Frost dates for Westby, MT
At Westby, the median last spring frost (32°F) is May 18 and the median first fall frost is Sep 21, for a typical growing season of 125 days. For a 90% chance that the last frost has passed, wait until May 30. Nearby ZIP codes are in USDA hardiness zone 3b.
- May 18Last spring frost (median)50% chance of 32°F after this date
- May 30Safer planting date10% chance of 32°F after this date
- Sep 21First fall frost (median)50% chance of 32°F by this date
- 125 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 15 | May 27 | Jun 9 |
| 32°F | May 6 | May 18 | May 30 |
| 28°F | Apr 26 | May 8 | May 20 |
| 24°F | Apr 13 | Apr 29 | May 11 |
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 31 | Sep 12 | Sep 24 |
| 32°F | Sep 7 | Sep 21 | Oct 1 |
| 28°F | Sep 16 | Sep 28 | Oct 11 |
| 24°F | Sep 23 | Oct 7 | Oct 21 |
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 | 87 days | 106 days | 123 days |
| 32°F | 108 days | 125 days | 140 days |
| 28°F | 123 days | 141 days | 159 days |
| 24°F | 141 days | 161 days | 182 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 |
|---|---|---|---|
| 59275 | 3b | -35 to -30°F | 8 km |
| 58845 | 3b | -35 to -30°F | 28 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 USC00248777, at 48.8744, -104.0503, elevation 641 m (2,103 ft).
- Data quality: Standard (24+ years of data), 27 years of freeze data.
- Average days per year at or below 32°F: 202.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Plentywood (39 km)
- Raymond Border Station (41 km)
- Medicine Lake 3 SE (53 km)
- Redstone (66 km)
- Culbertson (87 km)
- Wolf Point 34 NE (95 km)
- Wolf Point 29 ENE (100 km)
- Bredette (100 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.