Mega Bird Migration

It has been a hard time for migrating birds over the region.  

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).

About an hour later (9:09 PM) and after sunset, things have really changed.  Lots of echoes and some very intense.  These are the birds.  Birds don't like to migrate offshore very far and you can see that in the echoes.

 12:22 PM the echo coverage has expanded.  Lots of birds on the move

 5:37 AM there are still some birds, but the numbers are dropping.

 And after sunrise at 6:39 AM, nearly all are gone and we are back to ground clutter

The Langley radar is a Doppler radar and it gives the velocity of the targets (in this case birds) towards or away from the radar.  Here is the Doppler image at 12:46 AM.   Green indicates approaching and red and orange going away.   Clearly the birds are heading south!

Now let me show you something interesting.   Here in Seattle we have a very special type of weather radar, called a radar-wind profiler, located at the NOAA facility at Sand Point.   Instead of sweeping horizontallly, this radar has three static beams, mostly facing upwards. This radar picks up birds as well.  Take a look at an image from this radar for the 24-h ending mid-day Sunday.  The y-axis is height in meters and time is on the x-axis (in Greenwich Mean Time (GMT, UTC), 06 is 11 PM, 12 is 5 AM, etc.).  Stronger returns are in purple, blue, and green.  The birds are obvious.  Around 03 UTC (8 PM) we we see the start of the bird echoes.  Lots of flying in the evening, which fades a bit in the middle of the night.  But you see a complete collapse after 5-6 AM as it starts getting light out.

 

It will be dry for the next several days, so expect to see a lot more migrating birds on the local radars.