Know your weather terms: onshore vs offshore flow
Jun 3, 2014, 4:07 PM | Updated: 4:18 pm
This image is from a webcam atop Building 1 on the NOAA Western Regional Center campus at Sand Point in northeast Seattle. The camera looks north up Lake Washington toward Lake Forest Park and Kenmore.
The forecast this week reads, ‘morning clouds with afternoon clearing each day.’ The morning clouds are the result of onshore flow – the flow of air near the surface from the Pacific Ocean into western Washington.
Onshore flow usually brings cooler ocean air inland during the warm season often accompanied with clouds and even some fog and drizzle.
In contrast, offshore flow is air near the surface moving from the interior of Washington to off the coast. This air flow usually results in clear dry conditions and during the summer, quite warm temperatures.
Most of our warmest temperatures in western Washington occur during periods of offshore flow. Here is an educational slide show on Heat Waves in Western Washington, good for the classroom and elsewhere that help prepare you in advance of these hot weather conditions.
The transition from offshore flow to onshore flow can result in quite dramatic weather changes. One of these changes can be strong westerly winds blowing from the west down the Strait of Juan de Fuca, sometimes reaching gale force or winds around 40 mph. Temperatures can plunge 20 degrees or so in a matter of a few hours during this transition, as well.
So the next time you hear the terms ‘onshore’ or ‘offshore flow,’ you will know what they mean and what kind of weather usually accompanies them.
