National Fire Danger Rating System

One of the principal differences between the two types in that the NW Canadian High episodes are more likely to occur in late August through early October while the Pacific High episodes are most frequent in July and early August.

The pattern aloft is usually meridional with a well developed ridge off the PAC NW coast, a short wave trough dropping southeast into the northern Plains, and cool but rather dry northerly flow aloft over the Pacific Northwest.

The charts below illustrate a typical NW Canadian High episode that evolved over a three day period in September of 1985.

In a typical case, a front, usually oriented nearly east-west, passes trough the region moving in a southeast direction. The front is followed by a strong high pressure system from Northwest Canada. Surface winds initially are from the north, but as the high slips further southeast, winds veer to northeast and eventually to the east as the high slips into the Intermountain area. A strong east to west pressure gradient develops across Western Washington producing gusty downslope winds from the east (these may be gap winds or mountain waves, or a combination of the two). As this occurs a thermal trough begins to develop north along the coasts of Oregon and Washington. The east winds end when the thermal trough moves to the interior of Western Washington, but high fire danger usually persists for one or two more days due to the hot dry air mass associated with the thermal trough.