U.S. Frost Dates by Weather Station

NOAA station USC00420086 · Utah

Frost Dates for Alton, UT

NOAA weather station Alton (USC00420086), Utah · elevation 7098 ft (2164 m) · hardiness zone 6b*

  • Lower-risk planting dateafter June 15only 1 spring in 10 still has a frost later
  • Typical last frostJune 150% chance of a frost after it
  • Typical first fall frostSeptember 3050% chance of a frost before it
  • Frost-free season120 daystypical year, 32°F
  • Hardiness zone6bcalculated*

The year at a glance

Frost season at Alton, UTSpring: a frost after May 16 in 9 years out of 10, after June 1 in half of the years, after June 15 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before September 30 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 120 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 1First frost ~Sep 30Frost season at Alton, UTSpring: a frost after May 16 in 9 years out of 10, after June 1 in half of the years, after June 15 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before September 30 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 120 days.JFMAMJJASONDLast frost ~Jun 1First frost ~Sep 30
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 15September 16
50% (typical)June 1September 30
90% (likely)May 16October 12
Frost-free season (32°F, 50% chance)
120 days
Hard frost (28°F), last in spring, 50% chance after
May 17
Hard frost (28°F), first in fall, 50% chance before
October 11

The last 15 years (2006–2020 normals)

Typical last frost May 31 (1 day earlier than the 30-year normal), typical first fall frost October 1 (1 day later). Frost-free season 123 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Alton: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 15, 20% June 9, 30% June 6, 40% June 3, 50% June 1, 60% May 29, 70% May 24, 80% May 21, 90% May 16. Observed last spring frost in recent years: 2021 May 26, 2022 May 31, 2023 May 9, 2024 May 22, 2025 June 5, 2026 May 30.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jun 15Jun 1May 162021: May 262022: May 312023: May 92024: May 222025: June 52026: May 30Fall at Alton: chance of a frost before this dateFall, chance of a frost before the date: 10% September 16, 20% September 22, 30% September 24, 40% September 27, 50% September 30, 60% October 1, 70% October 5, 80% October 7, 90% October 12. Observed first fall frost in recent years: 2021 October 11, 2022 October 20, 2023 October 3, 2024 October 18, 2025 September 13.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 16Sep 30Oct 122021: October 112022: October 202023: October 32024: October 182025: September 13
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there. ◆ marks the actual date in each recent year.

What actually happened in recent years

Observed dates of the last spring and first fall frost (32°F) at this station; ◆ marks them on the charts above
YearLast spring frostFirst fall frost
2021May 26October 11
2022May 31October 20
2023May 9October 3
2024May 22October 18
2025June 5September 13
2026May 30season not over yet

What these dates mean

Setting out tender crops on the early spring date of May 16 in Alton carries a 90 % chance that a light frost will still occur later, so it is a gamble for growers.

The 50 % date of June 1 marks the point at which the odds are even for a frost later in spring, and the 50 % date of September 30 does the same for a frost earlier in fall. For a more cautious plan, the June 15 spring date has only a 10 % chance of a later frost, and the September 16 fall date has only a 10 % chance of an earlier frost, providing the lowest-risk windows for tender crops.

The hard-frost median dates are May 17 in spring and October 11 in fall; a hard frost at 28 °F can damage plants that would survive a light frost at 32 °F.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after June 15" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before September 16" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Alton, UTNumbered nearest stations: 1 — Bryce Canyon National Park Headquarters, UT — 22 mi; 2 — Kanab, UT — 29 mi; 3 — Zion National Park, UT — 32 mi; 4 — Pipe Spring National Monument, AZ — 42 mi; 5 — New Harmony, UT — 44 mi.UTNV25 mi · 40 km1. Bryce Canyon National Park Headquarters, UT — 22 mi12. Kanab, UT — 29 mi23. Zion National Park, UT — 32 mi34. Pipe Spring National Monument, AZ — 42 mi45. New Harmony, UT — 44 mi5
  1. Bryce Canyon National Park Headquarters, UT 22 mi (35 km)
  2. Kanab, UT 29 mi (46 km)
  3. Zion National Park, UT 32 mi (51 km)
  4. Pipe Spring National Monument, AZ 42 mi (68 km)
  5. New Harmony, UT 44 mi (71 km)

Sources

* Hardiness zone 6b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 37.4403, -112.4819.