The Impact of Snowoff Timing and Associated Atmospheric Drivers on the Alaska Wildfire Season

Abstract Wildfires occur each summer in the boreal forests of Alaska, with an increasing frequency of extreme fire seasons in recent decades. The wildfire season typically begins after the snow melts (i.e., snowoff) in April–May, which is trending toward earlier dates since 1959 (ranging from 2 to 4 days decade −1 over Alaska). This study evaluates snowoff dates in Alaska and related synoptic-scale atmospheric drivers in spring over 1959–2020 to assess possible linkages with the summer wildfire season. Many of the largest fire seasons followed regional snowoff dates in the earliest tercile accounting for 56%–95% of the total historical area burned by the region. Snowoff is best correlated with average daily maximum temperatures during April–May with weaker correlations thereafter. In some years, early snowoff and associated warm temperature anomalies persisted later into the summer influencing fire danger indices. This occurred in many years with the largest area burned. More limited instances of persistent lower precipitation anomalies were also found. The persistent temperature anomalies primarily occurred during El Niño conditions and the positive phase of the east Pacific/North Pacific pattern. Precipitation anomalies were most strongly associated with the Atlantic multidecadal oscillation. Blocking high pressure associated with these teleconnections was a likely driving mechanism, particularly for temperature persistence. The results indicated that the snowoff date and concurrent springtime synoptic climate conditions may provide predictability of fire weather conditions during the wildfire season when this persistence occurs. These subseasonal surface–atmosphere linkages could be used to inform wildfire-related seasonal forecasts. Significance Statement The purpose of this study is to understand how snowoff timing impacts seasonal wildfire activity in Alaska to inform wildfire-related forecasts needed for operational planning. Snowoff timing was found to directly impact fire weather conditions only during the early season. However, early snowoff was correlated with sustained warm temperature anomalies during several large wildfire years. These conditions were linked with El Niño and other teleconnections. Sustained dry conditions were most strongly associated with wildfire on the North Slope. Our results show that climate conditions during the spring snowoff period may serve as a potential predictor of summer temperature, precipitation, and wildfire in Alaska. These precursor relationships can therefore contribute to improvements in the seasonal prediction of wildfire danger.