WX BRIEF

Climate of the Pacific Northwest

Variable conditions and numerous hazards in this area can create challenges for pilots.

By Karsten Shein
Comm-Inst.
Climate Scientist


Oregon, Washington and British Columbia comprise the core of the Pacific Northwest of North America. This region's weather can have a significant impact on aviation operations.

The copilot had already warned the boss and his family in the back that they may have to divert from their intended landing at BVS (Skagit WA) due to a cloud deck that remained well below minimums. They'd shoot the GPS approach, but after the nonstop flight home from OGG (Maui HI), they wouldn't waste too much time and fuel making multiple attempts.

Sure enough, as they approached the IAF at SOCLO intersection to start their approach to runway 11, the AWOS report put the ceiling at 200 ft with 1 mile visibility and rain – well below minimums. Carrying on, they reached the decision altitude but could see no trace of the runway or its lights. Pushing the throttle, the crew flew the missed approach and informed ATC that they'd try their luck at PAE (Everett WA) where the ILS and MALSR on runway 16 should give them a better chance.

Needless to say, the boss was not happy. The diversion would mean a long car ride back to the family home in the midst of a rainy Friday evening rush hour out of Seattle. But he couldn't blame the pilots for the bad weather as such conditions are typical for the region at this time of year, so he called ahead to the FBO at PAE to arrange ground transport. By the time they taxied in, a golf cart met them at the airstair and whisked them to a waiting town car, while the pilots made arrangements to park the aircraft for a couple of days until the weather cleared up and they could reposition back to their home airport.

The Pacific Northwest

Thick advection fog blankets Vancouver and the Fraser river. Cool maritime air flowing over cold surfaces often produces these fogs along the Pacific Northwest coast.

Like most regions of the world, the area considered the Pacific Northwest is not so much based on political boundaries as it is defined by a shared geography – physical, climatic, and even cultural commonalities. The Pacific Northwest is most commonly described as the part of North America from Oregon in the south to British Columbia to the north. Often, the Alaskan peninsula is included as well. From west to east, the region extends from the Pacific coast inland to the Cascade mountains, giving rise to its alternate name – Cascadia. Of course, with no definitive boundaries, the region is sometimes considered to include inland areas as far east as the continental divide.

The fact that the majority of the region lies between a major ocean and a major mountain range, and rests squarely beneath the westerly flow of the polar jet throughout much of the year, means that the Pacific Northwest is heavily influenced by an onshore flow of cool maritime air that is forced to rise as it encounters the mountains to the east. The position and track of the polar jet also favors the development of frequent and intense cyclonic storm systems in the Gulf of Alaska, which contribute a great deal of extreme weather to the area.

Globally, atmospheric scientists identify areas of different climates by their temperature and precipitation patterns. These patterns help us to understand and even forecast the day-to-day and seasonal weather conditions the area is likely to encounter.

Climatologically, however, the Pacific Northwest includes several climate zones. Along the coast, especially around the Olympic Peninsula and Vancouver Island, the climate is a "Marine West Coast" climate. Due to basin-wide circulation patterns in the ocean, most west coast places in the middle latitudes are influenced by a cold ocean current offshore that helps to suppress summer temperatures. A Marine West Coast climate is a temperate oceanic climate (abbreviated Cfb by climatologists) where the coldest month averages above freezing (0° C | 32° F), but no month averages above 22° C (72° F) and only 4 months are above 10° C

(50° F). The 'f' in the abbreviation indicates that there is no distinct dry season, rather rain and snow is distributed more-or-less evenly throughout the year. Of course, there are some distinctive variations in precipitation throughout this region, such as the rain shadow occurring to the ENE of Mt Olympus on the Olympic peninsula.

Rainy November evening at PDX (Portland OR). While larger airports in the region have instrument approaches that allow most modern jets to land under even the lowest of ceilings and visibility, most of the 500 or so airports in the Pacific Northwest require higher approach minima, and therefore are more prone to weather issues.

South of the Olympic Peninsula and inland of Puget Sound, the climate switches over to what is most closely a warm-summer Mediterranean climate (or Csb). While little of the area around Seattle and Bellingham could be considered Mediterranean in the classical sense, it does share some general temperature and precipitation characteristics with that European region. Temperature wise, this Csb region is similar to the coastal Cfb, but summer average temperatures in areas away from the coast will tend to be a few degrees higher.

For example, as with the Cfb climate, the average temperature of the coldest month in a Csb climate is above freezing. But, whereas the coastal climate is characterized by even rainfall totals throughout the year, the Mediterranean climate experiences a winter maximum of precipitation in which the wettest winter month has at least 3 times more precipitation than the driest summer month, and that driest summer month receives less than 30 mm (1.2 in). At SEA (Seattle WA), July sees just 16 mm (0.63 in) of rain, while Dec averages 138 mm (5.43 in).

Throughout this Csb region, the amount of precipitation can vary greatly. For example, while SEA averages just 864 mm (34 in) of precipitation per year, ONP (Newport OR) along the coast experiences nearly 1778 mm (70 in) per year. Even with such abundant rainfall, ONP sees most of its rain in winter, with a July average of just 26 mm (1.02 in), compared to 289 mm (11.38 in) for Dec.

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