Why are there often strange, fixed radar echoes along the Washington Coast?

Almost every week someone asks me about a strange weather radar feature that occurs along the Washington coast.

UFOs? Chinese Balloons? A flock of birds standing still? Or could it be the ocean? The answer is revealed below.

Here is an example of April 11 at around 7 a.m. from the Langley Hill National Weather Service radar near Hoquiam Washington (radar shown above).

See that bluish area west of Hoquiam, just off the central Washington coast? That’s the feature.

Other features are areas of precipitation.

They leave. The area outside Hoquim remains the same.

The image above is the lowest sweep angle commonly used in US weather radars, half a degree (0.5 degrees) from the horizontal. However, the Langley Hill radar allowed a lower scanning angle (about 0.2 degrees) due to the intervention of Senator Maria Cantwell.

An image from that angle at the same time is shown below.

This feature has become stronger and bigger! A big suggestion.

Reflection out of the ocean

The Langley Hill radar is close enough to the coast that the radar beam at lower scanning angles actually hits the ocean surface and reflects back to the radar receiver. That creates the wrong echo. If you climb the tower (which I did during the dedication) one can actually see the Pacific Ocean (see image below taken about 2/3 on top of the tower).

One way to confirm a radar beam is looking at the ocean is to view the Doppler velocity field from the radar, which provides the speed of the target towards or away from the radar (see below).

The velocity of the “target” is essentially zero…so the object is at rest (this is not usually true for precipitation, being blown around by the wind).

The intensity of the return to the ocean is influenced by the lower atmospheric temperature structure. When there is a strong low-level inversion (temperature increases with altitude), the radar beam tends to be bent towards the surface, resulting in a strong signal. This is what is called a stable situation.

As the temperature drops rapidly with height, the beam is higher and less likely to return to the ocean (less stable condition).

You can be sure of one thing: that odd feature along the coast is no sign of coastal rain. Or UFOs.