,19 tools,20], with[21]. otherIn the countries U.S., systems like the such United as the Kingdom Severe Fire in the Danger process Index of developing and Santa Anasimilar Wind tools Threat [21]. Index, In the leverage U.S., systems advancements such as the in gridded Severe Fire met Dangereorological Index forecast and Santa data along Ana Wind with Threatfuels and Index, fire leveragebehavior advancements information to in griddedprovide meteorologicallocal-to-regional forecast predictions data along of fire with danger fuels and[22,23 fire], whilebehavior other information systems predict to provide fire local-to-regional danger directly predictionsfrom fire weather of fire danger conditions [22,23, ],independent while other systemsof fuels predict[24,25]. fireSimilarly danger, utility directly providers from fire incorporate weather conditions, a matrix independentof meteorological of fuels and [24 fuel,25]. moisture Similarly, criteria utility providersto de-energize incorporate electrical a matrix lines ofas meteorologicala fire-prevention and strategy fuel moisture [26]. criteriaWhile these to de-energize systems electricalserve as importantlines as a fire-prevention tools for understanding strategy [26 hazardous]. While these fire risk, systems many serve are asgenerated important independent tools for understanding of real-time humanhazardous input fire that risk, may many be areimportant generated for independent deciphering ofrapidly real-time changing human fire input environment that may be factors important that drivefor deciphering the most extreme rapidly fire changing activity. fire environment factors that drive the most extreme fire activity. Red Flag Warnings (RFWs),(RFWs), which are issued by meteorologists at the U.S. National Oceanic and AtmosphericAtmospheric Administration (NOAA) National Weather Weather Service (NWS), are forecasts of critical fire fire weather conditions that could lead to abundant new ignitions and/or and/or rapid spread and growth on existing fires fires ( (i.e.,i.e., extreme fire fire behavior) behavior),, but are not actuall actuallyy forecasts forecasts of of fire fire activity activity ( (FigureFigure 1)[) [2727]].. Forecasters at at local local weather weather forecast forecast offices offices (WFOs) (WFOs) issue issue RFWs RFWs on onshort short timescales timescales (imminent (imminent to 48 to- hr48-hr lead lead times) times) and and consider consider numerous numerous quantitative quantitative and qualitative and qualitative meteorological meteorological parameters parameters (e.g., (e.g.,relative relative humidity, humidity, wind windspeed, speed, atmospheric atmospheric stability, stability, dry frontal dry frontal passage, passage, lightning) lightning) along alongwith some with measuresome measure of fuel of dryness. fuel dryness. These Theseparameters parameters are considered are considered as criteria as criteria for RFW for issuance RFW issuance (Table (Table1) based1) onbased the onexpert the expertopinion opinion and analyses and analyses at WFOs at and WFOs vary and geographica vary geographicallylly by sub-regional by sub-regional fire weather fire weatherzones (FWZs). zones (FWZs). Although Although this arrangement this arrangement allows allows for forgreater greater flexibility flexibility to to accommodate accommodate local conditions conduciveconducive toto area area fire fire initiation initiation and and growth, growth, it has it resultedhas resulted in many in many different different criteria criteria across acrossFWZs, FWZs, complicating complicating the messaging the messaging intent intent of these of forecasts.these forecasts Furthermore,. Furthermore, limited limited studies studies have have been beenconducted conducted to verify to theverify performance the performance of RFWs of and RFWs we areand not we aware are not of any aware peer-reviewed of any peer studies-reviewed that studiesdiscuss that possible discuss refinements possible refinements to these forecasts. to these forecasts. Figure 1. AnAn example example of of a a Red Red Flag Flag Warning Warning issued by the National Weather Service (NWS) Spokane forecast oofficeffice for windy andand drydry conditionsconditions onon 2828 AugustAugust 2015.2015. Fire 2020, 3, 60 3 of 17 Table 1. Select criteria for issuing Red Flag Warnings (RFWs) at different weather forecast offices (WFOs) taken from the 2019 Northwest Area Fire Weather Annual Operating Plan available at https://gacc.nifc.gov/nwcc/content/pdfs/2020_Region_6_AOP_public.pdf. “LIGHTNING: Abundant lightning in conjunction with sufficiently dry fuels (fuels remain dry or critical during and after a lightning event). Warnings are not typically issued for WFO Pendleton isolated coverage events. Warnings not typically issued for events that will be All FWZs accompanied by significant rain (greater than 0.25 inches). However, if a lightning event will occur with significant rain, but is then followed by very hot and dry conditions,
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