Ocean and land interaction: Onshore vs offshore flow

KOIN 6 Weather Kids

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PORTLAND, Ore. (KOIN) – This next topic is near and dear to the Pacific Northwest and our response to the direction of the general wind flow. This lesson we are going to take a dive into onshore flow and offshore flow concepts and some of the related terms that come with it. The graphic below does most of the explaining, but there is definitely more to the process than meets the eye. We can begin with the basics:

Onshore Flow: When the wind is generally moving from west to east. That means it is coming from the direction of the ocean and moving east towards the land.

Offshore Flow: When the wind is moving from east to west. Coming from the interior sections of the state/states and moving towards the ocean.


The direction of the wind can significantly alter our conditions in the Willamette Valley and across the state. With that onshore flow cranking, it can move what we call a marine layer into the valley where moisture moves in from the west and finds it’s way into the valley. This can create low clouds and precipitation for many.

If you live near the coast, you probably have experienced a sea breeze or land breeze. A sea breeze is when cool ocean air is drawing to the land because of high pressure over the ocean and low pressure over land. You’ve guessed it, a land breeze is the exact opposite. While high pressure over land will direct the wind out towards the ocean where low pressure is in place.

Weekend Meteorologist Joseph Dames


What is unique about this process, is the time of the year and the season will also play a part in how we respond to the general flow.

For example, offshore flow in the winter can bring in cold and windy conditions for the Columbia River Gorge and areas of the Willamette Valley. It’s that easterly wind that aids in bringing our temperatures down to support snow. Because most of the cold winter air is banked to the east in areas of the Columbia Basin and it takes an offshore flow to push that cold air our direction. Additionally, a strong onshore flow in the winter can spell heavy rain and mountain snow because of the moisture content that is being produced with the warmer Pacific ocean.

In the summer, that offshore flow will heat us up and dry out our local conditions. When we see a strong easterly wind during a summer month, we are usually bumping up our daytime high temperature and concerned for fire weather conditions. When we get the onshore flow during the summer, we sometimes talk about that being “natures air conditioning”. This is because during the summer the cooler air coming from the Pacific Ocean can help drop the temperatures significantly. Especially for those communities along the coast, where it may be 90 degrees in Portland, but only the upper 60s at Seaside.

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