Use Historical Data to Plan Tomato and Vegetable crops.

The best data is old data. This link will show you all you need to know for timing your plantings. For tomatoes it looks like May 20th.  Keep in mind the 37 degree night was frost in my area. I remember it. The city areas tend to stay warmer and Im in a lower area.
 
Tomatoes want warm nights.
 
 
LAST YEARS temperatures for May 2009