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Frost forecast
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Frost and freeze probabilities
Freeze dates shift from year to year. Spring percentages describe the chance of a freeze after the stated date; fall percentages describe the chance of a freeze by that date. The 10% spring date is the cautious one: a later freeze happens about 1 year in 10.
These dates describe historical probabilities, not this year’s forecast or the earliest and latest possible freezes. Different weather stations or probability thresholds can produce different published dates for the same city.
Average first hard freeze in Rock Springs
The median first fall hard-freeze date (28°F) is October 3, from NOAA’s 1991–2020 Climate Normals at Rock Springs Fire Dept. This is the 50% probability date, not a forecast for this year.
| Temperature | Last spring date 90% · 50% · 10% chance of a later freeze | First fall date 10% · 50% · 90% chance of an earlier freeze | Season |
|---|---|---|---|
| 36°F frost possible | May 24 · Jun 6 · Jun 21 | Sep 2 · Sep 14 · Sep 25 | 97 days |
| 32°F freeze | May 8 · May 24 · Jun 9 | Sep 9 · Sep 24 · Oct 4 | 121 days |
| 28°F hard freeze | Apr 25 · May 9 · May 26 | Sep 19 · Oct 3 · Oct 17 | 146 days |
Average high temperature by month
1991–2020 normals at Rock Springs Fire Dept
Average low temperature by month
Months with lows near or below freezing carry frost risk
Where these frost dates come from
The frost dates for Rock Springs, WY come from the nearest weather station with freeze statistics in NOAA’s 1991–2020 Climate Normals: Rock Springs Fire Dept (station USC00487847, elevation 6,270 ft), 1.5 mi from Rock Springs. Local conditions (valleys, hilltops, distance to water, city heat) can shift frost dates by days or weeks. The hardiness zone is from the 2023 USDA Plant Hardiness Zone Map (USDA ARS and Oregon State University PRISM). See our methodology.
Frost dates near Rock Springs
| City | Distance | Last spring frost | First fall frost |
|---|---|---|---|
| Green River, WY | 14 mi | May 24 | Sep 22 |
| Vernal, UT | 81 mi | May 7 | Oct 6 |
| Lander, WY | 89 mi | May 18 | Sep 27 |
| Evanston, WY | 93 mi | Jun 12 | Sep 12 |
| Roosevelt, UT | 99 mi | May 23 | Sep 22 |
| Rawlins, WY | 104 mi | May 17 | Oct 2 |
| Riverton, WY | 108 mi | May 12 | Sep 24 |
| Craig, CO | 114 mi | Jun 6 | Sep 12 |
| North Logan, UT | 134 mi | Apr 30 | Oct 16 |
| Providence, UT | 134 mi | Apr 30 | Oct 16 |
Frequently asked questions
When is the last frost in Rock Springs?
On average, the last spring frost (32°F) in Rock Springs is around May 24. In about 1 year in 10 it comes after June 9, and in 9 years in 10 it comes after May 8.
When is the first frost in Rock Springs?
The first fall frost (32°F) usually arrives around September 24. There’s a 10% chance of a freeze by September 9 and a 90% chance by October 4.
Can it frost in Rock Springs after the last frost date?
Yes. Frost can form on clear, calm nights even when the air stays above 32°F. Temperatures of 36°F or lower, cold enough for frost on exposed plants, have a 50% chance of occurring after June 6.
What growing zone is Rock Springs?
Rock Springs is in USDA Plant Hardiness Zone 5a (average annual extreme minimum temperature −20 to −15°F), based on the 2023 USDA map for ZIP code 82943. See growing zones in Wyoming.
More: all Wyoming frost dates · Wyoming growing zones · frost vs. freeze