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Critical Fire Weather Patterns

In western Washington, strong east winds have long been associated with high fire danger. Historically, some of the largest, most destructive wildfires in Western Washington were caused by strong east winds that fanned slash burns (intentionally set fires associated with land-clearing operations) into uncontrolled wildfires. While there are numerous synoptic weather patterns that can produce strong offshore flow, the ones most critical (in terms of high fire danger) occur in late summer and early fall at the end of our normal, summertime, drying cycle. When surface high pressure systems are situated to the north and northeast of Washington, moderate to strong northeast to southwest pressure gradients develop across the state. When this condition exists, our normal, marine-type climate is interrupted by an invasion of a very dry and sometimes cool, continental air mass.

Strong east winds seldom occur in July or the first half of August in Western Washington. However, weak offshore flow during this period will block marine air from moving to the interior lowlands. The result is hot, dry weather over the interior lowlands, the Cascades, and the higher elevations of the Olympics, but coastal lowland areas remain rather cool with night and morning low clouds, partial afternoon clearing, and fairly high relative humidities.

In the late summer or early fall, surface high pressure systems moving southeast out of British Columbia can actually intensify as they reach the northern Rocky Mountains. In these cases, east to west pressure gradients actually increase across Western Washington over the period of several days. Warm surface temperatures resulting from adiabatic, downslope warming off the Cascades will cause the California thermal trough to expand north along the Oregon-Washington coastline. When pressure gradients across the Cascades reach 8 millibars or more, east winds of 40-60 mph can be expected in east, windprone drainages or through terrain gaps in the Cascades and the Olympics. Afternoon temperatures along the coast will soar to nearly 100 degree while relative humidities drop to between 10 and 20 percent.

Synoptic patterns which result in high fire danger across western Washington due to strong, offshore flow can be broken into two main categories. The two synoptic patterns are 1) the Pacific High with post-frontal, east winds, and 2) the Northwest Canadian High with post-frontal, east winds. The main differences between the two categories are the source region of the surface high pressure systems and the time of year each one occurs. The Pacific High pattern occurs most frequently in July while the Northwest Canadian high is most frequent from late August through mid-October.

Pacific High Synoptic Pattern

The Pacific High synoptic pattern that causes high fire danger is Western Washington can occur with either zonal or meridional flow aloft. In meridional flow a north-south, upper-level ridge is centered just offshore with northerly flow aloft over Washington, or the ridge is centered inland over the western states with southerly flow aloft over Washington.

The development of high fire danger due to Pacific High synoptic pattern would typically follow the scenario listed below.

A strong surface high pressure system builds over the NE Pacific or the Gulf of Alaska.

Following the passage of an upper-level, shortwave trough, a nose of the surface high pressure system will fill in behind a weak, surface cold front and then push inland over southern British Columbia.

The surface high pressure system then shifts southeast into the northern Intermountain region.

The examples shown below are based on composite 500mb and sea level pressure maps for similar flow patterns that resuted in high fire danger due to strong foehn or east to northeast winds across Western Washington.



	Pacific High with the upper-level ridge at 130W



	Pacific High with the upper-level ridge centered inland over the Western States

Northwest Canadian High Synoptic Pattern

Extremely high fire danger can also occur from late August through mid-October when surface high pressure systems over northwest Canada move southeast into the southern portion of British Columbia. Look for high amplitude, meridional flow over the Gulf of Alaska, northerly flow aloft into the Pacific Northwest, and a strong surface high pressure system situated over northwest Canada. Strong offshore flow will develop across western Washington as the surface high slips southeast into southern British Columbia. The pattern begins with strong outflow winds from the Frasier River Valley into northwest Washington. Look for northeast winds 20-40 mph, cool temperatures, and very low relative humidities at Bellingham and Sumas Mountain first. This flow will then cross the northern portion of the Olympic Peninsula through the Soleduck drainage, resulting in gusty east to northeast winds and extremely warm temperatures and very low relative humidities along the north coast of Washington. Click on the case scenario for an example of how this synoptic pattern evolves over a three day period.

Case Scenario of Northwest Canadian High

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