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Thanks in advance -David
With winds out of the south and west last night, birds should have taken the night off from migration. A little peak at the radar, though, left me with a few questions to ponder. Here’s the radar from 7:00pm last night through 5:00am this morning.
Frames are every 1/2 hour. Click on the thumbnail to view the full-sized animation.
Okay, so migration conditions last night were clearly poor (unless you’re a bird migrating north, when everyone else is heading south). The winds at the surface were southwesterly (~8 kts), while those aloft (3000 feet) were from the south (over DE,MD and southern NJ) and west (over PA and the rest of NJ) at about 15 – 20 kts. If we look at the base reflectivity images we do see a fair amount of “noise”, that is, low signal values (mostly gray in color), most of which is clumped near the radar (indicating that the objects (if they even are objects) are likely ground clutter. There does seem to be a pulse of activity after sunset, which could be cause by temperature inversion, which is usually identifiable using the velocity image (it will tend to show zero velocity). Instead, the velocity image shows movement across the radar consistent with the wind direction BUT apparently faster than the prevailing winds which usually suggests powered flight.
If the direction had been N->S, I could easily have said these were low-flying birds and it would have been tough to dispute it (there are some blue values mixed into the “noisy” background, which could help corroborate my story). My point is that even when the radar indicates objects moving at speeds 15-20 kts faster than the prevailing winds, it’s possible that part of the data represents something other than birds. This is especially true in the fall, when the atmosphere is a biotic stew, full of insects and pollen. So if some of this is a non-bird signal, how to explain the objects moving faster than the prevailing wind? Well, I don’t know. Right now I’m just going to wave my hands and say, it’s most likely not birds, given the direction of travel together with the very low reflectivity values and proximity to the radar. Based on intuition I’d say that what we’re seeing is a mix of ground clutter and other biotic and abiotic reflectance, mixed with some gusty winds. Of course, if anyone has any reason to believe “reverse migration” could explain part of this, I’m all ears.
P.S. Come check out my migration forecast for the Mid-Atlantic on Birdcapemay.org