A new calibration for Fire Weather Index in Spain (AEMET)

Abstract

The Fire Weather Index (FWI) is an index based on meteorology. The system consists of six components and it depends on weather variables taken each day at 12 UTC (or forecasted for 12 UTC): temperature, relative humidity, wind speed and rain during the previous 24 hours. FWI is an accumulative index, that is, subindexes values for a day D are used for the calculation of the final index the following day D+1. The procedures for the calculation for Iberian Peninsula and Balearic Islands were initialized by AEMET in 2008 March, being executed daily without interruption since then. Canarias Islands procedures are being executed since 2013 May. Procedures include analysis for day D and forecasts for day D+1, D+2 and D+3 in a 0.5ºx0.5º horizontal resolution grid cells with data provided by the HIRLAM numerical weather prediction model. FWI values calculated for a determinate localization have no meaning for themselves. It’s necessary to make a correspondence between the danger classes and those FWI values in order to calibrate it. For this, five danger classes split has been calculated only from a climatological point of view. Each class or risk level corresponds with a range of values of FWI between different percentiles. Thus, fire low risk corresponds with FWI values below its percentile 40; moderate risk with FWI values between percentile 40 and 65; high risk between percentile 65 and 85; very high risk between percentile 85 and 95 and extreme fire risk above its percentile 95. In the calculation of the different percentiles, a data period from May 2008 until December 2013 has been used. This period will be updated with most recent values according to the month pass.