One of my favorite combos is begonias and 'Silver Falls' Dichondra. (Fibrous begonias of either cocktail or ambassador series are usually what I've used.) Scaevola does OK, too.

I discovered angelonia last year and mix it with wave petunias. Bothat can take the late afternoon heat well.

What you should do is look for a few annual plants at your local independant garden center....
Use Profusion Zinnias mixed with neat foliage plants like
Euphorbia 'Diamond Frost',Gaura 'Passionate Rainbow',Ipomoea 'Marguerite',and Iresine 'Bloodleaf'.
Ivy geraniums,Heliotrope,and Ageratum will all blend well together in your zone.
Happy planting!

I agree your local garden center - even the seasonal garden centers at Home Depot, Lowes, Wal-mart, etc. - is the best place to find a wide variety of colorful annuals that can take a Western exposure.

Simple, readily available and surefire - in pots on my west side I've used combinations of the common zonal geraniums (red is my favorite but they come in white, pink, salmon, etc.), sweet potato vine (comes in burgundy/black, variegated, lime green), Dianthus Chinensis (red, white, pink, salmon, or combinations of colors, ruffled, fringed, double flowered), petunias, Gerbera daisies, hybrid verbena...

You're pretty much only limited by your imagination when it comes to annuals that can take the sun as long as you water them regularly.

I'd take a good look at the boxes several times a day to make certain about the amount of direct sun. A little bit of late afternoon sun is not enough to support all of the sun loving plants listed above, but you may find out that the site gets more light than you think.

I don't think you can beat Wave or Avalanche petunias. They reach down to anywhere between 2' and 3' from my window boxes. Put something with them that contrasts to the shape of the plant and the shape of the flower. If there's room, something like blue annual salvia would be gorgeous.

The ivy type geraniums are nice for window boxes too.