Some of the earliest emerging broadleaf weeds have begun to emerge. The biggest problem with weeds in turfgrass is reduced aesthetic value, although some weeds can out compete turfgrass when management is reduced. Smooth and large crabgrass, yellow foxtail, and annual bluegrass are the most frequent annual grass weeds in turfgrass. 
Smooth crabgrass emerges in the spring before large crabgrass. Smooth crabgrass emergence begins slowly when soil temperatures in the upper inch of soil reaches 54F for seven days and moisture is available. This 54F soil temperature occurs many times when the dogwood begin to flower and the forsythia flowers begin to fade. Visit this website to track soil temperature for your area: http://www.greencastonline.com/tools/soil-temperature .
Waiting to apply crabgrass preventer until just before emergence will ensure control of smooth and large crabgrass later into the season. Peak crabgrass emergence is from mid-May to July 1st. Crabgrass preventer must be applied before plants emerge, otherwise it will not be effective. After applying the crabgrass preventer irrigate the lawn to get the herbicide incorporated into the soil.
If crabgrass densities are high, a postemergence herbicide application may be required. If you have used preemergence crabgrass preventer for many years and have successfully controlled the crabgrass, it may be wise to stop applying the crabgrass preventer and scout to see whether any crabgrass will emerge. If it does emerge then apply a postemergence herbicide. Crabgrass can be controlled with some postemergence herbicides, but timing and rate are very important to effectively control crabgrass. Effective postemergence herbicides include Dimension, Methar 30, Acclaim Extra, MSMA Turf Herbicide, and Drive 75 DF, but some of these may be difficult to obtain. When applying postemergence herbicides be sure to obtain thorough coverage and do not mow for two days before and after the herbicide application.