U.S. Frost Dates by Weather Station

NOAA station USC00246157 · Montana

Frost Dates for Norris Madison Power House, MT

NOAA weather station Norris Madison Power House (USC00246157), Montana · elevation 4745 ft (1446 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 14only 1 spring in 10 still has a frost later
  • Typical last frostApril 3050% chance of a frost after it
  • Typical first fall frostOctober 1050% chance of a frost before it
  • Frost-free season162 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Norris Madison Power House, MTSpring: a frost after April 15 in 9 years out of 10, after April 30 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 27 in 1 year out of 10, before October 10 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 162 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 30First frost ~Oct 10Frost season at Norris Madison Power House, MTSpring: a frost after April 15 in 9 years out of 10, after April 30 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 27 in 1 year out of 10, before October 10 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 162 days.JFMAMJJASONDLast frost ~Apr 30First frost ~Oct 10
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 14September 27
50% (typical)April 30October 10
90% (likely)April 15October 27
Frost-free season (32°F, 50% chance)
162 days
Hard frost (28°F), last in spring, 50% chance after
April 17
Hard frost (28°F), first in fall, 50% chance before
October 22

The last 15 years (2006–2020 normals)

Typical last frost April 29 (1 day earlier than the 30-year normal), typical first fall frost October 13 (3 days later). Frost-free season 166 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 Norris Madison Power House: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 14, 20% May 9, 30% May 6, 40% May 3, 50% April 30, 60% April 28, 70% April 25, 80% April 20, 90% April 15. Observed last spring frost in recent years: 2021 May 22, 2022 May 22, 2023 April 23, 2024 May 1, 2025 April 19, 2026 May 6.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 14Apr 30Apr 152021: May 222022: May 222023: April 232024: May 12025: April 192026: May 6Fall at Norris Madison Power House: chance of a frost before this dateFall, chance of a frost before the date: 10% September 27, 20% October 1, 30% October 4, 40% October 7, 50% October 10, 60% October 14, 70% October 18, 80% October 22, 90% October 27. Observed first fall frost in recent years: 2021 October 12, 2022 October 24, 2023 October 25, 2024 October 30, 2025 October 28.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 27Oct 10Oct 272021: October 122022: October 242023: October 252024: October 302025: October 28
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 22October 12
2022May 22October 24
2023April 23October 25
2024May 1October 30
2025April 19October 28
2026May 6season not over yet

What these dates mean

The 162 days between the 50% dates in Norris Madison Power House, Montana, bracket the core growing season. The median light-frost date in spring is April 30, when there is a 50 % chance of a frost later; the median light-frost date in fall is October 10, when there is a 50 % chance of a frost earlier.

For a cautious planting schedule, the spring date with only a 10 % chance of a later frost is May 14, making it the recommended day to set out tender crops. In the autumn, the cautious date with only a 10 % chance of an earlier frost is September 27, the point at which tender crops should be finished.

The 50% hard-frost dates are April 17 in spring and October 22 in fall. Hard frost at 28 °F is more damaging than a light frost at 32 °F because it can injure plants that survive the milder temperature.

How to read the probabilities

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

  • Spring: "10% after May 14" 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 27" 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 Norris Madison Power House, MTNumbered nearest stations: 1 — Ennis, MT — 11 mi; 2 — near Four Corners, MT — 26 mi; 3 — near Bozeman, MT — 31 mi; 4 — Bozeman (Gallatin Field), MT — 31 mi; 5 — Trident, MT — 33 mi.MTWY25 mi · 40 km1. Ennis, MT — 11 mi12. near Four Corners, MT — 26 mi23. near Bozeman, MT — 31 mi34. Bozeman (Gallatin Field), MT — 31 mi45. Trident, MT — 33 mi5
  1. Ennis, MT 11 mi (18 km)
  2. near Four Corners, MT 26 mi (43 km)
  3. near Bozeman, MT 31 mi (50 km)
  4. Bozeman (Gallatin Field), MT 31 mi (50 km)
  5. Trident, MT 33 mi (53 km)

Sources

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

Station coordinates: 45.4875, -111.6336.