Posted by Looking at weather radar this time of year can be a bit confusing, as radar returns often appear seeming out of nowhere just after sunset and disappear just before sunrise. A radar loop of the past two nights over the PNW shows this amazing phenomenon, which can be attributed to migrating birds headed north for the summer.
First of all, are these really birds? The hydrometeor classification algorithm on RadarScope uses the dual-polarization of weather radar to classify particles based on their size and orientation. The large size and random orientation of birds makes it easy to classify them as ‘biological’ as opposed to raindrops which tend to be shaped like hamburger buns (wider than they are flat).
Another interesting characteristic of bird migrations is that the birds preferentially fly over land. In the first GIF above you can see the birds do not fly over the Pacific Ocean. In the GIF below, which shows the birds taking off after 9 PM Sunday evening, the initial returns are all over land and not over the surrounding water.
The birds appear to disperse near the end of the above loop, but in reality they are just flying too high to be seen by the lowest beam (Tilt 1) of the Camano Island radar. Similar to precipitation, we can only see the birds where we have radar coverage.
Doppler radar also gives us the ability to measure the speed of the birds’ flight. The Camano Island radar clocked the birds around 25-30 mph. Not surprisingly, the direction of moment is south to north, as green colors indicate movement toward the radar and red colors indicate movement away from the radar.
The birds were not getting any help from wind, as the wind direction in the lower levels was light and variable on Sunday night.
BirdCast
Ornithologists (bird experts) have developed an incredible tool to keep track of bird migration patterns with weather data. The BirdCast project uses an algorithm baed on radar data to keep track of migration patterns in real time and make predictions about migration for upcoming days. This tracking project is helping researchers learn what kinds of environmental factors influence bird migration.
The live migration map from Sunday night confirms what we saw on radar — medium bird migration traffic moving northward over the PNW. East of the Rockies there are incredible numbers of migrating birds.
What does the bird migration forecast look like for upcoming nights? Continued migration along the west coast, as well as an increase in migration activity in the Great Plains.
What species of birds are currently migrating over the PNW?
That’s a great question that I don’t have the answer to. Please reach out by leaving a comment or on social media if you know where to find that information! I’m sure our local bird watchers are keeping an eye out for new visitors to their feeders.
The Convergence Zone 