U.S. Frost Dates by Weather Station

NOAA station USC00324091 · North Dakota

Frost Dates for Heart Butte Dam, ND

NOAA weather station Heart Butte Dam (USC00324091), North Dakota · elevation 2134 ft (650 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 28only 1 spring in 10 still has a frost later
  • Typical last frostMay 1450% chance of a frost after it
  • Typical first fall frostSeptember 2950% chance of a frost before it
  • Frost-free season136 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Heart Butte Dam, NDSpring: a frost after May 3 in 9 years out of 10, after May 14 in half of the years, after May 28 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before September 29 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 136 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 14First frost ~Sep 29Frost season at Heart Butte Dam, NDSpring: a frost after May 3 in 9 years out of 10, after May 14 in half of the years, after May 28 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before September 29 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 136 days.JFMAMJJASONDLast frost ~May 14First frost ~Sep 29
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 28September 16
50% (typical)May 14September 29
90% (likely)May 3October 10
Frost-free season (32°F, 50% chance)
136 days
Hard frost (28°F), last in spring, 50% chance after
May 5
Hard frost (28°F), first in fall, 50% chance before
October 6

The last 15 years (2006–2020 normals)

Typical last frost May 15 (1 day later than the 30-year normal), typical first fall frost October 1 (2 days later). Frost-free season 139 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 Heart Butte Dam: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 28, 20% May 23, 30% May 20, 40% May 17, 50% May 14, 60% May 12, 70% May 9, 80% May 7, 90% May 3. Observed last spring frost in recent years: 2021 May 27.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 28May 14May 32021: May 27Fall at Heart Butte Dam: chance of a frost before this dateFall, chance of a frost before the date: 10% September 16, 20% September 20, 30% September 24, 40% September 26, 50% September 29, 60% October 1, 70% October 4, 80% October 7, 90% October 10. Observed first fall frost in recent years: 2021 October 20, 2024 October 14.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 16Sep 29Oct 102021: October 202024: October 14
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 27October 20
2022incomplete recordincomplete record
2023incomplete recordincomplete record
2024incomplete recordOctober 14
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

There are 136 days between the 32 °F 50% dates in Heart Butte Dam, North Dakota, framing the interval in which light frosts shift from being unlikely to more likely. The 50% spring date is May 14, when a light frost has a 50 % chance of occurring later, and the 50% fall date is September 29, when a light frost has a 50 % chance of occurring earlier. These dates mark the midpoint of the frost-risk window for a gardener.

For a cautious plan, the spring LATE date of May 28 carries only a 10 % chance of a later light frost, making it the preferred day to set out tender crops. In the fall, the EARLY date of September 16 holds a 10 % chance of an earlier light frost, signaling when to finish harvesting tender crops. The hard-frost median dates are May 5 and October 6; a hard frost at 28 °F can damage plants that survive a 32 °F light frost, so its threshold is more critical.

How to read the probabilities

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

  • Spring: "10% after May 28" 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 16" 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 Heart Butte Dam, NDNumbered nearest stations: 1 — Hebron, ND — 24 mi; 2 — New Salem, ND — 26 mi; 3 — Pretty Rock, ND — 29 mi; 4 — Richardton Abbey, ND — 32 mi; 5 — Mandan, ND — 45 mi.NDSD25 mi · 40 km1. Hebron, ND — 24 mi12. New Salem, ND — 26 mi23. Pretty Rock, ND — 29 mi34. Richardton Abbey, ND — 32 mi45. Mandan, ND — 45 mi5
  1. Hebron, ND 24 mi (39 km)
  2. New Salem, ND 26 mi (41 km)
  3. Pretty Rock, ND 29 mi (46 km)
  4. Richardton Abbey, ND 32 mi (51 km)
  5. Mandan, ND 45 mi (73 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: 46.5917, -101.8075.