Why is the coast sometimes the warmest place in the Northwest?

It happened again on Wednesday: the warmest location in the Northwest was along the coast (see max temp map for that day below) and not inland.  At North Bend, Oregon it got to 91F!  The Washington coast was warmer than eastern Washington!

We are used to thinking about the coast being a cool place.   Pacific water temperatures (around 50F) are generally much cooler than inland temperatures virtually the entire summer and if the air is coming off the water, cool temperatures and low clouds are the rule.

But the strange thing is that during the run up to many of our heat waves, the coast is often the warmest place for several days.  But why?

It all has to do with offshore flow and sinking down mountain slopes!

With cold water offshore, the only way for the coast to warm up is to isolate it from the cold water:  that means offshore, EASTERLY flow.   And that happened on Wednesday.

Here are the surface observations and 9-h model forecast for the Oregon coast at 2 PM on Wednesday.  Yep...we have offshore flow along the coast!

This offshore flow is associated with high pressure inland and an area of low pressure (the thermal trough), moving northward from California. (see map showing sea level pressure, surface winds, and lower atmosphere temperature below).

Higher pressure inland and lower pressure along the coast helps produce easterly flow at low levels.

But there is something else:  flow down the terrain. As air sinks down terrain, it is compressed (air pressure is greater lower down) and warmed (air warms when compressed as in your bicycle pump).  As shown in the figure below, under easterly flow there are three chances for air to sink:  on the Rockies, on the Cascades, and finally on the coastal mountains.   The air progressively warms as we move past one barrier after another to the west.  The result is that the warmest air in this situation will be west of the final barrier:  the coastal mountains.

So you want to imagine being on the coast of Maui or the Cayman Islands, slathering on sunblock while you drink something tropical in the warm sun?   Just pick an offshore flow day along the Northwest coast in spring or summer.  Close your eyes and imagine.

But one thing:  don't go into water.  If you do, the tropical illusion will be replaced by hypothermic shock.