U.S. Frost Dates by Weather Station

NOAA station USW00014946 · South Dakota

Frost Dates for Watertown, SD

NOAA weather station Watertown Regional Airport (USW00014946), South Dakota · elevation 1748 ft (533 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 20only 1 spring in 10 still has a frost later
  • Typical last frostMay 750% chance of a frost after it
  • Typical first fall frostSeptember 3050% chance of a frost before it
  • Frost-free season145 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Watertown, SDSpring: a frost after April 26 in 9 years out of 10, after May 7 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 19 in 1 year out of 10, before September 30 in half of the years, before October 13 in 9 years out of 10. Typical frost-free season: 145 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 7First frost ~Sep 30Frost season at Watertown, SDSpring: a frost after April 26 in 9 years out of 10, after May 7 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 19 in 1 year out of 10, before September 30 in half of the years, before October 13 in 9 years out of 10. Typical frost-free season: 145 days.JFMAMJJASONDLast frost ~May 7First frost ~Sep 30
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 20September 19
50% (typical)May 7September 30
90% (likely)April 26October 13
Frost-free season (32°F, 50% chance)
145 days
Hard frost (28°F), last in spring, 50% chance after
April 29
Hard frost (28°F), first in fall, 50% chance before
October 9

The last 15 years (2006–2020 normals)

Typical last frost May 7 (same as the 30-year normal), typical first fall frost October 5 (5 days later). Frost-free season 148 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 Watertown: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 20, 20% May 16, 30% May 12, 40% May 10, 50% May 7, 60% May 4, 70% May 2, 80% April 30, 90% April 26. Observed last spring frost in recent years: 2021 May 11, 2022 April 27, 2023 May 3, 2024 April 24, 2025 May 3, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 20May 7Apr 262021: May 112022: April 272023: May 32024: April 242025: May 32026: May 7Fall at Watertown: chance of a frost before this dateFall, chance of a frost before the date: 10% September 19, 20% September 23, 30% September 26, 40% September 28, 50% September 30, 60% October 2, 70% October 5, 80% October 8, 90% October 13. Observed first fall frost in recent years: 2021 October 16, 2022 September 28, 2023 October 7, 2024 October 13, 2025 October 21.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 19Sep 30Oct 132021: October 162022: September 282023: October 72024: October 132025: 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 11October 16
2022April 27September 28
2023May 3October 7
2024April 24October 13
2025May 3October 21
2026May 7season not over yet

What these dates mean

May 20 is the cautious spring date, carrying a 10 % chance that a light frost will occur later and marking the point at which a Watertown gardener can set out tender crops with the lowest risk. The 50 % dates—May 7 for spring light frost and September 30 for fall light frost—are the median moments when a light frost is equally likely to have happened or not; they are not frost dates themselves.

For the fall season the cautious date is September 19, when there is only a 10 % chance that a light frost has arrived earlier, allowing the gardener to finish harvesting tender crops with confidence. The hard-frost median dates are April 29 in spring and October 9 in fall; a hard frost can damage or kill plants that survive a light frost, making it a more critical threshold.

How to read the probabilities

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

  • Spring: "10% after May 20" 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 19" 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 Watertown, SDNumbered nearest stations: 1 — Clear Lake, SD — 25 mi; 2 — Summit, SD — 28 mi; 3 — Webster, SD — 35 mi; 4 — Waubay National Wild Life, SD — 37 mi; 5 — De Smet, SD — 41 mi.SDMN25 mi · 40 km1. Clear Lake, SD — 25 mi12. Summit, SD — 28 mi23. Webster, SD — 35 mi34. Waubay National Wild Life, SD — 37 mi45. De Smet, SD — 41 mi5
  1. Clear Lake, SD 25 mi (40 km)
  2. Summit, SD 28 mi (45 km)
  3. Webster, SD 35 mi (56 km)
  4. Waubay National Wild Life, SD 37 mi (59 km)
  5. De Smet, SD 41 mi (66 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 44.9047, -97.1494.