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. the

Pacific High with the upper level ridge inland and southerly flow aloft

The maps to the left are composites of five similar synoptic scale patterns that resulted in high fire danger across Western Washington. In each case, a portion of the Pacific high pressure system moved inland over southern B.C. following the passage of a short wave trough across western Canada. Strong east to northeast winds developed across Western Washington as the surface high slipped southeast into the northern Intermountain region. However, in these cases, the upper level ridge was centered inland over the Western States instead of offshore at 130W.

This synoptic pattern is usually characterized by 1) high amplitude, meridional flow, 2) a deepening upper level trough between 140W-150W, and 3) an intensifying ridge of high pressure aloft downstream over the Western States. At times, this pattern can also form when a strong split develops in the upper level flow pattern along the west coast. The composite 500mb height chart includes two cases of split flow when a subtropical ridge in the southern branch phased with an upper level ridge in the northern branch to form a full latitude ridge over the Western States.

After several days, offshore flow gradually weakens as the upper level ridge shifts further east and the "California" thermal trough shifts inland over Western Washington. Fire danger will remains high though under the influence of a hot, dry air mass; however, by this time, the east winds have ended. The critical fire weather pattern usually ends with a marine push as the thermal trough shifts into Eastern 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.

Please refer any questions or comments about this web site to: john.werth@noaa.gov