Northern California has multiple critical fire weather patterns that result with three different types of critical fire weather events: dry thunderstorms, northeast Foehn winds, and southwest Foehn winds. Dry thunderstorms can occur under an upper level ridge that brings monsoonal moisture north, and it can occur on the edge of an upper level trough that is moving into the Pacific Northwest. Northeast Foehn winds affect the western slopes of the Sierra and Cascades and occur when an upper level trough moves inland over the Pacific Northwest or over the Great Basin. Southwest Foehn winds affect the eastern slopes of the Sierra and Cascades and develop as an upper level trough moves into the Pacific Northwest with strong winds and dry air moving over the Sierra and Cascades.
Dry thunderstorms develop over northern California due to two predominant patterns, an upper level trough approaching the northern Pacific coast of the United States and under an upper level ridge. Variations of these patterns occur, especially with the subtropical ridge pattern. For the subtropical ridge pattern, dry thunderstorms usually occur on the west side of the upper level ridge where southerly to southeasterly flow is present that brings moisture into northern California. During some of the cases, an upper level trough is present to the west, off the coast, but it does not break down the ridge. An upper level trough approaching the Pacific coast, usually the Pacific Northwest coast can trigger dry thunderstorms. As previously stated, dry thunderstorms occur on the fringe of the greatest instability and the deepest moisture. Thunderstorms form in this environment, but they tend to be drier. Wallmann et al. (2010) and Nauslar et al. (2013) are additional resources for dry thunderstorm in northern California.
During 20-21 June 2008, in northern California, predominantly dry thunderstorms produced over 5,000 lightning strikes (Map 1), which ranked in the top 5 of lightning strikes ever observed during a 24-hour time period for northern California. The lightning ignited 602 fires by 00 UTC June 22 throughout northern California which overwhelmed the local and regional fire management. Twelve firefighters lost their lives and the monetary cost of fighting the fires reached nearly $300 million (Wallmann et al. 2010). Thunderstorms developed offshore in the previous twenty-four hours providing evidence of the approaching moisture and instability. Map 2 outlines High Level Total Totals, which quantifies elevated thunderstorm potential for values greater than 26 deg C, over the region indicating the potential is present for elevated thunderstorms. This moisture and instability progressed onshore during the afternoon and evening of 20 June 2008 helping initiate mostly dry thunderstorms across northern California. Due to the offshore thunderstorms, the upper level jet was affected and actually created a smaller appendage of the jet that helped the thunderstorms develop over the coast and further onshore during the event (Map 3). Map 4 shows the upper level trough off the coast with some modest relative humidity (RH) at 500 mb, with less RH lower in the atmosphere at 700 mb (Map 5), and especially at the lowest levels at 850 mb (Map 6). More information about this event can be found in Wallmann et al. (2010) and Nauslar et al. (2013).
Northeast Foehn Winds
Similar to Santa Ana Winds, Northeast Foehn winds are downslope winds that are developed and accelerated due to cross-mountain flow. Northeast Foehn winds develop when a strong upper level trough moves inland in the Pacific Northwest or Great Basin. On the backside of the upper level trough, winds need to be orientated northeast to southwest or close to it (north to south can be acceptable). This usually means the upper level trough is tilted positively, similar to a forward slash (/). Due to the position of the trough and jet, sinking motion is present, and sinking motion coupled with strong cross-mountain flow helps produce a mountain wave, which triggers strong downslope winds on the west slopes of the Cascades and Sierra. RH also decreases as an effect of the mountain wave and downslope winds, which creates the potential for extreme fire behavior and rapid fire growth. Two of the fires affected these winds include the Fern and 49er fires. The 49er fire environment is examined below.
The 49er fire developed on 11 September 1988, east of Yuba City and north of Auburn, CA. It burned over 33,000 acres and destroyed 312 structures. A strong upper level trough moved from eastern Washington into eastern Idaho with an upper level ridge building northeastward in the eastern Pacific (Map 1; Map 2). A jet on the backside of the trough brought strong northeasterly, cross-mountain flow and sinking motion to northern California (Map 3). This helped fuel strong winds and low RH on the west side of the Sierra and Cascades that fueled the 49er fire (Map 4). A strong surface height (proxy for surface pressure) gradient is also apparent over northern California, which is a good indicator for stronger winds especially with this particular orientation of mostly north to south (Map 5).
Southwest Foehn Winds
The Southwest Foehn Winds are very similar to Northeast Foehn winds, the only differences being that the upper level trough is just offshore or in the Pacific Northwest, and the downslope winds affect the eastern slopes of the Cascades and Sierra. The strong southwest winds are accompanied by dry air, via the dry slot of the upper level trough. The dry air and strong winds create the same critical fire weather environment as the Northeast Foehn winds, but on the eastern slopes. We examine the Fountain Fire as an example.
The Fountain Fire began on 20 August 1992, 40 miles northeast of Redding, and lasted for about a week. The first few days had the greatest fire spread. The fire burned more than 64,000 acres and destroyed more than 300 homes. An upper level trough moved onshore in the Pacific Northwest bringing strong southwest, cross-mountain flow to northern California (Map 1). A strong upper level jet was situated directly over northern California, which enhanced the sinking motion and cross-mountain flow to create a nearly ideal environment for Southwest Foehn winds across northern California on the east slopes of the Cascades and Sierra (Map 2). The surface map shows a strong height gradient from west to east over northern California, which is another indicator for strong southwest winds (Map 3).