U.S. Frost Dates by Weather Station

NOAA station USC00101671 · Idaho

Frost Dates for Chilly Barton Flat, ID

NOAA weather station Chilly Barton Flat (USC00101671), Idaho · elevation 6260 ft (1908 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 30only 1 spring in 10 still has a frost later
  • Typical last frostJune 1450% chance of a frost after it
  • Typical first fall frostSeptember 850% chance of a frost before it
  • Frost-free season86 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Chilly Barton Flat, IDSpring: a frost after May 27 in 9 years out of 10, after June 14 in half of the years, after June 30 in 1 year out of 10. Fall: the first frost comes before August 26 in 1 year out of 10, before September 8 in half of the years, before September 22 in 9 years out of 10. Typical frost-free season: 86 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 14First frost ~Sep 8Frost season at Chilly Barton Flat, IDSpring: a frost after May 27 in 9 years out of 10, after June 14 in half of the years, after June 30 in 1 year out of 10. Fall: the first frost comes before August 26 in 1 year out of 10, before September 8 in half of the years, before September 22 in 9 years out of 10. Typical frost-free season: 86 days.JFMAMJJASONDLast frost ~Jun 14First frost ~Sep 8
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 30August 26
50% (typical)June 14September 8
90% (likely)May 27September 22
Frost-free season (32°F, 50% chance)
86 days
Hard frost (28°F), last in spring, 50% chance after
May 24
Hard frost (28°F), first in fall, 50% chance before
September 20

The last 15 years (2006–2020 normals)

Typical last frost June 13 (1 day earlier than the 30-year normal), typical first fall frost September 9 (1 day later). Frost-free season 89 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 Chilly Barton Flat: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 30, 20% June 25, 30% June 21, 40% June 17, 50% June 14, 60% June 10, 70% June 6, 80% June 2, 90% May 27. Observed last spring frost in recent years: 2021 June 11, 2022 June 15, 2023 June 22, 2024 June 20, 2025 June 23, 2026 June 11.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 30Jun 14May 272021: June 112022: June 152023: June 222024: June 202025: June 232026: June 11Fall at Chilly Barton Flat: chance of a frost before this dateFall, chance of a frost before the date: 10% August 26, 20% September 1, 30% September 3, 40% September 6, 50% September 8, 60% September 11, 70% September 14, 80% September 18, 90% September 22. Observed first fall frost in recent years: 2021 September 4, 2024 August 29, 2025 September 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 26Sep 8Sep 222021: September 42024: August 292025: September 23
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
2021June 11September 4
2022June 15incomplete record
2023June 22incomplete record
2024June 20August 29
2025June 23September 23
2026June 11season not over yet

What these dates mean

June 30 is the cautious spring date for light frost, giving a gardener a 10% chance that any frost will occur later and allowing the safe setting out of tender crops. The 50% date is June 14, which means a 50% chance of a frost later than that day. Planting before May 27 carries a 90% chance of frost, so early sowing is a gamble.

In the autumn, August 26 serves as the cautious fall date, with only a 10% chance that a frost has arrived earlier, so it marks the latest reliable time to finish harvesting tender crops. The 50% date is September 8, indicating a 50% chance of a frost earlier than that day. The hard-frost 50% dates are May 24 in spring and September 20 in fall; the lower temperature of 28 °F can damage plants more severely than 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 30" 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 August 26" 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 Chilly Barton Flat, IDNumbered nearest stations: 1 — Mackay Lost River Ranger Station, ID — 11 mi; 2 — Idaho Falls 46 W, ID — 54 mi; 3 — Richfield, ID — 66 mi; 4 — Shoshone, ID — 78 mi; 5 — Dubois, ID — 83 mi.ID25 mi · 40 km1. Mackay Lost River Ranger Station, ID — 11 mi12. Idaho Falls 46 W, ID — 54 mi23. Richfield, ID — 66 mi34. Shoshone, ID — 78 mi45. Dubois, ID — 83 mi5
  1. Mackay Lost River Ranger Station, ID 11 mi (18 km)
  2. Idaho Falls 46 W, ID 54 mi (87 km)
  3. Richfield, ID 66 mi (106 km)
  4. Shoshone, ID 78 mi (125 km)
  5. Dubois, ID 83 mi (133 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.9778, -113.8292.