U.S. Frost Dates by Weather Station

NOAA station USC00324571 · North Dakota

Frost Dates for Keene, ND

NOAA weather station Keene 3 S (USC00324571), North Dakota · elevation 2470 ft (753 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 2050% chance of a frost before it
  • Frost-free season121 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Keene, NDSpring: a frost after May 8 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 20 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 121 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Sep 20Frost season at Keene, NDSpring: a frost after May 8 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 20 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 121 days.JFMAMJJASONDLast frost ~May 20First frost ~Sep 20
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 2September 5
50% (typical)May 20September 20
90% (likely)May 8October 2
Frost-free season (32°F, 50% chance)
121 days
Hard frost (28°F), last in spring, 50% chance after
May 10
Hard frost (28°F), first in fall, 50% chance before
September 28

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 22 (2 days later). Frost-free season 124 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 Keene: 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 22, 50% May 20, 60% May 17, 70% May 15, 80% May 12, 90% May 8. Observed last spring frost in recent years: 2021 May 10, 2022 June 3, 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 2May 20May 82021: May 102022: June 32023: May 192024: May 42025: May 222026: May 19Fall at Keene: chance of a frost before this dateFall, chance of a frost before the date: 10% September 5, 20% September 9, 30% September 14, 40% September 17, 50% September 20, 60% September 22, 70% September 25, 80% September 28, 90% October 2. Observed first fall frost in recent years: 2021 October 13, 2022 September 21, 2023 October 6, 2024 October 3, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 5Sep 20Oct 22021: October 132022: September 212023: 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 10October 13
2022June 3September 21
2023May 19October 6
2024May 4October 3
2025May 22September 6
2026May 19season not over yet

What these dates mean

The 121 days between the 50% dates in Keene, May 20 and September 20, bracket the central growing window recorded over 30 years. The 50% date is May 20, marking a 50% chance of a light frost later in spring, and the 50% date is September 20, marking a 50% chance of a light frost earlier in fall. Gardeners can use these points to gauge the midpoint of frost risk.

For a low-risk plan, the cautious spring date is June 2, when there is only a 10% chance of a frost later, and the cautious fall date is September 5, when there is only a 10% chance of a frost earlier. The median hard-frost dates are May 10 and September 28; a hard frost at 28 °F can injure plants more severely than a light frost at 32 °F.

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 Keene, NDNumbered nearest stations: 1 — Grassy Butte, ND — 37 mi; 2 — Williston (Sloulin Airport), ND — 38 mi; 3 — near Williston, ND — 41 mi; 4 — Sidney, MT — 58 mi; 5 — Kenmare, ND — 65 mi.NDMT25 mi · 40 km1. Grassy Butte, ND — 37 mi12. Williston (Sloulin Airport), ND — 38 mi23. near Williston, ND — 41 mi34. Sidney, MT — 58 mi45. Kenmare, ND — 65 mi5
  1. Grassy Butte, ND 37 mi (59 km)
  2. Williston (Sloulin Airport), ND 38 mi (62 km)
  3. near Williston, ND 41 mi (66 km)
  4. Sidney, MT 58 mi (93 km)
  5. Kenmare, ND 65 mi (105 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: 47.8967, -102.9208.