Upper Level Ridge and Breakdown
The building of an upper level ridge and the subsequent breakdown of the ridge is Alaska's most critical fire weather pattern. As the ridge builds into Alaska from northwestern Canada, hot and dry conditions develop over much of Alaska, especially across the interior. The hot and dry conditions cure fuels, which make the fuels more receptive to ignitions and fire spread. Eventually, an upper level trough will move into the area from the west breaking down the ridge while producing thunderstorms. The lightning from the thunderstorms can ignite many fires across the interior of Alaska. The lightning can last a couple of days to more than a week.
An example of the upper level ridge breakdown occurred in late May and early June of 2013. A ridge was entrenched over Alaska with the axis running northwest to southeast into northwestern Canada on 24 May 2010 (Map 1). An upper level trough slowly approached Alaska from the west over the last week of May helping to break down the ridge (Map 2; Map 3). As the ridge broke down over the interior of Alaska, instability combined with enough moisture produced thunderstorms as evident by the elevated values of high-level total totals (HLTT) (Map 4). Lightning ignited fires included Gilles Creek, Big Swede, Healy River, South Fork Healy, and Upper Gerstle. All of these fires made up the Delta Complex, which burned more than 27,000 acres outside of Delta Junction, Alaska. This pattern is also responsible for most of the largest fires in Alaska including the Railbelt (2012) and Taylor (2004) Complex fires.