Weather conditions for concurrent widespread fires in boreal forests were examined by various weather maps and temperature charts. The four study regions in boreal forests are three in East Siberia and one in Alaska. We applied preliminary analysis method for Sakha proposed by the authors to show the effectiveness of our approach. More than 12 very active fire-periods were identified from satellite hotspot data. Analysis results clearly showed fires during all active fire-periods became very active as warm air masses from south approached four study regions. These movements of warm air masses were mainly related to the meandering of large westerlies. To explain the large increase of daily hotspots (fires) during active fire-periods, a preliminary wind analysis was carried out. Strong wind conditions occurred when warm and dry air masses were approaching, stagnating, and passing over Southern Sakha under various weather conditions at lower air. During the top fire-period in Southern Sakha, wind velocity at lower air (925 hPa) changed from about 1 to 8 m/s while number of hotspot increased from around 1000 to 9000.