From NOAA 1991–2020 normals
Why Frost Date Websites Give Different Dates
The same town can have a last frost a week or more apart on two websites. The reason is the probability each one prints, and the station it reads.
NOAA does not publish one last-frost date per station. It publishes nine: the dates after which the last 32°F night of spring came in 90%, 80% and so on down to 10% of the years 1991–2020. A website has to pick one. The Old Farmer's Almanac prints the 30% date; many others print the 50% date, the middle of the record. Across the 4,701 stations behind this site's indexed pages where frost comes every year, the 30% date is a median 7 days later than the 50% date, and the 10% date 16 days later. From the 90% date to the 10% date is a median 32 days.
The second reason is the station. A town can be read from the airport, from a cooperative observer in its centre or from a station in the next valley, and frost depends on height and on whether cold air drains into the spot. Each ZIP page here names its station, the distance to it, and how the nearest other station differs.
The largest place of eight states, with the last spring frost at five of the nine probability levels:
| Place | Station | 90% | 70% | 50% | 30% | 10% |
|---|---|---|---|---|---|---|
| Katy, TX | Houston Sugarland Mem, TX | December 28 | January 20 | February 2 | February 20 | March 7 |
| Lakewood, NJ | Toms River, NJ | April 8 | April 16 | April 23 | April 30 | May 12 |
| Chicago, IL | Chicago Midway Airport, IL | March 30 | April 5 | April 11 | April 17 | April 27 |
| Corona, NY | New York Laguardia Airport, NY | March 12 | March 20 | March 26 | April 1 | April 6 |
| Antioch, TN | Nashville Berry Field, TN | March 19 | March 29 | April 4 | April 10 | April 21 |
| Lawrenceville, GA | Atlanta Peachtree Airport, GA | March 5 | March 17 | March 25 | April 1 | April 12 |
| Bakersfield, CA | Bakersfield 5 NW, CA | December 31 | January 17 | January 31 | February 12 | February 27 |
| Queen Creek, AZ | Chandler Heights, AZ | January 2 | January 17 | February 1 | February 12 | February 27 |
Read a column as: in that share of springs, the last 32°F night came after the date. 1991–2020, NOAA NCEI.
Which date to use depends on what a frost would cost. For tomatoes set out from a nursery, the 10% date leaves one spring in ten with a frost after planting; for peas and lettuce, which survive a light frost, the 50% date or earlier is enough. The frost risk by date tool gives the chance for any day.
Computed by ArtUp Labs from NOAA NCEI U.S. Climate Normals for the stations behind this site's indexed ZIP pages, one row per station. Methodology · All guides