Is there a preemergent to use on Poa annua grass in the lawn and if so what is the name and when should it be applied?
According to The Lawn Bible by David R. Mellor (Hyperion, 2003), your best defense against Poa annua (annual bluegrass) is to mow high (2 inches, usually) to shade out weed seedlings; aerate the soil to improve drainage, because weeds thrive in waterlogged soil; and let the surface of the soil dry out between waterings. If one were to apply preemergent herbicide, this would be done in late summer to early fall, but we strongly recommend that you avoid use of herbicides and pesticides in lawn care due to environmental and health concerns. There are effective non-toxic approaches to weed management. Northwest Center for Alternatives to Pesticides has useful information on maintaining a lawn without chemicals.
For another good discussion of this issue, you might want to refer to the book, What the Experts May Not Tell You about Growing the Perfect Lawn by Tom Ogren (Warner Books, 2004).
The University of California, Davis Extension has a document about the management of Poa annua, including specific information on the various herbicides that have been used to treat it but, as mentioned before, it is best to avoid the use of toxic chemicals in the garden.