U.S. Frost Dates by Weather Station

NOAA station USC00327704 · North Dakota

Frost Dates for Rugby, ND

NOAA weather station Rugby (USC00327704), North Dakota · elevation 1550 ft (472 m) · hardiness zone 3b*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1750% chance of a frost after it
  • Typical first fall frostSeptember 2350% chance of a frost before it
  • Frost-free season129 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Rugby, NDSpring: a frost after May 3 in 9 years out of 10, after May 17 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 11 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: 129 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 17First frost ~Sep 23Frost season at Rugby, NDSpring: a frost after May 3 in 9 years out of 10, after May 17 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 11 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: 129 days.JFMAMJJASONDLast frost ~May 17First 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.Today

Frost dates (32°F)

Probability of frost, based on 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 31September 11
50% (typical)May 17September 23
90% (likely)May 3October 5
Frost-free season (32°F, 50% chance)
129 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 12 (5 days earlier than the 30-year normal), typical first fall frost September 27 (4 days later). Frost-free season 136 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 Rugby: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 23, 40% May 20, 50% May 17, 60% May 14, 70% May 11, 80% May 8, 90% May 3. Observed last spring frost in recent years: 2021 May 29, 2023 May 3, 2024 May 25, 2025 April 30, 2026 May 11.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 17May 32021: May 292023: May 32024: May 252025: April 302026: May 11Fall at Rugby: chance of a frost before this dateFall, chance of a frost before the date: 10% September 11, 20% September 15, 30% September 18, 40% September 21, 50% September 23, 60% September 25, 70% September 28, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 October 16, 2022 September 22, 2023 October 7, 2024 October 4, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 11Sep 23Oct 52021: October 162022: September 222023: October 72024: October 42025: 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
2021May 29October 16
2022incomplete recordSeptember 22
2023May 3October 7
2024May 25October 4
2025April 30October 6
2026May 11season not over yet

What these dates mean

Setting out tender crops on the early spring date of May 3 is a gamble; there is a 90 % chance that a light frost will occur later than this date in Rugby. The cautious spring date for planting tender crops is May 31, when only a 10 % chance of a later frost remains. The 50 % date is May 17; at that point there is an even chance of a light frost occurring later.

In the autumn the cautious date for finishing harvest of tender crops is September 11, with only a 10 % chance that a light frost has already arrived. The 50 % date is September 23, marking an even likelihood of a frost earlier than that date. The 50 % hard-frost dates are May 6 and October 1, and a hard frost at 28 °F is more damaging to tender crops 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 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 11" 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 Rugby, NDNumbered nearest stations: 1 — Towner, ND — 18 mi; 2 — Willow City, ND — 22 mi; 3 — Drake, ND — 26 mi; 4 — Harvey, ND — 38 mi; 5 — Upham, ND — 38 mi.25 mi · 40 km1. Towner, ND — 18 mi12. Willow City, ND — 22 mi23. Drake, ND — 26 mi34. Harvey, ND — 38 mi45. Upham, ND — 38 mi5
  1. Towner, ND 18 mi (29 km)
  2. Willow City, ND 22 mi (36 km)
  3. Drake, ND 26 mi (41 km)
  4. Harvey, ND 38 mi (61 km)
  5. Upham, ND 38 mi (61 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: 48.3542, -99.9925.