I just checked the radar on the NCAR site here: http://weather.rap.ucar.edu/radar  and could see the telltale sign of migration on both the Fort DIX (KDIX) and Dover, DE (KDOX) radar stations. Here’s the 11:55pm base reflectivity image for KDIX and the corresponding velocity image here. For those of you new to this, or if you’re just dusting off your radar cap- the base reflectivity image is the scan from the lowest elevation angle (the radar will scan multiple angles to complete a “volume scan” but we only use the base angle (0.5 degrees) for studying birds since 99.9% of the migration activity occurs within this elevation angle). Looking at the reflectivity I can see a few things: 1) we get a bloom after sunset consistent with migratory birds. 2) there is some anomalous propagation present, anomalous in that the beam is being bent in some areas causing it to reach out further at an angle lower than predicted by the scan angle alone. This is not uncommon especially after sunset when temperature inversions cause the beam to bend (usually toward earth). In some cases this can be particularly bad  and cause the beam to hit the ground (causing ground clutter) or even split so some of the beam hits the ground while some travels up at some angle. Anyway- not entirely important except that anomalous propagation (or AP for short) makes things look “messy”- which requires us to take a hard look at the velocity image.

Looking at the velocity image we see that the main trajectory (targets are moving from cool colors to hot colors) is from the NW->SE, and the velocities along the major axis indicate that these targets are moving >25kts. If we look at the radiosonde data for KOKX (the nearest radiosonde-collecting station to KDIX; radiosonde is also known as weather balloon data; taken twice per day at 12:00 and 00:00 UTC, or 8:00am and 8:00pm Daylight Savings Time (eastern)) we’ll see that the prevailing winds at ~3000ft (or 925mb) were 10-15kts out of the NW. Therefore these targets are traveling at 10+ kts faster than the prevailing winds- which can only mean one thing… migration is happening!

So what does this mean for birding tomorrow? Well, with decent movement out of Central NY State and New England, I’d expect to see some birds pushed to the coast of New Jersey tomorrow. Birds in the interior of the state will be well distributed and therefore harder to pin down- while even a moderate flight along the coast will be apparent to birders. The radiosonde data from Wallops Island, VA, indicates some NE winds at the surface which may serve to redirect birds inland over Delaware (although how far north that effect is felt is up for grabs). The Wildwood Airport is reporting light and variable wind currently, with a slight leaning toward SW (3kts) at Midnight. I’ll be heading to the Higbee Dike to see what early migrants engage in morning flight and to say hello to Tom Johnson who has returned for another season of “extreme birding for scientific research”. I will probably not post the radar until later in the morning/afternoon- so consider this “the post” until then.

Good Birding
David