The conditions necessary to produce high fire danger in Western Washington are closely related to the surface pressure pattern over the Pacific Northwest and western Canada. Periods of high fire danger occur when the region's marine-type climate is interrupted by the occasional invasion of a continental air mass or by subsiding air from aloft over the Pacific anticyclone. During these periods warm, dry, low-level winds blow across Western Washington from the east or northeast. Strong, offshore flow develops across the Cascade Mountains as surface pressure begins to lower along or just off the coast of Washington and Oregon.

More than three-fourths of the high fire danger periods in Western Washington occur when a surface high pressure system over the Gulf of Alaska or the Northeast Pacific moves inland over the southern half of British Columbia. This pattern results in moderate to strong, low-level, offshore flow across Western Washington. While this synoptic pattern can develop during any month of the year, in terms of high fire danger, the most critical months are June through September, when fuels are their driest.

Either meridional or zonal flow aloft can produce the Pacific High surface pressure that results in high fire danger across Western Washington. The mean 500mb height chart shown above is a composite plot of 5 meridional flow cases which produced strong offshore flow across Western Washington. In the meridional flow cases, the upper level ridge is usually oriented north to south off the coast, centered at approximately 130W longitude. Upper level winds over the region are generally from the north; however, if the upper level ridge noses inland over southern B.C., the upper level winds can be veer to the east or northeast. (See example below.)

Pacific High cases with zonal flow aloft tend to be more short-lived than cases with meridional flow because of the steady movement eastward of surface pressure systems under zone flow. Regardless of the flow patter aloft, an offshore component to the surface winds is necessary for high fire danger. In the typical case, an extensive area of surface high pressure develops over the Gulf of Alaska or the NE Pacific, and then moves inland over southern B.C. following the passage of a short-wave trough. As the high pressure ridge settles south east of the Cascades, the pressure pattern becomes favorable for easterly winds across the Cascade Mountains into Western Washington.

If the high pressure cell shifts southeast into the Intermountain region, the original northeast to southwest pressure gradient evolves to a more east to west pressure gradient across Western Washington. Additional downslope, adiabatic warming off the Cascades lowers surface pressures over Western Washington and Western Oregon, causing the "California" thermal trough to build north along the coasts of Oregon and Washington. A large surface pressure difference of 10-12 millibars can develop between the east slopes of the Cascades and the coast .

If the meridional ridge aloft is centered near or along the coast, much of the air in the easterly wind flow may come aloft, after having surfaced in eastern Washington due to the large-scale subsidence associated with the upper level ridge. When the subsiding air reaches the surface it is extremely warm and dry. Further warming and drying occurs when the air descends the west slopes of the Cascades, becoming the hot, desiccating "east winds" that bring high fire danger to Western Washington.

Pacific High with the upper level ridge nosing inland over southern British Columbia

The most severe fire weather is associated with the meridional flow pattern when the southern portion of a transitory short-wave trough develops into a closed, upper level low over the desert southwest, or off the coast of California as the upper level ridge builds inland over the Pacific Northwest and British Columbia. This pattern is often referred to as a "West Coast Block". It can persist for as long as a week to ten days, producing strong east winds over all of Western Washington.


NOTE:

The antecedent weather and its effect on fuels in the forecast district must be considered in each case. If the area has experienced considerable rainfall just before the onset of one of these critical fire weather patterns, fire danger will not be as severe as if the previous period had been hot and dry.

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