An Observational Analysis of Red Flag Warning Criteria in Minnesota with Respect to Daily Wildfire Occurrences

On 2 November 2022, the Minnesota Department of Natural Resources (MnDNR) received 34 new wildfire reports. This was the greatest number of new fire starts reported to the MnDNR on one Fall day since 4 November 2011. There were no active Red Flag Warnings (RFW) from the National Weather Service (NWS) on this day. In Minnesota, the weather criteria for a RFW is a minimum relative humidity of 25% or less with a sustained wind speed or frequent gusts of 9 ms-1 or greater. On 2 November 2022, temperatures were well above 22° C, with winds gusting over 14 ms-1 and occasional gusts to 22 ms-1. However, afternoon relative humidity values only fell to between 30% and 35%, which remained above the RFW criteria, with no RFW headlines being issued as a result. On 6 November 2022, a higher fire risk was anticipated once again in southern Minnesota, but this time a RFW was issued as relative humidity values fell to as low as 20% along with wind gusts as high as 25 ms-1, but with temperatures only from 10-12° C. The MnDNR only had 5 wildfire starts reported to them on 6 November 2022.

The large discrepancy in the number of wildfire starts between the two dates mentioned led to a simple question; Does the RFW criteria do an adequate job of capturing high fire danger days in the state of Minnesota? To identify significant wildfire fire days in the state of Minnesota, data for daily wildfire starts was collected for the state on the Wildland Fire Interagency Geospatial Services platform from 24 May 2014 through 2022. Days exceeding 25 new wildfire starts in the state were flagged as active wildfire days. With a list of active wildfire days compiled, weather data from Remote Automated Weather Stations across the state was obtained from those days to create observation based statistics. These observations were then compared to current RFW weather criteria and additional variables, such as the Vapor Pressure Deficit and indices from the National Fire Danger Rating System (NFDRS) and the Canadian Forest Fire Danger Rating System (CFFDRS). From this analysis, the effectiveness of our current RFW criteria at highlighting active wildfire days was measured, with additional variables and indices evaluated to begin the discussion of how RFW criteria could be modified in the future to improve the ability of our fire weather products to better highlight the potential for active wildfire days in the state of Minnesota.