Hard freeze and wind chill

It's 29F this morning with a windchill of 22F. This isn't unusual for Minnesota April, but it's been so warm the last few weeks. If we could get one more great big snow, I'd be happy, but I don't think the rest of folk would like it.
It will be in the 70's in two or three weeks. A week or so of that and it will be OK to plant.

I'm letting the seedlings get a little outside air. Before they go in the ground, I've got to "harden" them. I didn't do this last year and had a lot of failed seedlings:

Tomato Transplanting Tips (from http://www.vegetableseedwarehouse.com/)

1. Harden off tomato plants for about 10 days before transplanting outside. The first day take the plants outside to a shady, protected spot for a few hours. Gradually increase the time each day and expose the plants to more sun and wind until they are outside all day and night.
2. Be sure the soil in containers is moist before transplanting.
3. Transplant tomatoes on a cloudy, wind free day or wait until the late afternoon.
4. Set plants out in well drained soil in a sunny location. If you live in the south, 1/2 day of sun is OK.
5. Try growing tomatoes in large containers on a patio or plant some tomato plants among flowers or shrubbery in a border. You don't have to grow them in a vegetable garden.
6. Set the tomato plants out deep. Remove the lower leaves and bury the stem so that only the top leaves are sticking out.