How we find your frost dates
For each ZIP code we use the nearest NOAA weather station (within 60 km) that has freeze statistics in the official 1991 to 2020 U.S. Climate Normals. The average last frost is the median date of the last 32°F night in spring: half of years have a later one. The 1 in 10 date is later still: only 1 year in 10 has a frost after it, so it is a safer date for tender plants. The first fall frost works the same way in autumn.
Frost dates can differ a lot over short distances because of elevation, valleys and nearby water, so treat them as a guide and watch the forecast.