Every spring, I get this question from people who are champing at the winter bit. It usually goes something like this, "it's so warm outside, can't I start my seeds early?" While you can start them early, you are really shooting yourself (and your plants) in the foot (leaf?) by doing so.
The weather is always unpredictable, particularly in spring. From what I read, though I'm not a scientist, while it's tempting to think that global warming means that we can plant earlier, it also means that there will be more weather surprises, including wild temperature variations.
There are two issues here, planting seeds indoors early, and planting seeds outdoors early. The risks are somewhat different but tied. Let's take planting indoors early first. When you plant indoors early, it means that the chances are that your plants will be stressed, and/or rootbound by the time you put them outdoors. While it is really tempting to start early, it makes much more sense to wait. Besides, if your plants are ready to go out, you will be tempted to plant them outside before your last frost date and then your chances are pretty high, that unless you protect them, your plants will get whacked.
Now let's talk about planting seeds outside early, sometimes it does work out if you plant, but you are taking a risk. You don't want to plant and then have a frost, which can happen quite often. Also, the soil temperature can be too cold to have the seeds germinate, and then you've wasted both your time and your seeds.
So my message is, patience, grasshopper. Wait.