U.S. Frost Dates by Weather Station

NOAA station USC00245340 · Montana

Frost Dates for Malta, MT

NOAA weather station Malta 35 S (USC00245340), Montana · elevation 2615 ft (797 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 29only 1 spring in 10 still has a frost later
  • Typical last frostMay 1650% chance of a frost after it
  • Typical first fall frostSeptember 2350% chance of a frost before it
  • Frost-free season128 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Malta, MTSpring: a frost after May 4 in 9 years out of 10, after May 16 in half of the years, after May 29 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 23 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 128 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 16First frost ~Sep 23Frost season at Malta, MTSpring: a frost after May 4 in 9 years out of 10, after May 16 in half of the years, after May 29 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 23 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 128 days.JFMAMJJASONDLast frost ~May 16First frost ~Sep 23
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)May 29September 10
50% (typical)May 16September 23
90% (likely)May 4October 4
Frost-free season (32°F, 50% chance)
128 days
Hard frost (28°F), last in spring, 50% chance after
May 5
Hard frost (28°F), first in fall, 50% chance before
September 30

The last 15 years (2006–2020 normals)

Typical last frost May 17 (1 day later than the 30-year normal), typical first fall frost September 26 (3 days later). Frost-free season 130 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 Malta: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 29, 20% May 24, 30% May 21, 40% May 19, 50% May 16, 60% May 14, 70% May 11, 80% May 8, 90% May 4. Observed last spring frost in recent years: 2022 May 22, 2023 May 19, 2024 May 20, 2025 May 6, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 29May 16May 42022: May 222023: May 192024: May 202025: May 62026: May 19Fall at Malta: chance of a frost before this dateFall, chance of a frost before the date: 10% September 10, 20% September 15, 30% September 18, 40% September 21, 50% September 23, 60% September 25, 70% September 27, 80% October 1, 90% October 4. Observed first fall frost in recent years: 2021 September 17, 2022 September 21, 2023 October 6, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 10Sep 23Oct 42021: September 172022: September 212023: October 62024: October 12025: October 6
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
2021incomplete recordSeptember 17
2022May 22September 21
2023May 19October 6
2024May 20October 1
2025May 6October 6
2026May 19season not over yet

What these dates mean

Setting out tender crops on the early spring date of May 4 is a gamble, because there is a 90 % chance a light frost will still occur. The cautious spring date for tender crops is May 29, when only a 10 % chance of a light frost remains.

The 50% date is May 16, meaning there is a 50 % chance of a light frost later than this date, and the 50% date is September 23, meaning there is a 50 % chance of a light frost earlier than this date. The cautious fall date to finish harvesting tender crops is September 10, with only a 10 % chance that a light frost has arrived earlier. The 50% hard-frost dates are May 5 and September 30; a hard frost at 28 °F can injure plants that survive a light frost at 32 °F. These probabilities are drawn from 28 years of NOAA records for Malta, Montana.

How to read the probabilities

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

  • Spring: "10% after May 29" 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 10" 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 Malta, MTNumbered nearest stations: 1 — Content, MT — 26 mi; 2 — Valentine, MT — 43 mi; 3 — Saco, MT — 51 mi; 4 — Glasgow #2, MT — 66 mi; 5 — Glasgow, MT — 67 mi.MT25 mi · 40 km1. Content, MT — 26 mi12. Valentine, MT — 43 mi23. Saco, MT — 51 mi34. Glasgow #2, MT — 66 mi45. Glasgow, MT — 67 mi5
  1. Content, MT 26 mi (42 km)
  2. Valentine, MT 43 mi (69 km)
  3. Saco, MT 51 mi (83 km)
  4. Glasgow #2, MT 66 mi (105 km)
  5. Glasgow, MT 67 mi (108 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: 47.8417, -107.9556.