Pacific Northwest
Pacific Northwest Predictive Services have identified three critical fire weather patterns: 1) thermal troughs producing hot, dry, and unstable conditions, 2) mass ignitions from lightning, 3) East/Foehn Winds, and 4) dry cold frontal passage. As with most of the areas, some form of the breakdown of an upper level ridge precipitates most of the critical fire weather patterns. Patterns 1, 3, and 4 usually exacerbate current fires although strong winds can ignite new fires by downing power lines. The East Winds and thermal trough critical fire weather patterns can also become a hybrid when the pressure change caused by a thermal trough produces strong easterly winds on the west side of the Cascades. Lightning outbreaks is the pattern that deals directly with ignitions, but usually do not exacerbate current fires unless outflow winds increase fire spread or the shear number of fires overwhelms local and regional resources. The severity of each pattern depends on fuel and atmospheric conditions and there are different varieties of each critical fire weather pattern especially lightning outbreaks.
Lightning Outbreaks
Wet and dry thunderstorms pose problems for the Pacific Northwest. The sheer number of lightning ignitions can overwhelm initial attack resources. Antecedent fuel conditions must be dry enough to support fire, but many times the vast number of lightning strikes can start enough fires to cause problems at the local and regional levels. A couple of patterns can spark lightning outbreaks in the Pacific Northwest. An approaching upper level trough off the coast that breaks down an existing upper level ridge is one pattern. Another thunderstorm pattern is a shortwave rotating around a larger upper level low further up the coast in British Columbia. Finally, a much less frequent pattern for thunderstorm activity is when an upper level ridge builds up into the Pacific Northwest that allows enough Monsoonal and some Pacific moisture to advect into the area helping produce thunderstorms.
A shortwave rotated south around a larger upper level trough located further north on the British Columbia coast during 8-9 September 2012 (Map 1; Map 2). The shortwave advected Pacific moisture along with some subtropical moisture that was brought northwest by the ridge that was over the Pacific Northwest. Elevated HLTT values (Map 3) coincided with higher quantities of precipitable water (Map 4), which indicated the potential for thunderstorms, especially high-based thunderstorms across parts of the Pacific Northwest (Map 5). As the shortwave moved onshore (Map 2), the collocation of moisture and instability and generated thunderstorms that moved further to the east and northeast (Map 5; Map 7). This event produced more than 50 fires including several large fires by 11 September 2012.
Dry Cold Front
A dry cold front is associated with an upper level trough that moves through British Columbia and into the Pacific Northwest. It occurs in late spring, summer, and fall. A dry slot within the upper level low, usually associated with southwest flow just ahead of the cold front contributes to the critical fire weather environment. As the cold front approaches, the pressure gradient increases, thus increasing winds just ahead, along, and behind the cold front. Due to the trajectory or the thermodynamic profile of the upper level trough, not enough moisture may be present to combine with the instability to produce precipitation resulting in a dry cold front. Strong winds and low RH from dry cold fronts can spread existing fires or ignite new fires by downing power lines.
The weather leading up to the fires of northeastern Washington 10 July 2008 was characterized by a building upper ridge between 6 July and 10 July. Temperatures rose and humidity fell over northeastern Washington as the upper ridge amplified. The ridge was flattened by a sharp upper level short wave, which passed over the British Columbia and Washington border on 10 July (Map 1). A strong upper level jet was present over the area as well (Map 2). The upper level shortwave was accompanied by a dry surface cold front, which moved across northeastern Washington during the afternoon and evening of 10 July (Map 3). Maps 4-6 illustrate the strong, dry front throughout the lower levels of atmosphere. RH values were below 30% (Maps 4 and 5) with winds exceeding 30 mph including at the surface (Map 6). According to Pacific Northwest GACC Predictive Services, this was a 99th percentile wind event for the area including the 4th windiest day in the past decade.
Thermal Trough
Hot, dry, and unstable conditions characterize thermal troughs migrating from west to east across the Cascades. Thermal troughs don't ignite new fires but can cause rapid fire spread on existing fires when fuels are dry and plentiful, particularly in timber type fuels. Thermal troughs are areas of surface low pressure that reside under an upper level ridge axis. Due to the heating below an upper level ridge, surface temperatures increase, which yields upward motion that creates an area of low pressure near the surface. The lower part of the atmosphere also becomes very unstable producing a Haines Index of 6. Due to the increase of temperatures, RH drops thus creating a hot, dry, and unstable environment. Gusty winds can be associated with thermal troughs due to the increased pressure gradient induced by the heating. Additionally, the thermal trough can produce a dramatic wind shift due to it being an area of surface convergence.
East Winds
East winds are Foehn winds that affect the western slopes of the Cascade Mountains. East winds are easterly winds that are orientated perpendicular to the Cascades that induce a mountain wave thus produce strong downslope winds for the western slopes of the Cascades. East winds develop when a strong, positively tilted trough sweeps through the Pacific Northwest. These winds are similar to Santa Ana Winds in southern California and Northeast and Southwest winds in northern California. East winds do not start many fires, but do spread existing fires where terrain and wind alignment is best. These winds mostly occur in late summer and early fall. East winds can develop due to the presence of a thermal trough as well. When the thermal trough is west of the crest of the Cascades and strong enough, gusty easterly winds affect the western slopes of the Cascades.
The final example for the Pacific Northwest area includes a combination of the thermal trough and East winds critical fire weather patterns. This combination of patterns occurred during 21-23 September 2009. A surface thermal trough on the Oregon/Washington coast created dry offshore flow, which boosted fire activity on several ongoing fires in the Oregon Cascades. The thermal trough then moved inland on 23 September directly over the ongoing fires in the Cascades and further boosted fire behavior because of instability. The easterly winds were not very strong at upper levels, but the winds were offshore which brought in very dry air over the area (Map 1; Map 2). Additionally, wind speeds increased near the surface as downslope winds emerged with the tightening gradient of the thermal trough accelerating flow (Map 3). The thermal trough moved further onshore over the Cascades increasing the Haines Index (Map 4), which led to large fire growth.