U.S. Frost Dates by Weather Station

NOAA station USC00245870 · Montana

Frost Dates for Moorhead, MT

NOAA weather station Moorhead 9 NE (USC00245870), Montana · elevation 3220 ft (982 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1750% chance of a frost after it
  • Typical first fall frostSeptember 2250% chance of a frost before it
  • Frost-free season126 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Moorhead, MTSpring: a frost after May 5 in 9 years out of 10, after May 17 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 126 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 17First frost ~Sep 22Frost season at Moorhead, MTSpring: a frost after May 5 in 9 years out of 10, after May 17 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 126 days.JFMAMJJASONDLast frost ~May 17First frost ~Sep 22
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 31September 7
50% (typical)May 17September 22
90% (likely)May 5October 5
Frost-free season (32°F, 50% chance)
126 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 13 (4 days earlier than the 30-year normal), typical first fall frost September 24 (2 days later). Frost-free season 131 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 Moorhead: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 23, 40% May 20, 50% May 17, 60% May 14, 70% May 12, 80% May 9, 90% May 5. Observed last spring frost in recent years: 2021 May 11, 2023 May 2, 2024 May 5, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 17May 52021: May 112023: May 22024: May 52025: May 22026: May 19Fall at Moorhead: chance of a frost before this dateFall, chance of a frost before the date: 10% September 7, 20% September 12, 30% September 16, 40% September 19, 50% September 22, 60% September 25, 70% September 28, 80% October 2, 90% October 5. Observed first fall frost in recent years: 2021 September 17, 2022 October 15, 2023 October 6, 2024 September 22, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 22Oct 52021: September 172022: October 152023: October 62024: September 222025: September 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
2021May 11September 17
2022incomplete recordOctober 15
2023May 2October 6
2024May 5September 22
2025May 2September 6
2026May 19season not over yet

What these dates mean

May 31 is the cautious spring light-frost date, giving a Moorhead gardener a low-risk window to set out tender crops. The 50% date is May 17, which represents a 50% chance of a frost later than this date. Setting out tender crops on May 5 would be a gamble, because there is a 90% chance of frost later than that date. The median light-frost date in fall is September 22, indicating a 50% chance of a frost earlier than this date.

September 7 is the cautious fall light-frost date, allowing a gardener to finish harvesting tender crops with only a 10% chance that a frost has already arrived. The hard-frost 50% dates are May 6 and October 2; a hard frost at 28 °F can damage plant tissues that survive a light frost at 32 °F, so it is a more critical threshold.

How to read the probabilities

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

  • Spring: "10% after May 31" 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 7" 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 Moorhead, MTNumbered nearest stations: 1 — Sonnette, MT — 13 mi; 2 — Biddle, MT — 16 mi; 3 — Leiter, WY — 35 mi; 4 — Brandenberg, MT — 50 mi; 5 — Clearmont, WY — 54 mi.MTWY25 mi · 40 km1. Sonnette, MT — 13 mi12. Biddle, MT — 16 mi23. Leiter, WY — 35 mi34. Brandenberg, MT — 50 mi45. Clearmont, WY — 54 mi5
  1. Sonnette, MT 13 mi (21 km)
  2. Biddle, MT 16 mi (26 km)
  3. Leiter, WY 35 mi (56 km)
  4. Brandenberg, MT 50 mi (80 km)
  5. Clearmont, WY 54 mi (86 km)

Sources

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

Station coordinates: 45.1758, -105.7517.