U.S. Frost Dates by Weather Station

NOAA station USC00329430 · North Dakota

Frost Dates near Williston, ND

NOAA weather station Williston Experiment Farm (USC00329430), North Dakota · elevation 2105 ft (642 m) · hardiness zone 4a*

Williston is the nearest town to the station (5.5 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter May 25only 1 spring in 10 still has a frost later
  • Typical last frostMay 1350% chance of a frost after it
  • Typical first fall frostSeptember 2650% chance of a frost before it
  • Frost-free season135 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season near Williston, NDSpring: a frost after May 1 in 9 years out of 10, after May 13 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before September 26 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 135 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 13First frost ~Sep 26Frost season near Williston, NDSpring: a frost after May 1 in 9 years out of 10, after May 13 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before September 26 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 135 days.JFMAMJJASONDLast frost ~May 13First frost ~Sep 26
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)May 25September 14
50% (typical)May 13September 26
90% (likely)May 1October 8
Frost-free season (32°F, 50% chance)
135 days
Hard frost (28°F), last in spring, 50% chance after
May 3
Hard frost (28°F), first in fall, 50% chance before
October 3

The last 15 years (2006–2020 normals)

Typical last frost May 11 (2 days earlier than the 30-year normal), typical first fall frost September 27 (1 day later). Frost-free season 138 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 near Williston: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 25, 20% May 21, 30% May 18, 40% May 15, 50% May 13, 60% May 11, 70% May 8, 80% May 5, 90% May 1. Observed last spring frost in recent years: 2021 May 28, 2022 May 21, 2024 April 29, 2025 April 29, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 25May 13May 12021: May 282022: May 212024: April 292025: April 292026: May 19Fall near Williston: chance of a frost before this dateFall, chance of a frost before the date: 10% September 14, 20% September 19, 30% September 22, 40% September 24, 50% September 26, 60% September 28, 70% September 30, 80% October 3, 90% October 8. Observed first fall frost in recent years: 2021 September 24, 2023 October 6, 2024 October 13.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 14Sep 26Oct 82021: September 242023: October 62024: October 13
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 28September 24
2022May 21incomplete record
2023incomplete recordOctober 6
2024April 29October 13
2025April 29incomplete record
2026May 19season not over yet

What these dates mean

May 25 is the cautious spring date for the Williston area; with only a 10 % chance of a light frost after that day, a gardener can plan to set out tender crops with minimal risk. The 50 % date of May 13 marks the median point where a light frost is equally likely to occur later or not, while the hard-frost median of May 3 indicates the point at which a 28 °F freeze has an even chance of having happened.

In the fall, September 14 serves as the cautious date for finishing harvest of tender crops, because there is just a 10 % chance that a light frost has already arrived. The median light-frost date of September 26 shows an even-odds situation, and the hard-frost median of October 3 marks the equivalent point for a 28 °F freeze.

A hard frost matters more than a light frost because the lower temperature can damage plant tissue that survives the milder 32 °F chill.

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 14" 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 Williston, NDNumbered nearest stations: 1 — Williston (Sloulin Airport), ND — 5 mi; 2 — Sidney, MT — 34 mi; 3 — Culbertson, MT — 36 mi; 4 — Keene, ND — 41 mi; 5 — Westby, MT — 53 mi.NDMT25 mi · 40 km1. Williston (Sloulin Airport), ND — 5 mi12. Sidney, MT — 34 mi23. Culbertson, MT — 36 mi34. Keene, ND — 41 mi45. Westby, MT — 53 mi5
  1. Williston (Sloulin Airport), ND 5 mi (8 km)
  2. Sidney, MT 34 mi (55 km)
  3. Culbertson, MT 36 mi (57 km)
  4. Keene, ND 41 mi (66 km)
  5. Westby, 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: 48.1375, -103.7372.