U.S. Frost Dates by Weather Station

NOAA station USW00094014 · North Dakota

Frost Dates for Williston (Sloulin Airport), ND

NOAA weather station Williston Sloulin International Airport (USW00094014), North Dakota · elevation 1902 ft (580 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 3only 1 spring in 10 still has a frost later
  • Typical last frostMay 2150% chance of a frost after it
  • Typical first fall frostSeptember 2150% chance of a frost before it
  • Frost-free season123 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Williston (Sloulin Airport), NDSpring: a frost after May 8 in 9 years out of 10, after May 21 in half of the years, after June 3 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 21 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 123 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 21First frost ~Sep 21Frost season at Williston (Sloulin Airport), NDSpring: a frost after May 8 in 9 years out of 10, after May 21 in half of the years, after June 3 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 21 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 123 days.JFMAMJJASONDLast frost ~May 21First frost ~Sep 21
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)June 3September 9
50% (typical)May 21September 21
90% (likely)May 8October 2
Frost-free season (32°F, 50% chance)
123 days
Hard frost (28°F), last in spring, 50% chance after
May 11
Hard frost (28°F), first in fall, 50% chance before
September 28

The last 15 years (2006–2020 normals)

Typical last frost May 18 (3 days earlier than the 30-year normal), typical first fall frost September 22 (1 day later). Frost-free season 126 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 Williston (Sloulin Airport): chance of a frost after this dateSpring, chance of a frost after the date: 10% June 3, 20% May 29, 30% May 26, 40% May 24, 50% May 21, 60% May 18, 70% May 16, 80% May 13, 90% May 8.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 3May 21May 8Fall at Williston (Sloulin Airport): chance of a frost before this dateFall, chance of a frost before the date: 10% September 9, 20% September 13, 30% September 17, 40% September 19, 50% September 21, 60% September 23, 70% September 25, 80% September 28, 90% October 2.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 9Sep 21Oct 2
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.

What these dates mean

The 123 days between the 50% dates in Williston (Sloulin Airport)—May 21 and September 21—frame the period where a light frost is no more likely than not. During this span a gardener can expect the probability of a frost later than May 21 or earlier than September 21 to be around fifty percent.

For tender crops the cautious spring date is June 3, when there is only a ten-percent chance of a light frost later, while the early-season gamble date is May 8 with a ninety-percent chance of frost later. In the fall the cautious date to finish harvesting tender crops is September 9, with a ten-percent chance of a light frost earlier, and the late-season gamble date is October 2 with a ninety-percent chance of frost earlier. A hard frost (28°F) matters more than a light frost because it can damage plants that survive a 32°F event. The median hard-frost dates are May 11 in spring and September 28 in fall.

How to read the probabilities

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

  • Spring: "10% after June 3" 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 9" 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 (Sloulin Airport), NDNumbered nearest stations: 1 — near Williston, ND — 5 mi; 2 — Keene, ND — 38 mi; 3 — Sidney, MT — 39 mi; 4 — Culbertson, MT — 40 mi; 5 — Fortuna, ND — 51 mi.NDMT25 mi · 40 km1. near Williston, ND — 5 mi12. Keene, ND — 38 mi23. Sidney, MT — 39 mi34. Culbertson, MT — 40 mi45. Fortuna, ND — 51 mi5
  1. near Williston, ND 5 mi (8 km)
  2. Keene, ND 38 mi (62 km)
  3. Sidney, MT 39 mi (62 km)
  4. Culbertson, MT 40 mi (65 km)
  5. Fortuna, ND 51 mi (83 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: 48.1739, -103.6367.