Last night’s radar imagery was notable for its widespread contamination by a notorious NEXRAD bugaboo called anomalous propagation, or simply ‘anaprop’ by those who wrestle with it often.  Here’s the national animation, showing apparent bird movement over a large swath of the eastern US, but with a certain degree of ‘sloppiness’ added by the anaprop:

And here’s an extreme instance of the anaprop itself, from the Mobile station at 1 AM this morning:

The phantom ‘spokes’ of high reflectivity emanating from the station are caused by the radar beam being refracted downward, so what you’re actually seeing is the reflectivity of the ground.  A common cause of this phenomenon is an atmospheric event known as a temperature inversion, which occurs when a layer of warmer air forms above a layer of cooler air, reversing the normal altitudinal trend and interfering with the radar beam’s path.  This often happens on calm, clear nights within areas of high-pressure; after sunset, the ground begins to lose heat quickly, cooling the near-surface air, while subsiding air aloft (associated with the high) warms as it sinks, eventually forming a relatively stable blanket atop the cooler air below.  These conditions prevailed over most of the Southeast last night, and soundings confirm that an inversion developed.  Here is the temperature profile over Mobile at 1 AM.  The red line follows the change in temperature with altitude; note the near-surface jag representing a warming trend (a pretty substantial one, of about 8 degrees F) from the ground up to about 1200 feet.

Jackson’s temperature inversion was slightly less dramatic, with a maximum differential of about 6 degrees.  Now let’s check out the bird activity over Jackson, using filtered velocities to eliminate insects and other slow-moving targets.  As often happens, peak movement here occurred in the middle of the night, once a wave of coastal departees made it to our area; here’s a snapshot of their passage at 12:30 AM:

What stands out to me is the tight clustering of targets near the radar station (mostly 20-30 miles away), reflecting birds traveling at unusually low altitudes (<1000 ft).  With no cloud layers or headwinds aloft to keep them down, my best guess is that they were favoring the highly stable, slightly cooler air below the temperature inversion – or, more precisely, striking the best balance between staying low (to keep cool and avoid turbulence) and flying high (to catch stronger tailwinds).  This is a hypothesis that could be evaluated by keeping close tabs on bird movements and temperature profiles . . .

On a side note, I was interested to see high reflectivities last night over parts of Georgia and Florida, despite easterly winds prevailing aloft.  Here in the mid-South, this could be a good time to look for those elusive Florida migrants – birds like Cape May Warbler, Black-throated Blue Warbler, even (dare I speak its name?) Connecticut Warbler . . .