Frost dates for Lewistown 11ese, MT
At Lewistown 11ese, the median last spring frost (32°F) is May 19 and the median first fall frost is Sep 25, for a typical growing season of 128 days. For a 90% chance that the last frost has passed, wait until Jun 1. Nearby ZIP codes are in USDA hardiness zone 4b.
- May 19Last spring frost (median)50% chance of 32°F after this date
- Jun 1Safer planting date10% chance of 32°F after this date
- Sep 25First fall frost (median)50% chance of 32°F by this date
- 128 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 20 | Jun 1 | Jun 16 |
| 32°F | May 7 | May 19 | Jun 1 |
| 28°F | Apr 23 | May 7 | May 22 |
| 24°F | Apr 5 | Apr 26 | May 10 |
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 | Sep 2 | Sep 16 | Sep 27 |
| 32°F | Sep 11 | Sep 25 | Oct 9 |
| 28°F | Sep 22 | Oct 5 | Oct 21 |
| 24°F | Sep 29 | Oct 16 | Oct 31 |
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 | 85 days | 105 days | 124 days |
| 32°F | 108 days | 128 days | 146 days |
| 28°F | 129 days | 150 days | 172 days |
| 24°F | 149 days | 173 days | 197 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 |
|---|---|---|---|
| 59441 | 4b | -25 to -20°F | 23 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 USC00244979, at 47.0069, -109.2231, elevation 1532 m (5,028 ft).
- Data quality: Representative (10+ years), 12 years of freeze data.
- Average days per year at or below 32°F: 172.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Lewistown (16 km)
- Lewistown 11 SSE (18 km)
- Lewistown Muni Airport (18 km)
- Grass Range (32 km)
- Judith Gap 13 E (48 km)
- Ryegate 18 NNW (53 km)
- Judith Gap (54 km)
- Moccasin Exp. Station (55 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.