Frost dates for Fortine 1 N, MT
At Fortine 1 N, the median last spring frost (32°F) is May 16 and the median first fall frost is Sep 18, for a typical growing season of 121 days. For a 90% chance that the last frost has passed, wait until Jun 3. Nearby ZIP codes are in USDA hardiness zone 5a.
- May 16Last spring frost (median)50% chance of 32°F after this date
- Jun 3Safer planting date10% chance of 32°F after this date
- Sep 18First fall frost (median)50% chance of 32°F by this date
- 121 daysGrowing season (median)between 32°F freezes
Last spring frost dates
The chance that the temperature drops to the threshold after each date.
| Temperature | 90% chance later | 50% chance later | 10% chance later |
|---|---|---|---|
| 36°F | May 18 | Jun 2 | Jun 23 |
| 32°F | May 3 | May 16 | Jun 3 |
| 28°F | Apr 14 | May 1 | May 20 |
| 24°F | Mar 24 | Apr 12 | May 4 |
First fall frost dates
The chance that the temperature has dropped to the threshold by each date.
| Temperature | 10% chance by | 50% chance by | 90% chance by |
|---|---|---|---|
| 36°F | Aug 20 | Sep 4 | Sep 22 |
| 32°F | Sep 1 | Sep 18 | Oct 5 |
| 28°F | Sep 15 | Oct 2 | Oct 19 |
| 24°F | Sep 30 | Oct 19 | Nov 5 |
Growing season length
Days between the last spring and first fall freeze, as the length reached in 9, 5 and 1 out of 10 years.
| Temperature | 9 in 10 years at least | 5 in 10 years at least | 1 in 10 years at least |
|---|---|---|---|
| 36°F | 64 days | 90 days | 114 days |
| 32°F | 97 days | 121 days | 144 days |
| 28°F | 129 days | 154 days | 178 days |
| 24°F | 159 days | 186 days | 213 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 100% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 59918 | 5a | -20 to -15°F | 10 km |
| 59933 | 5a | -20 to -15°F | 12 km |
| 59934 | 5a | -20 to -15°F | 16 km |
Not the official USDA Plant Hardiness Zone Map. Zones shown here are the values published in the USDA-ARS / Oregon State University PRISM Group 2023 ZIP code tables, shown unaltered. For the official interactive map, visit planthardiness.ars.usda.gov.
About this station
- NOAA station ID USC00243139, at 48.7783, -114.8997, elevation 914 m (3,000 ft).
- Data quality: Representative (10+ years), 15 years of freeze data.
- Average days per year at or below 32°F: 183.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Eureka Ranger Station (18 km)
- Olney (35 km)
- Polebridge (45 km)
- Libby Dam (base) (51 km)
- Yaak 9nne (57 km)
- Whitefish (57 km)
- Libby 1 NE Ranger Station (63 km)
- Libby (66 km)
Sources
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal normals by station (version 1.0.1), file dated June 27, 2023; documentation.
- USDA Agricultural Research Service and PRISM Group, Oregon State University, 2023 Plant Hardiness Zone Map ZIP code tables (CONUS, Alaska, Hawaii, Puerto Rico), files dated November 17, 2023. Data © PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed November 1, 2026. Official map: planthardiness.ars.usda.gov.
- U.S. Census Bureau, 2024 Gazetteer ZIP Code Tabulation Areas file, dated August 30, 2024 (ZIP centroids used to find the nearest station).
Freeze dates are 30-year probabilities, not forecasts. Check your local forecast before planting tender plants.