U.S. Frost Dates by Weather Station

NOAA station USC00323196 · North Dakota

Frost Dates for Fortuna, ND

NOAA weather station Fortuna 1 W (USC00323196), North Dakota · elevation 2350 ft (716 m) · hardiness zone 3b*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1950% chance of a frost after it
  • Typical first fall frostSeptember 1750% chance of a frost before it
  • Frost-free season121 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Fortuna, NDSpring: a frost after May 8 in 9 years out of 10, after May 19 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 17 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 121 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 19First frost ~Sep 17Frost season at Fortuna, NDSpring: a frost after May 8 in 9 years out of 10, after May 19 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 17 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 121 days.JFMAMJJASONDLast frost ~May 19First frost ~Sep 17
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 31September 4
50% (typical)May 19September 17
90% (likely)May 8September 28
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 26

The last 15 years (2006–2020 normals)

Typical last frost May 18 (1 day earlier than the 30-year normal), typical first fall frost September 19 (2 days later). Frost-free season 121 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 Fortuna: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 27, 30% May 24, 40% May 22, 50% May 19, 60% May 17, 70% May 15, 80% May 11, 90% May 8. Observed last spring frost in recent years: 2021 May 27, 2022 May 22, 2025 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 19May 82021: May 272022: May 222025: May 22Fall at Fortuna: chance of a frost before this dateFall, chance of a frost before the date: 10% September 4, 20% September 9, 30% September 12, 40% September 15, 50% September 17, 60% September 19, 70% September 22, 80% September 25, 90% September 28. Observed first fall frost in recent years: 2021 September 24, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 4Sep 17Sep 282021: September 242025: 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 27September 24
2022May 22incomplete record
2023incomplete recordincomplete record
2024incomplete recordno data
2025May 22September 6
2026no dataseason not over yet

What these dates mean

September 4 is the cautious fall date to finish harvesting tender crops in Fortuna. It carries a 10% chance that a light frost has arrived earlier, while September 28 carries a 90% chance that a light frost will already be present, making it a gamble for tender crops.

In spring, May 31 is the cautious date for setting out tender crops, offering a 10% chance of a later light frost; May 8 carries a 90% chance and is a gamble. The 50% dates, May 19 and September 17, represent the point of even odds. The hard-frost 50% dates are May 10 in spring and September 26 in fall.

Hard frost at 28 °F can injure plants that would survive 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 May 31" 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 4" 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 Fortuna, NDNumbered nearest stations: 1 — Westby, MT — 11 mi; 2 — Crosby, ND — 23 mi; 3 — Williston (Sloulin Airport), ND — 51 mi; 4 — near Williston, ND — 53 mi; 5 — Culbertson, MT — 61 mi.NDMT25 mi · 40 km1. Westby, MT — 11 mi12. Crosby, ND — 23 mi23. Williston (Sloulin Airport), ND — 51 mi34. near Williston, ND — 53 mi45. Culbertson, MT — 61 mi5
  1. Westby, MT 11 mi (18 km)
  2. Crosby, ND 23 mi (37 km)
  3. Williston (Sloulin Airport), ND 51 mi (83 km)
  4. near Williston, ND 53 mi (86 km)
  5. Culbertson, MT 61 mi (99 km)

Sources

* Hardiness zone 3b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.9081, -103.8056.