First we had some sleet or small hail today and now we're talking about a frost or freeze tonight.
It may sound like it's late in the season for a frost, so I did some digging. If we hit 32 at Lambert airport it would tie for the 5th latest frost on record in St. Louis. However, the latest frost was on May 10th back in 1966. I checked with the Weather Service today and they said the average last frost for St. Louis is around April 7th (which roughly means that 50% of the last frosts in Spring occur after that date).
So, tonight our forecast is for 35 at Lambert, but 29-34 in outlying areas. However, when you see those morning temperatures, keep in mind that those are temperatures at about 5 feet off the ground. After all, that's where most people "experience" the weather. That also means that in the morning the ground level temperature can be one to a few degrees colder than what you hear on television. Just something to keep in mind, and it explains why you may see frost when we're telling you the low temperature is 35.
At least it won't be as bad as this picture sent to KMOV.com back on March 21st 2006.