U.S. Frost Dates by Weather Station

NOAA station USC00137678 · Iowa

Frost Dates for Sigourney, IA

NOAA weather station Sigourney (USC00137678), Iowa · elevation 732 ft (223 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 9only 1 spring in 10 still has a frost later
  • Typical last frostApril 2450% chance of a frost after it
  • Typical first fall frostOctober 1550% chance of a frost before it
  • Frost-free season172 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Sigourney, IASpring: a frost after April 9 in 9 years out of 10, after April 24 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 30 in 1 year out of 10, before October 15 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 172 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 24First frost ~Oct 15Frost season at Sigourney, IASpring: a frost after April 9 in 9 years out of 10, after April 24 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 30 in 1 year out of 10, before October 15 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 172 days.JFMAMJJASONDLast frost ~Apr 24First frost ~Oct 15
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.Today

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 9September 30
50% (typical)April 24October 15
90% (likely)April 9October 27
Frost-free season (32°F, 50% chance)
172 days
Hard frost (28°F), last in spring, 50% chance after
April 12
Hard frost (28°F), first in fall, 50% chance before
October 24

The last 15 years (2006–2020 normals)

Typical last frost April 26 (2 days later than the 30-year normal), typical first fall frost October 17 (2 days later). Frost-free season 172 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 Sigourney: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 9, 20% May 4, 30% May 1, 40% April 28, 50% April 24, 60% April 21, 70% April 18, 80% April 14, 90% April 9. Observed last spring frost in recent years: 2021 May 13, 2022 April 27, 2023 May 3, 2024 April 21, 2025 April 17.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 9Apr 24Apr 92021: May 132022: April 272023: May 32024: April 212025: April 17Fall at Sigourney: chance of a frost before this dateFall, chance of a frost before the date: 10% September 30, 20% October 5, 30% October 9, 40% October 12, 50% October 15, 60% October 17, 70% October 20, 80% October 23, 90% October 27. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 10, 2024 October 16, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 30Oct 15Oct 272021: October 222022: September 282023: October 102024: October 162025: October 23
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 13October 22
2022April 27September 28
2023May 3October 10
2024April 21October 16
2025April 17October 23
2026incomplete recordseason not over yet

What these dates mean

The 172 days between the 50% dates in Sigourney, April 24 and October 15, are the span in which light frosts have an even chance of occurring. The 50% dates, April 24 and October 15, are the points at which the chance of a light frost is exactly fifty percent. These probabilities derive from a 29-year record (1991-2020).

The cautious spring date for setting out tender crops is May 9, which carries a ten-percent chance that a light frost will occur later. The cautious fall date for finishing tender crops is September 30, which carries a ten-percent chance that a light frost has already arrived earlier.

A hard frost at 28 °F, represented by the 50% dates of April 12 and October 24, is more damaging than a light frost because the colder temperature can injure plant tissue that survives a 32 °F frost.

How to read the probabilities

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

  • Spring: "10% after May 9" 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 30" 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 Sigourney, IANumbered nearest stations: 1 — near Ottumwa, IA — 20 mi; 2 — Oskaloosa, IA — 22 mi; 3 — Williamsburg, IA — 26 mi; 4 — Fairfield, IA — 26 mi; 5 — Washington, IA — 27 mi.IL25 mi · 40 km1. near Ottumwa, IA — 20 mi12. Oskaloosa, IA — 22 mi23. Williamsburg, IA — 26 mi34. Fairfield, IA — 26 mi45. Washington, IA — 27 mi5
  1. near Ottumwa, IA 20 mi (31 km)
  2. Oskaloosa, IA 22 mi (35 km)
  3. Williamsburg, IA 26 mi (41 km)
  4. Fairfield, IA 26 mi (42 km)
  5. Washington, IA 27 mi (44 km)

Sources

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

Station coordinates: 41.3372, -92.2278.