Soil temp is very important, it is the catalyst for everything that happens. That's why Forsythia budding is like a turkey pop-up thermometer, it's a rule-of-thumb method for knowing soil temp is approaching 50 deg.
When forsythia are in full bloom, the average daily soil temp is already at 55 deg and seeds are already germinating. By this time, the efficacy of pre-emergent is greatly diminished.
Think of crabgrass control the same as birth control pills. They are used BEFORE "germination" to prevent germination. Pre-emergent protection typically lasts 60 to 90 days, so, if you put it down 30 days before soil temps are in the 50's, it is already there doing its thing. You don't want to put any granular products down while the ground is still hard. One heavy rainfall can wash it off your lawn and into the storm drains.
Pre-emergents work differently than weed killer in two ways.
Seeds on and under the soil surface absorb dissolved pre-emergent like a sponge when watered. That is why you can't put down grass seed the same time pre-emergent is present . You have to "water in" pre-emergents.
On the other hand, weed killer is absorbed into the entire plant through the leaf of a seedling or mature weed. Most weed killers are not absorbed through the root, so watering-in does nothing but wash the herbicide off the plant you are trying to kill.