U.S. Frost Dates by Weather Station

NOAA station USC00326255 · North Dakota

Frost Dates for Napoleon, ND

NOAA weather station Napoleon (USC00326255), North Dakota · elevation 1980 ft (604 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 26only 1 spring in 10 still has a frost later
  • Typical last frostMay 1350% chance of a frost after it
  • Typical first fall frostSeptember 2850% chance of a frost before it
  • Frost-free season136 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Napoleon, NDSpring: a frost after May 1 in 9 years out of 10, after May 13 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 28 in half of the years, before October 11 in 9 years out of 10. Typical frost-free season: 136 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 13First frost ~Sep 28Frost season at Napoleon, NDSpring: a frost after May 1 in 9 years out of 10, after May 13 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 28 in half of the years, before October 11 in 9 years out of 10. Typical frost-free season: 136 days.JFMAMJJASONDLast frost ~May 13First frost ~Sep 28
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 26September 15
50% (typical)May 13September 28
90% (likely)May 1October 11
Frost-free season (32°F, 50% chance)
136 days
Hard frost (28°F), last in spring, 50% chance after
May 4
Hard frost (28°F), first in fall, 50% chance before
October 6

The last 15 years (2006–2020 normals)

Typical last frost May 14 (1 day later than the 30-year normal), typical first fall frost September 30 (2 days 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 at Napoleon: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 26, 20% May 22, 30% May 18, 40% May 16, 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.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 26May 13May 12021: May 28Fall at Napoleon: chance of a frost before this dateFall, chance of a frost before the date: 10% September 15, 20% September 19, 30% September 23, 40% September 26, 50% September 28, 60% October 1, 70% October 3, 80% October 6, 90% October 11. Observed first fall frost in recent years: 2021 October 21.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 15Sep 28Oct 112021: October 21
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 28October 21
2022incomplete recordincomplete record
2023incomplete recordincomplete record
2024incomplete recordincomplete record
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

May 26 is the cautious spring date for light frost, meaning there is only a 10% chance of a frost later than that day and it is the date gardeners use to set out tender crops. The 50% date in spring, May 13, marks the point at which a frost is equally likely to occur later, while the 50% date in fall, September 28, marks an even chance of a frost occurring earlier. The hard-frost 50% dates, May 4 and October 6, indicate when a 28 °F frost is as likely as not, and they remind gardeners that hard frost can be more damaging than a light 32 °F frost because it can injure plants that survive milder frost. For the fall season, September 15 is the cautious date, with only a 10% chance that a frost has arrived earlier, and it is the reference point for finishing harvest of tender crops.

How to read the probabilities

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

  • Spring: "10% after May 26" 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 15" 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 Napoleon, NDNumbered nearest stations: 1 — Streeter, ND — 21 mi; 2 — Hazelton, ND — 28 mi; 3 — Steele, ND — 28 mi; 4 — Ashley, ND — 37 mi; 5 — Pollock, SD — 49 mi.SD25 mi · 40 km1. Streeter, ND — 21 mi12. Hazelton, ND — 28 mi23. Steele, ND — 28 mi34. Ashley, ND — 37 mi45. Pollock, SD — 49 mi5
  1. Streeter, ND 21 mi (34 km)
  2. Hazelton, ND 28 mi (44 km)
  3. Steele, ND 28 mi (45 km)
  4. Ashley, ND 37 mi (60 km)
  5. Pollock, SD 49 mi (78 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: 46.5067, -99.7692.