Awesome!

I am in zone 5b as well, my recommendation would be and others please chime in, would be to do any cole crops now and perennial herbs.

Cole crops like broccoli, kale, collards, cabbage, beets, swiss chard, spinach,onions, anything that likes cool weather to start. Tomatoes do surprisingly well with this method, I had some I sowed in December and March. The March ones sprouted a month earlier for me. Just when I thought the December ones didn't I discovered a new sprout. My tomato plants were small when planted, but quickly caught up during the summer. They plants were still fruiting into the fall. I pulled them when the first signs of frost came, but probably could have kept going.

Perennial herbs like sage, thyme, oregano, marjoram, chives can be started now. The warmer weather ones like basil could be started later. Last year I sowed them late in containers, I think it was May and they did well - the spring we had was cold so I didn't worry too much.

Experiment and have fun with it, and document what you can.

I second what Gardenunusual2 says: Any cold weather crops as well as perennials are fine to sow now. Possibly hardy annuals like pansies.

Personally I use pro-mix but as long as it is labeled potting mix, I am sure it is fine to sow. Though from what I understand, steer clear of the cheap stuff that certain drugstores sell. They are apparently quite terrible and potentially full of weed seeds.

As for how thick to sow, you'd have to look at the age of the seeds. (Older = less germination for most seeds = sow thicker) I'd rather sow too thin than too thick to be honest. I put 5-9 large seeds per container (like peas, squash and the like) and a pinch of tiny seeds that I try to spread out as good as possible. Remember that these plants have to have space to spread out a bit before they get transplanted.

The concern is, if you sow it now and if they sprout let's say by end of feb or end of march because of unusually warm weather, then what do you do? Even cold hardy veggies such as lettuce, kale, collard can not go in ground till mid April or end of April and last frost date in my area is Mid May.

No worries with WS veggies because they'll sprout when ready & be able to take what weather comes their way. I've let kale self sow & replant if too thick in one spot. Last fall I let lettuce, parsley, radish, & arugula self sow. I have sprouts from all of them & we've had plenty of winter weather with 13 degree lows. I recently side dressed with some composted manures & put up a few bottomless jugs or clear rubbermaid containers over them to help speed things along, so I can ID the rest of the sprouts & move them where I want them to finish growing.

Last week we had 55 degrees and sunshine for 7 days straight. Today it's 37 degrees with wind, hail, & heavy rain showers. The sprouts will be fine.

I agree about the cold hardy vegies. They'll be fine in the containers until they can be planted in the garden. You can add a very diluted fertilizer to them.

In fact last year I gave lettuce sprouts in a large container (size of a shoe box) to a friend, and she kept them in the container all season, and cut them, and they grew back. I wouln't recommend that, but it worked.