Feb 20, 2018. This multi-day rain event we're about to get is going to activate the spring and summer weed seeds. Now is the time to put down your preemergent herbicides for summer crabgrass and other summer annuals.

Complicating this, and confusing the issue, there are other plants already germinated and in a dormant state, which will come alive with this rain event. I call these wild flowers because they include bluebonnets as well as other more serious plants (thistle). These will not be controlled by a preemergent herbicide at this time. These weeds sprouted last fall during the 5-day rain event the last week in September. Their nature is to sprout and go dormant until the spring rains come. These are those rains. Here is a picture I took last Oct 5, a week after those rains.

Zoom in and you can see those tiny green dots are very small plants. Here is a much more recent picture, Feb 16, of those same plants.
You can see they are not dots any more. We have had 2 weeks of mist and fog which did not accumulate to anything, but it was enough to prep the soil for the coming rain. Zoom in and you can see they have grown considerably, but they would still be hidden under turf. I suspect these things are going to bolt with the rain coming over the next few days.