They don't like headwinds as they head south, and we have had persistent southerly (from the south) flow for weeks due to a persistent low off the coast (see image). Hard going for the little fellows.
For over a week, the typical lower-atmospheric weather map (here shown at 850 hPa, around 5000 ft) had this configuration, with a low right off our coast, resulting in persistent southerly flow and showers.
They don't like rain, and particularly heavy showers, and I don't have to tell you about our recent thunderstorms! I guess birds are a lot like people in that way.
But something magical for birds happened last night: as the low moved through, the winds in the lower atmosphere turned to northerly (see image) and the skies cleared. Optimal conditions for migration. And our bird friends were ready!
Weather map at 850 hPa (roughly 5000 ft). High pressure has built off the coast resulting in modest northerly flow over the western side of the Cascades. Tail wind for birds going south.
Let me show you, using a variety of weather radars, this pulse of bird migration in action.
The new Langley Hill coastal radar is a wonderful bird-watching tool. When little precipitation is around, these radars are in a highly-sensitive mode that really shows the birds. And remember, many migrating birds, particularly the smaller ones, like to fly at night when they are safer from predators.
Starting with the radar image (composite of all altitudes) at 7:49 PM Saturday night, we see a lot of ground-clutter returns (the lower radar beams hitting the surface mainly).
It will be dry for the next several days, so expect to see a lot more migrating birds on the local radars.