It was a classic, with the skies filling with bird echoes right after sunset and then disappearing near sunrise.
Let me show you.
9:30 PM that night. Lots of birds. They don't like to fly offshore. Smart birds.
Our radars are Doppler radars, which means we can get velocities towards or away from the radar. The Langley Hill radar at 2 AM shows the birds are moving northward, with green colors to the south (approaching) and yellow to the north (going away). Right direction.
A few years ago, the National Weather Service radars were upgraded to dual-polarization, which allows them to tell the shape of "targets." This information can be used to determine what the targets are composed of. Here is the output hydrometeor classification algorithm at 2 AM. It is going for BI or birds (gray color)..undoubtedly, a correct interpretation.
So what kind of weather do birds like to fly in? The famous Cornell Lab of Ornithology has a a wonderful site called BirdCast, which has a lot of this information, including migration forecasts.
According to this site, birds don't like flying in rain (who does?) and they prefer to have winds at their back (ditto).
With a ridge of high pressure over us now, there is no precipitation...so their feathers will be dry.
The winds? Here is a time-height cross section of the winds above Seattle-Tacoma Airport for the period. Time advances from right to left. The vertical axis is height, from sea level up to about 10,000 ft (700 stands for a pressure of 700 hPa). The wind barbs (blue color) show light winds, from varied directions. Time is in UTC (GMT). Good conditions for birds. So with ideal weather in early spring, the birds were ready to move northward last night.
|"El Niño, and the Rise of the Pacific as Global Climate Pacemaker"|
Thursday, April 7, 2016
7:30 – 8:30 p.m.