U.S. Frost Dates by Weather Station

NOAA station USC00245668 · Montana

Frost Dates for Mildred, MT

NOAA weather station Mildred 5 N (USC00245668), Montana · elevation 2482 ft (757 m) · hardiness zone 4a*

  • 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 2250% chance of a frost before it
  • Frost-free season124 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Mildred, 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 7 in 1 year out of 10, before September 22 in half of the years, before October 6 in 9 years out of 10. Typical frost-free season: 124 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Sep 22Frost season at Mildred, 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 7 in 1 year out of 10, before September 22 in half of the years, before October 6 in 9 years out of 10. Typical frost-free season: 124 days.JFMAMJJASONDLast frost ~May 20First 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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 2September 7
50% (typical)May 20September 22
90% (likely)May 6October 6
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 17 (3 days earlier than the 30-year normal), typical first fall frost September 25 (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 Mildred: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 2, 20% May 28, 30% May 25, 40% May 23, 50% May 20, 60% May 17, 70% May 14, 80% May 11, 90% May 6. Observed last spring frost in recent years: 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 2May 20May 62025: May 22026: May 19Fall at Mildred: 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 20, 50% September 22, 60% September 25, 70% September 28, 80% October 2, 90% October 6. Observed first fall frost in recent years: 2024 October 3, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 22Oct 62024: October 32025: 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
2021no datano data
2022no datano data
2023no dataincomplete record
2024incomplete recordOctober 3
2025May 2October 6
2026May 19season not over yet

What these dates mean

There are 124 days between the 32°F 50% dates of May 20 and September 22 in Mildred, a span that defines the core growing window. The 50% date is May 20, meaning a light frost is as likely as not around that time, and the 50% date is September 22, with a similar even chance for a light frost later in the season. The 50% hard-frost dates are May 7 and October 1, marking the points where a 28°F frost is equally probable.

For a cautious spring plan, the June 2 date carries only a 10 % chance of a light frost later, making it the recommended moment to set out tender crops, whereas the May 6 date carries a 90 % chance and is a gamble. In the fall, September 7 is the cautious date with a 10 % chance of a light frost earlier, suitable for finishing tender harvests, while October 6 bears a 90 % chance and is risky.

A hard frost at 28°F can damage or kill plants that would survive a 32°F light frost, so it represents a greater threat to tender crops.

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 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 Mildred, MTNumbered nearest stations: 1 — Terry, MT — 16 mi; 2 — Glendive, MT — 26 mi; 3 — Carlyle, MT — 31 mi; 4 — Plevna, MT — 32 mi; 5 — Mizpah, MT — 36 mi.NDSD25 mi · 40 km1. Terry, MT — 16 mi12. Glendive, MT — 26 mi23. Carlyle, MT — 31 mi34. Plevna, MT — 32 mi45. Mizpah, MT — 36 mi5
  1. Terry, MT 16 mi (26 km)
  2. Glendive, MT 26 mi (43 km)
  3. Carlyle, MT 31 mi (50 km)
  4. Plevna, MT 32 mi (51 km)
  5. Mizpah, MT 36 mi (59 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: 46.7614, -104.9622.