Here is some science about nitrogen fate urea applied at different times of the year. It is clear the 5 lbs urea results in excess in ground water...but...less than 4 pounds applied leaves the ground water at lower nitrate levels, drinkable levels.
Referring to the original posters question about timing for crabgrass control, air temps of 70 consistently would be OK, but perhaps a bit late. Companies that have several weeks worth of crabgrass control to complete in the spring, usually start well before that...more like at the date of the first mowing. This link shows the best dates for crabgrass control--divided into early, target, and late. Other pests treatment dates are also shown. Around here we used to say:"Anytime in April."