U.S. Frost Dates by Weather Station

NOAA station USC00323705 · North Dakota

Frost Dates for Grassy Butte, ND

NOAA weather station Grassy Butte 2 ENE (USC00323705), North Dakota · elevation 2670 ft (814 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 1only 1 spring in 10 still has a frost later
  • Typical last frostMay 1850% chance of a frost after it
  • Typical first fall frostSeptember 2350% chance of a frost before it
  • Frost-free season125 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Grassy Butte, NDSpring: a frost after May 7 in 9 years out of 10, after May 18 in half of the years, after June 1 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 23 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 125 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 23Frost season at Grassy Butte, NDSpring: a frost after May 7 in 9 years out of 10, after May 18 in half of the years, after June 1 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 23 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 125 days.JFMAMJJASONDLast frost ~May 18First frost ~Sep 23
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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 1September 9
50% (typical)May 18September 23
90% (likely)May 7October 5
Frost-free season (32°F, 50% chance)
125 days
Hard frost (28°F), last in spring, 50% chance after
May 9
Hard frost (28°F), first in fall, 50% chance before
October 3

The last 15 years (2006–2020 normals)

Typical last frost May 18 (same as the 30-year normal), typical first fall frost September 26 (3 days later). Frost-free season 129 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 Grassy Butte: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 1, 20% May 28, 30% May 24, 40% May 21, 50% May 18, 60% May 16, 70% May 14, 80% May 11, 90% May 7. Observed last spring frost in recent years: 2021 May 11, 2022 May 22, 2023 May 19, 2024 May 4, 2025 May 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 1May 18May 72021: May 112022: May 222023: May 192024: May 42025: May 222026: May 19Fall at Grassy Butte: chance of a frost before this dateFall, chance of a frost before the date: 10% September 9, 20% September 14, 30% September 18, 40% September 21, 50% September 23, 60% September 26, 70% September 29, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 October 13, 2022 September 22, 2023 October 6, 2024 October 3, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 9Sep 23Oct 52021: October 132022: September 222023: October 62024: October 32025: 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 11October 13
2022May 22September 22
2023May 19October 6
2024May 4October 3
2025May 22September 6
2026May 19season not over yet

What these dates mean

The two 32°F 50% dates, May 18 and September 23, are 125 days apart, defining the central window for tender gardening activities in Grassy Butte. The 50% dates represent the median point at which an equal chance of a light frost exists, giving a reference for the length of the growing season.

In spring, June 1 carries a 10% chance that a light frost will occur later, making it the cautious date for setting out tender crops; planting on May 7 carries a 90% chance of a frost later and is a gamble. In fall, September 9 has a 10% chance that a light frost has already arrived, so it is the cautious date to finish harvesting tender crops; counting on crops through October 5 carries a 90% chance of a frost earlier and is a gamble. The hard-frost 50% dates are May 9 and October 3, marking the median moments when a 28°F frost becomes as likely as not.

A hard frost matters more because the 28°F temperature can damage plants that survive a light 32°F frost.

How to read the probabilities

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

  • Spring: "10% after June 1" 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 9" 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 Grassy Butte, NDNumbered nearest stations: 1 — Medora, ND — 37 mi; 2 — Keene, ND — 37 mi; 3 — near Dickinson, ND — 46 mi; 4 — Sidney, MT — 49 mi; 5 — Savage, MT — 53 mi.NDMT25 mi · 40 km1. Medora, ND — 37 mi12. Keene, ND — 37 mi23. near Dickinson, ND — 46 mi34. Sidney, MT — 49 mi45. Savage, MT — 53 mi5
  1. Medora, ND 37 mi (59 km)
  2. Keene, ND 37 mi (59 km)
  3. near Dickinson, ND 46 mi (74 km)
  4. Sidney, MT 49 mi (79 km)
  5. Savage, MT 53 mi (85 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: 47.4011, -103.2072.