U.S. Frost Dates by Weather Station

NOAA station USC00247560 · Montana

Frost Dates for Sidney, MT

NOAA weather station Sidney (USC00247560), Montana · elevation 1933 ft (589 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 25only 1 spring in 10 still has a frost later
  • Typical last frostMay 1150% chance of a frost after it
  • Typical first fall frostSeptember 2750% chance of a frost before it
  • Frost-free season137 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Sidney, MTSpring: a frost after April 30 in 9 years out of 10, after May 11 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 13 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 137 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 11First frost ~Sep 27Frost season at Sidney, MTSpring: a frost after April 30 in 9 years out of 10, after May 11 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 13 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 137 days.JFMAMJJASONDLast frost ~May 11First frost ~Sep 27
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 25September 13
50% (typical)May 11September 27
90% (likely)April 30October 9
Frost-free season (32°F, 50% chance)
137 days
Hard frost (28°F), last in spring, 50% chance after
May 2
Hard frost (28°F), first in fall, 50% chance before
October 6

The last 15 years (2006–2020 normals)

Typical last frost May 13 (2 days later than the 30-year normal), typical first fall frost September 30 (3 days later). Frost-free season 139 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 Sidney: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 25, 20% May 20, 30% May 17, 40% May 14, 50% May 11, 60% May 8, 70% May 6, 80% May 3, 90% April 30. Observed last spring frost in recent years: 2022 May 2, 2023 April 30, 2024 April 21, 2025 April 19, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 25May 11Apr 302022: May 22023: April 302024: April 212025: April 192026: May 19Fall at Sidney: chance of a frost before this dateFall, chance of a frost before the date: 10% September 13, 20% September 18, 30% September 22, 40% September 24, 50% September 27, 60% September 29, 70% October 1, 80% October 5, 90% October 9. Observed first fall frost in recent years: 2021 October 13, 2022 October 7, 2023 October 25, 2024 October 30.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Nov 1Sep 13Sep 27Oct 92021: October 132022: October 72023: October 252024: October 30
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 recordOctober 13
2022May 2October 7
2023April 30October 25
2024April 21October 30
2025April 19incomplete record
2026May 19season not over yet

What these dates mean

Setting out tender crops on the early spring date of April 30 in Sidney risks a 90 % chance of a frost later, so planting then is a gamble. The cautious alternative for spring is the May 25 date, which carries only a 10 % chance that a frost will occur later.

The 50 % spring date is May 11, when there is a 50 % chance of a frost later; the 50 % fall date is September 27, when there is a 50 % chance of a frost earlier. For a careful schedule, tender crops should be finished by the September 13 date, which has only a 10 % chance that a frost has arrived earlier.

A hard frost at 28 °F, marked by the median dates of May 2 in spring and October 6 in fall, can damage plants that would survive a light frost at 32 °F, making the hard-frost thresholds especially important for growers in this location.

How to read the probabilities

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

  • Spring: "10% after May 25" 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 13" 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 Sidney, MTNumbered nearest stations: 1 — Savage, MT — 21 mi; 2 — Culbertson, MT — 34 mi; 3 — near Williston, ND — 34 mi; 4 — Williston (Sloulin Airport), ND — 39 mi; 5 — Grassy Butte, ND — 49 mi.NDMT25 mi · 40 km1. Savage, MT — 21 mi12. Culbertson, MT — 34 mi23. near Williston, ND — 34 mi34. Williston (Sloulin Airport), ND — 39 mi45. Grassy Butte, ND — 49 mi5
  1. Savage, MT 21 mi (34 km)
  2. Culbertson, MT 34 mi (54 km)
  3. near Williston, ND 34 mi (55 km)
  4. Williston (Sloulin Airport), ND 39 mi (62 km)
  5. Grassy Butte, ND 49 mi (79 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: 47.7283, -104.1469.