Most frost-date tables list three dates for each season: a 10% date, a 50% date and a 90% date. They look like confidence labels, but they are probability levels from NOAA's 1991-2020 climate normals, and the way they run is easy to read backwards.
The 50% date is the median
Over the 30 years NOAA analysed, half of the years had their last spring freeze before the 50% date and half after. It is a middle-of-the-road date, not a promise. In roughly one spring in two you will still see a freeze after it.
The other two levels bracket the range
For the last spring freeze, the 10% date is the later one: only about one spring in ten has had its last freeze after that date, so planting after it leaves a wide margin. The 90% date is the earlier one, the date by which the last freeze had already happened in most years.
For the first fall freeze, it runs the other way. The 10% date is the earliest first freeze (about one year in ten has seen a freeze by then) and the 90% date is the latest.
NOAA's own field description says "10% probability ... or earlier" for both seasons. The published values, however, are ordered as described above, and that is how our products label them. If you want to check, pick any station in the data and look: spring dates get earlier from 10% to 90%, fall dates get later.
Which one should I use?
- Cautious gardeners use the spring 10% date and the fall 10% date. The window between them is the shortest, and the margin is the widest.
- Typical is the 50% level on both ends.
- Optimistic uses the spring 90% and fall 90% dates. The window is the longest and you accept a real chance of an early or late freeze.
Tender crops that a single freeze would end usually deserve the cautious end. Hardy crops can use the optimistic one.
Remember what the numbers are
They are historical statistics from 1991-2020 at one weather station. Actual frost dates vary from year to year and from yard to yard, so the table is a planning aid and not a forecast. Check the current weather forecast before you plant out.
Want the dates for your own station? See the Garden Calendars workbook and regional packs, or the free sample PDF. Next guide: Last spring frost and first fall frost: how to read both.
Please note. Historical 1991-2020 averages from NOAA; actual frost dates vary year to year; not a forecast or a guarantee; not agricultural advice. Planting windows are generic offsets from historical dates; follow your seed packet, your local extension office and the current weather forecast. This product is AI-assisted, so errors are still possible; please report them to gb1@mail.grokbot.com.
Source: Palecki, M., Durre, I., Applequist, S., Arguez, A., Lawrimore, J. (2021). U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), version 1.0.1. NOAA National Centers for Environmental Information. doi:10.25921/4ek7-fk11. Quietcog is independent and is not affiliated with, endorsed by or sponsored by NOAA or the National Weather Service. Quietcog processed the station files; any processing errors are ours.
Support: gb1@mail.grokbot.com.