U.S. Frost Dates by Weather Station

NOAA station USC00137147 · Iowa

Frost Dates for Rock Rapids, IA

NOAA weather station Rock Rapids (USC00137147), Iowa · elevation 1350 ft (412 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 13only 1 spring in 10 still has a frost later
  • Typical last frostMay 150% chance of a frost after it
  • Typical first fall frostOctober 450% chance of a frost before it
  • Frost-free season156 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Rock Rapids, IASpring: a frost after April 18 in 9 years out of 10, after May 1 in half of the years, after May 13 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 4 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 156 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 1First frost ~Oct 4Frost season at Rock Rapids, IASpring: a frost after April 18 in 9 years out of 10, after May 1 in half of the years, after May 13 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 4 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 156 days.JFMAMJJASONDLast frost ~May 1First frost ~Oct 4
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 13September 21
50% (typical)May 1October 4
90% (likely)April 18October 19
Frost-free season (32°F, 50% chance)
156 days
Hard frost (28°F), last in spring, 50% chance after
April 21
Hard frost (28°F), first in fall, 50% chance before
October 16

The last 15 years (2006–2020 normals)

Typical last frost May 1 (same as the 30-year normal), typical first fall frost October 8 (4 days later). Frost-free season 161 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 Rock Rapids: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 13, 20% May 9, 30% May 6, 40% May 3, 50% May 1, 60% April 29, 70% April 26, 80% April 22, 90% April 18. Observed last spring frost in recent years: 2021 May 11, 2022 May 4, 2023 May 3, 2024 April 21, 2025 April 20, 2026 May 6.Spring: last frostChance of a frost after this date10%50%90%May 1May 13May 1Apr 182021: May 112022: May 42023: May 32024: April 212025: April 202026: May 6Fall at Rock Rapids: chance of a frost before this dateFall, chance of a frost before the date: 10% September 21, 20% September 26, 30% September 29, 40% October 2, 50% October 4, 60% October 8, 70% October 12, 80% October 15, 90% October 19. Observed first fall frost in recent years: 2021 October 17, 2022 September 28, 2023 October 7, 2024 October 7, 2025 October 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 21Oct 4Oct 192021: October 172022: September 282023: October 72024: October 72025: October 22
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 17
2022May 4September 28
2023May 3October 7
2024April 21October 7
2025April 20October 22
2026May 6season not over yet

What these dates mean

Setting out tender crops on April 18 in Rock Rapids carries a 90% chance of a light frost, so it is a gamble. The later spring date of May 13 has only a 10% chance of a light frost and is the cautious date that growers use to begin planting tender varieties.

The 50% dates—May 1 in spring and October 4 in fall—mark the point where a light frost is equally likely to occur. For a cautious fall plan the 10% date is September 21, while October 19 represents a 90% chance and is a gamble for late-season tender crops.

Hard frosts at 28 °F are more damaging than light frosts at 32 °F, so the 28 °F 50% dates—April 21 and October 16—signal a higher-risk threshold for plants.

How to read the probabilities

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

  • Spring: "10% after May 13" 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 21" 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 Rock Rapids, IANumbered nearest stations: 1 — Sibley, IA — 21 mi; 2 — Sheldon, IA — 23 mi; 3 — Sioux Center, IA — 26 mi; 4 — Sioux Falls, SD — 30 mi; 5 — Sanborn, IA — 31 mi.SDMN25 mi · 40 km1. Sibley, IA — 21 mi12. Sheldon, IA — 23 mi23. Sioux Center, IA — 26 mi34. Sioux Falls, SD — 30 mi45. Sanborn, IA — 31 mi5
  1. Sibley, IA 21 mi (34 km)
  2. Sheldon, IA 23 mi (38 km)
  3. Sioux Center, IA 26 mi (42 km)
  4. Sioux Falls, SD 30 mi (48 km)
  5. Sanborn, IA 31 mi (50 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.43, -96.1686.