U.S. Frost Dates by Weather Station

NOAA station USC00247214 · Montana

Frost Dates for Roundup, MT

NOAA weather station Roundup (USC00247214), Montana · elevation 3200 ft (975 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 2only 1 spring in 10 still has a frost later
  • Typical last frostMay 2050% chance of a frost after it
  • Typical first fall frostSeptember 2150% chance of a frost before it
  • Frost-free season124 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Roundup, MTSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 2 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 21 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 124 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Sep 21Frost season at Roundup, MTSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 2 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 21 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 124 days.JFMAMJJASONDLast frost ~May 20First frost ~Sep 21
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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 2September 5
50% (typical)May 20September 21
90% (likely)May 6October 4
Frost-free season (32°F, 50% chance)
124 days
Hard frost (28°F), last in spring, 50% chance after
May 7
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 19 (1 day earlier than the 30-year normal), typical first fall frost September 23 (2 days later). Frost-free season 123 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 Roundup: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 2, 20% May 29, 30% May 26, 40% May 22, 50% May 20, 60% May 17, 70% May 14, 80% May 11, 90% May 6.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 2May 20May 6Fall at Roundup: chance of a frost before this dateFall, chance of a frost before the date: 10% September 5, 20% September 10, 30% September 14, 40% September 18, 50% September 21, 60% September 24, 70% September 26, 80% September 30, 90% October 4.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 5Sep 21Oct 4
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.

What these dates mean

Setting out tender crops on the early spring date of May 6 in Roundup carries a 90% chance of a frost later than that date, making it a gamble for growers. The cautious alternative is June 2, when only a 10% chance of a later frost remains.

The 50% spring date is May 20; there is a 50% chance of a frost later than this date. In the fall the 50% date is September 21, with a 50% chance of a frost earlier than that date. For a low-risk schedule, the cautious fall date is September 5, which has only a 10% chance that a frost has already arrived.

Hard-frost risk is higher because a temperature of 28 °F can damage tissue that survives a light 32 °F frost; the 50% hard-frost dates are May 7 in spring and October 1 in fall.

How to read the probabilities

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

  • Spring: "10% after June 2" 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 5" 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 Roundup, MTNumbered nearest stations: 1 — Flatwillow, MT — 28 mi; 2 — Melstone, MT — 33 mi; 3 — Ryegate, MT — 39 mi; 4 — Grass Range, MT — 43 mi; 5 — Billings (Logan Airport), MT — 44 mi.25 mi · 40 km1. Flatwillow, MT — 28 mi12. Melstone, MT — 33 mi23. Ryegate, MT — 39 mi34. Grass Range, MT — 43 mi45. Billings (Logan Airport), MT — 44 mi5
  1. Flatwillow, MT 28 mi (44 km)
  2. Melstone, MT 33 mi (54 km)
  3. Ryegate, MT 39 mi (63 km)
  4. Grass Range, MT 43 mi (69 km)
  5. Billings (Logan Airport), MT 44 mi (70 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: 46.4381, -108.5319.