Cooler Than Normal Water Off the Northwest Coast

One of the reasons Northwest weather is so temperate in summer is the cool coastal waters off our coast.

And this year, we start the summer with surface waters cooler than normal.

This is illustrated by the sea surface temperature anomalies (differences) from normal (below), with blue indicating cooler than normal water.

Below is a map of sea surface temperatures off the U.S. West Coast (°C).   The purple and blue colors indicate the coldest temperatures.   You will need a sweater to enjoy a coastal stroll!

Why such cold water along the coast? 

One reason is the upwelling of cold water from below, as illustrated by the figure below. Such upwelling is forced by northerly (from the north) winds along the West Coast.

Importantly, these northerly winds have been stronger than normal for the past month.

Why? 

Because of stronger-than-normal surface high pressure offshore of the northwest.  This is demonstrated by the figure below, which shows the sea-level pressure anomaly (difference) from normal over the past month.

The red area indicates a much higher-than-normal high-pressure area offshore.

Since winds move counterclockwise around high-pressure areas in the Northern Hemisphere, this implies enhanced northerly flow along the coast, something shown explicitly in the figure below (which shows the wind differences from normal of the past month).

With this persistent pressure/wind pattern in place, the current 5-day forecasts are for colder than normal conditions over western Washington, Oregon, and British Columbia (blue and green indicate colder than normal conditions)

Keep your sweaters handy!