Frost dates for Nampa, Idaho
Idaho · first & last freeze, from NOAA 1991–2020 climate normals
In a typical year, the last spring freeze around Nampa comes near April 29 and the first fall freeze near October 13. One year in ten, a freeze still arrives after May 18 in spring — and one year in ten, the first fall freeze has already landed by September 29. NCEI's own growing-season statistic (days between freezes, percentiled per year) puts the median at 164 days.
Freeze-date probabilities, three thresholds
| Threshold | Last spring — 9 in 10 years past by | Last spring — typical (median) | Last spring — 1 in 10 years later than | First fall — 1 in 10 years by | First fall — typical (median) | First fall — 9 in 10 years by |
|---|---|---|---|---|---|---|
| 36°F — near-freezing (frost can occur) | May 1 | May 16 | June 6 | September 15 | September 30 | October 14 |
| 32°F — freezing | April 15 | April 29 | May 18 | September 29 | October 13 | October 28 |
| 28°F — hard freeze | March 25 | April 11 | May 1 | October 9 | October 25 | November 10 |
Measured at Nampa Sugar Factory, ID (USC00106305), 1.4 mi from the city's census reference point, elev. 2,470 ft. NOAA/NCEI 1991–2020 normals. Read each date column as odds over the 30-year record — e.g. "9 in 10 years past by" means only one spring in ten saw a later freeze than that date.
Frost-free windows (computed)
| Window (32°F ends) | Opens | Closes | Length |
|---|---|---|---|
| Cautious (1-in-10 freeze risk at each end) | May 18 | September 29 | 134 days |
| Typical (median freeze dates) | April 29 | October 13 | 167 days |
| Gambler's (9-in-10 freeze risk at each end) | April 15 | October 28 | 196 days |
Pure arithmetic on the freeze-date percentiles above — the "cautious" window opens on the date only 1 year in 10 still sees a later spring freeze, and closes on the date by which only 1 year in 10 has already frozen.
About this city's numbers
- Census entity: Nampa city, Idaho — official 2024 population estimate 117,350.
- Weather station: Nampa Sugar Factory, ID (USC00106305), 1.4 mi from the city's census-gazetteer reference point at elevation 2,470 ft. Frost dates are that station's history — neighborhoods vary.
- Data pulled 2026-09-05; normals period 1991-2020 (replaced by NOAA roughly once a decade).