Why the Olympics Are So Wet: The Rain Shadow Explained

Drive from the Hoh Rain Forest to Sequim and you leave some of the wettest spots in the continental United States for a town that gets about 16 inches of rain a year. The west side of the Olympic Peninsula is drenched, while the northeast corner sits in a rain shadow so dry that farmers irrigate their fields. The cause is the Olympic Mountains themselves, standing in the path of storms off the Pacific.

How a rain shadow forms

The park’s weather brochure explains that the ocean is the source of most of the peninsula’s weather. Storm clouds full of moisture move inland, run into the foothills, and are forced upward. As the air rises it cools and the pressure drops, so it can hold less water. The surplus falls as rain in the lowlands and as snow in the high country.

The storms keep moving east, losing more moisture as they go. Once the air passes the crest it sinks down the far side. Sinking air warms, and because it has already lost much of its water, its humidity falls. Clouds thin out and rain becomes scarce, leaving a dry “shadow” downwind of the peaks.

Up high, the moisture comes down as snow. Mount Olympus, the highest point on the peninsula at 7,980 feet, is estimated to get more than 50 feet of snow a year. Our post on how the Olympic Mountains formed tells the story of the range itself.

The wet side: coast and rain forest valleys

The park’s weather page gives 100 to 170 inches of rain a year along the coast and in the west facing valleys. The brochure calls the west side valleys the wettest spots in the continental United States, and says the Hoh Rain Forest records an average of almost 12 feet of rain a year. Heavy fog can add more than 30 inches of moisture to the forest on top of that.

All that water is why the Hoh and Quinault valleys grow temperate rain forests, with moss hanging from the maples and spruce. Our guide to the temperate rainforests of Olympic National Park covers the trees and plants, and mosses and lichens of the Olympic rainforest looks at the smallest growth that thrives on the damp.

The dry side: Port Angeles, Sequim and the northeast

The drop off to the east is steep. Olympic National Park Headquarters in Port Angeles averages only 25 to 30 inches a year, according to the park. Farther east, the brochure says, the northeastern peninsula lies in a dry rain shadow and the town of Sequim gets roughly 16 inches of rain a year.

That gap is remarkable on the ground. The park notes that Sequim farmers irrigate their fields and that a few prickly pear cacti grow there, less than 35 miles from lush rain forest. Inside the park, the change shapes the forests too: the western valleys are dominated by temperate rain forest, while the eastern slopes hold drier lowland forests.

Olympic Peninsula rainfall compared

Here are the yearly averages the park publishes for four places, plus one from the state climate office. Different weather stations and record periods give different numbers, so treat these as a guide to the contrast rather than exact totals.

Average yearly precipitation, west to northeast

Place	Setting	Inches a year	Source
Quinault	Windward side of the Olympics	151.45	Washington State Climate Office
Hoh	Rain forest valley	135.0	NPS
Kalaloch	Pacific coast	103.00	NPS
Port Angeles	North shore	25.9	NPS
Sequim	Northeast rain shadow	15.80	NPS

The Washington State Climate Office notes that while the Cascades cause the state’s most distinct rain shadow, the Olympics produce one too. Its own figure for Sequim is 23.98 inches, higher than the park’s, which shows how much the station matters. Either way, Sequim gets a small fraction of what falls on the Quinault.

When the rain falls

The park says most precipitation falls between November and April. Summers are drier everywhere, but the west side is still wetter than the rain shadow in every season, as the brochure’s average monthly rainfall shows.

Average monthly rainfall by season, Hoh vs Sequim

Season	Hoh (inches a month)	Sequim (inches a month)
Winter	18.33	1.61
Spring	10.33	0.94
Summer	3.33	0.79
Autumn	13.00	1.91

Much of the winter moisture lands as snow up high. The park says Hurricane Ridge traditionally records 30 to 35 feet of snow a year, which is why our Hurricane Ridge in winter guide matters for anyone heading up the road from Port Angeles.

What the rain shadow means for your visit

• Pack for both sides. A sunny morning in Port Angeles can turn into a soaking afternoon at the Hoh. Our list of what to pack for Olympic National Park covers rain gear and layers.
• Know the gap. Over a year the northeast gets a small fraction of the coast’s rain, as the figures above show, so weather in Sequim tells you little about the Hoh.
• Expect rain any time. The park warns that rain is always possible and that hikers should be ready for thunderstorms, especially at higher elevations.
• Plan by season. Our best time to visit Olympic National Park guide walks through what each season brings.

How Friends of Olympic National Park helps

We are Friends of Olympic National Park (FONP), a volunteer led friends group based in Port Angeles, right on the edge of the rain shadow. Our volunteers adopt a trail, join cleanups and host the Winter Perspectives lecture series. If the park’s wild weather and the forests it feeds matter to you, please become a member.

Frequently asked questions