National Fire Danger Rating System

The high amplitude pattern is characterized by a strong ridge of high pressure aloft centered over the Gulf of Alaska, a closed, upper-level low near Hudson Bay, and northerly flow aloft along the west coast of Canada into the Pacific Northwest. Shortwave troughs embedded in the flow can produce widespread thunderstorm activity across both Western and Eastern Washington. The amount of thunderstorm activity and whether it is wet of dry depends on the strength of the shortwave trough and the amount of moisture available, Experience has shown that shortwaves entering Washington from north are usually dry if their trajectory is mainly over land and wet if they have an over-the-water trajectory.

As the trough progresses into the Pacific Northwest, it can result in strong and gusty southwest gradient winds, especially over higher mountain tops and ridges of the North Cascades. Gusty northwest winds usually follow the trough passage.

The shortwave usually forms a dynamically induced surface trough east of the Cascades that results in increasing onshore flow across Western Washington. When this happens, fire danger begins to decrease as fuel moisture in the lighter fuels begins to increase. This tends to dampen the effects of increasing winds and also helps to lower ignition probabilities across Western Washington. However, new fire starts from lightning activity can occur with this pattern if 1) weather conditions prior to the trough passage have been unusually warm and dry, 2) the trough is weak and doesn't produce much shower activity, and 3) the resultant onshore flow is weak.