Frost Dates: First and last frost dates for Twitchell Dam, CA
What's a safe temperature for tender plants? When the nightly temperature falls, frost can form, even above 32°, because the air is colder high above the ground and the frost can form up there and then fall down onto your garden and do some damage, even if it's 36° on the ground. So many factors come into play, including wind, concrete, houses, trees and other structures, etc etc etc. Because of all this, you might want to consider 36° as "the danger zone".
In your average springtime, you have a 90% chance that there will be no 36° nights by May 21. In other words, you can pretty much count on being safe from frost by that day. But we want to get those tomatoes in the ground as soon as possible, right? We see that there's still an 80% chance of 36° on March 2, so we don't dare plant that early. We wait a few days and by April 3 we are at the 50/50 point. At this point, we are close and we can start watching the weather forecast. If the upcoming week's forecast doesn't show below 40°, then it's probably okay to risk planting out your plants.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| Last 16° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 20° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 24° | Jan 15 | Dec 26 | -- | -- | -- | -- | -- | -- | -- |
| Last 28° | Feb 15 | Feb 2 | Jan 23 | Jan 13 | Jan 3 | Dec 21 | -- | -- | -- |
| Last 32° | Apr 3 | Mar 18 | Mar 6 | Feb 23 | Feb 13 | Feb 2 | Jan 20 | Dec 31 | -- |
| Last 36° | May 21 | May 4 | Apr 22 | Apr 12 | Apr 3 | Mar 24 | Mar 14 | Mar 2 | Feb 13 |
The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:
- You have a small 20% chance of getting 32° by November 18.
- There is a 50% chance of being hit by a 32° frost starting around December 8
- You have a 80% chance of seeing 32° by January 2
- Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| First 16° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| First 20° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| First 24° | Dec 20 | Jan 10 | -- | -- | -- | -- | -- | -- | -- |
| First 28° | Nov 22 | Dec 4 | Dec 14 | Dec 23 | Jan 2 | Jan 15 | -- | -- | -- |
| First 32° | Nov 9 | Nov 18 | Nov 25 | Dec 2 | Dec 8 | Dec 14 | Dec 22 | Jan 2 | -- |
| First 36° | Oct 20 | Oct 29 | Nov 4 | Nov 10 | Nov 15 | Nov 20 | Nov 25 | Dec 2 | Dec 11 |
Data is provided by the NOAA National Centers for Environmental Information.