Hard to believe, but monarchs regularly overwinter in small numbers (dozens to high hundreds) in arid desert locations such as in the Saline Valley in California - a moon crater shaped valley about midway inbetween Mount Whitney and Death Valley:
Distant aerial view (orange dot is the cluster site location):
http://i5.photobucket.com/albums/y189/m ... alinec.jpg
Closer aerial view (orange circle is the cluster site location):
http://i5.photobucket.com/albums/y189/m ... alined.jpg
Here's a January 27, 1990 midday view of the monarchs nectaring and sunning themselves on the mulefat bushes:
http://i5.photobucket.com/albums/y189/m ... alinea.jpg
Here's a early December 1996 midday view of some the monarchs at this same location clustering in Tamarisk bushes:
http://i5.photobucket.com/albums/y189/m ... alineb.jpg
The Saline Valley overwintering sites are just like the overwintering sites in Mexico that
most mating and cluster break up occurs in February. The butterflies are in good condition despite the very low daytime humidity (in the 5-25% range), scant winter rainfall and the fact that the only evergreen vegetation in mid and late winter are bushes such as tamarisk, mulefat and creosote.
One problem with the Saline Valley habitats, however, is that roughly once every five years, overnight temps dip into the teens and freeze most or all of the monarchs.