U.S. Frost Dates by Weather Station

NOAA station USC00244386 · Montana

Frost Dates for Ingomar, MT

NOAA weather station Ingomar 9 E (USC00244386), Montana · elevation 2800 ft (853 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 4only 1 spring in 10 still has a frost later
  • Typical last frostMay 2150% chance of a frost after it
  • Typical first fall frostSeptember 1950% chance of a frost before it
  • Frost-free season120 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Ingomar, MTSpring: a frost after May 8 in 9 years out of 10, after May 21 in half of the years, after June 4 in 1 year out of 10. Fall: the first frost comes before September 6 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 120 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 21First frost ~Sep 19Frost season at Ingomar, MTSpring: a frost after May 8 in 9 years out of 10, after May 21 in half of the years, after June 4 in 1 year out of 10. Fall: the first frost comes before September 6 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 120 days.JFMAMJJASONDLast frost ~May 21First frost ~Sep 19
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 4September 6
50% (typical)May 21September 19
90% (likely)May 8October 2
Frost-free season (32°F, 50% chance)
120 days
Hard frost (28°F), last in spring, 50% chance after
May 10
Hard frost (28°F), first in fall, 50% chance before
September 28

The last 15 years (2006–2020 normals)

Typical last frost May 12 (9 days earlier than the 30-year normal), typical first fall frost October 1 (12 days later). Frost-free season 140 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 Ingomar: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 4, 20% May 30, 30% May 26, 40% May 23, 50% May 21, 60% May 18, 70% May 15, 80% May 12, 90% May 8. Observed last spring frost in recent years: 2021 May 31, 2022 June 2, 2023 May 20, 2024 May 21, 2025 May 22, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 4May 21May 82021: May 312022: June 22023: May 202024: May 212025: May 222026: May 22Fall at Ingomar: chance of a frost before this dateFall, chance of a frost before the date: 10% September 6, 20% September 11, 30% September 14, 40% September 17, 50% September 19, 60% September 22, 70% September 25, 80% September 28, 90% October 2. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2023 October 6, 2024 October 1, 2025 October 7.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 6Sep 19Oct 22021: September 172022: September 102023: October 62024: October 12025: October 7
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 31September 17
2022June 2September 10
2023May 20October 6
2024May 21October 1
2025May 22October 7
2026May 22season not over yet

What these dates mean

The 120 days between the 50% light-frost dates in Ingomar—May 21 and September 19—encompass the core period when tender crops can be grown with an even chance of a light frost either side.

For a cautious planting plan, gardeners should wait until the June 4 light-frost date, which carries only a 10 % chance of a frost later, before setting out tender crops in spring. In the fall, the cautious date to finish harvesting tender crops is September 6, when there is only a 10 % chance that a frost has arrived earlier. The 50% dates, May 21 and September 19, simply mark the point at which a light frost is equally likely.

Hard frosts at 28 °F, represented by the median dates May 10 and September 28, can damage plants more severely than the 32 °F light frosts, so they are a higher risk to consider.

How to read the probabilities

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

  • Spring: "10% after June 4" 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 6" 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 Ingomar, MTNumbered nearest stations: 1 — Hysham, MT — 19 mi; 2 — Forsyth, MT — 32 mi; 3 — Melstone, MT — 32 mi; 4 — Cohagen, MT — 43 mi; 5 — Hysham 25 SSE, MT — 44 mi.25 mi · 40 km1. Hysham, MT — 19 mi12. Forsyth, MT — 32 mi23. Melstone, MT — 32 mi34. Cohagen, MT — 43 mi45. Hysham 25 SSE, MT — 44 mi5
  1. Hysham, MT 19 mi (31 km)
  2. Forsyth, MT 32 mi (52 km)
  3. Melstone, MT 32 mi (52 km)
  4. Cohagen, MT 43 mi (70 km)
  5. Hysham 25 SSE, MT 44 mi (70 km)

Sources

* Hardiness zone 4a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 46.5675, -107.195.