Frost dates for Lewistown 42 WSW, MT
At Lewistown 42 WSW, the median last spring frost (32°F) is Jun 15 and the median first fall frost is Sep 4, for a typical growing season of 77 days. For a 90% chance that the last frost has passed, wait until Jul 8. Nearby ZIP codes are in USDA hardiness zone 4b.
- Jun 15Last spring frost (median)50% chance of 32°F after this date
- Jul 8Safer planting date10% chance of 32°F after this date
- Sep 4First fall frost (median)50% chance of 32°F by this date
- 77 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 |
|---|---|---|---|
| 32°F | May 31 | Jun 15 | Jul 8 |
| 28°F | May 13 | May 25 | Jun 10 |
| 24°F | Apr 30 | May 11 | May 23 |
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 |
|---|---|---|---|
| 32°F | Aug 19 | Sep 4 | Sep 18 |
| 28°F | Sep 2 | Sep 18 | Oct 1 |
| 24°F | Sep 13 | Sep 30 | Oct 13 |
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 | 24 days | 41 days | 66 days |
| 32°F | 52 days | 77 days | 99 days |
| 28°F | 90 days | 111 days | 132 days |
| 24°F | 120 days | 140 days | 159 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 |
|---|---|---|---|
| 59452 | 4b | -25 to -20°F | 17 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 USW00004140, at 46.8847, -110.2894, elevation 1545 m (5,070 ft).
- Data quality: Representative (10+ years), 12 years of freeze data.
- Average days per year at or below 32°F: 212.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Stanford (30 km)
- Moccasin Exp. Station (32 km)
- Neihart 8 NNW (41 km)
- Harlowton 11nw (43 km)
- Martinsdale 3 NNW (43 km)
- Judith Gap (47 km)
- Denton (55 km)
- Raynesford 2 NNW (57 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.