Altocumulus castellanus transitions to stable altostratus on 12-07-2010

Here is a time lapse of altocumulus castellanus clouds passing overhead from the southwest ahead of a short-wave trough.  While the altocumulus were moving from the southwest, lower clouds were almost stationary.  Some of the altocumulus produced snow which sublimated before reaching the ground.  The loop concludes as more moisture came in at a slightly lower level creating a more stable cloud deck with embedded waves.
 
http://www.youtube.com/watch?v=4igb0MYgooM

The sounding below may explain some of the behavior we see here.  The steep lapse rates above the frontal inversion at 600 mb may be the level of the castellanus.  The slightly lower altostratus deck moves in at a slightly lower level, perhaps near 700 mb where the air is stable even when saturated.  Then the nearly stationary clouds may have occupied a layer where the sounding showed light winds at 800 mb.  All this is really conjecture at this point but it's fun to guess what's happening.

For an excellent discussion on Castellanus, take a look at Corfidi et al. 2008.

Corfidi, Stephen F., Sarah J. Corfidi, David M. Schultz, 2008: Elevated Convection and Castellanus: Ambiguities, Significance, and Questions. Wea. Forecasting, 23, 1280–1303.