When do NG depart Tucson

It appears that NG depart Tucson area around 1 April. I usually leave Tucson on April 1st, and my sightings usually occur up to the day I leave Tucson. This year - 2008 - I left Tucson on 14 April. 2 April was my last sighting. Of course, this is an assumption. Just because only 1 sighting was made after 1 April does not present concrete evidence.