The last spring freeze doesn’t happen on the same day every year. Instead of a single “average” date, NOAA’s National Centers for Environmental Information (NCEI) publishes freeze dates at several probabilities, computed from 30 years of daily low temperatures (1991–2020) at each weather station.
Spring: the last freeze
NCEI’s spring statistic (“PRBLST”) is the date by which the last freeze of the cold season will have occurred, at a given probability. On our pages we phrase it as the chance of a freeze after each date:
- 90% date (earliest): in 9 years out of 10 there is still a freeze after this date.
- 50% date (median): half the years have a freeze after it, half don’t. This is the “average last frost date”.
- 10% date (latest): only 1 year in 10 has a freeze after this date. This is the cautious date for planting frost-tender crops.
Example, Chicago O’Hare (32°F): 90% April 3, 50% April 16, 10% May 1.
Fall: the first freeze
The fall statistic (“PRBFST”) works the other way: the chance that the first freeze has already happened by each date.
- 10% date (earliest): 1 year in 10 has a freeze by this date.
- 50% date: the median first freeze.
- 90% date (latest): in 9 years out of 10 a freeze has happened by this date.
Example, Chicago O’Hare (32°F): 10% October 15, 50% October 28, 90% November 11.
Three temperatures
We show dates for 36°F (frost possible on exposed plants), 32°F (freeze) and 28°F (hard freeze). See frost vs. freeze.
Growing season
The growing season length is the number of days between the last spring and first fall freeze. NCEI publishes it as its own statistic (“PRBGSL”); we show the 50% value.
Limits
- These are climate statistics, not forecasts. Check the 7-night NWS forecast on each page during frost season.
- Values come from the nearest station with complete records, which may be miles away or at a different elevation.
- Warm locations that freeze in fewer than half of years have no 50% date.
Source: NOAA NCEI U.S. Climate Normals 1991–2020 documentation.