U.S. Frost Dates by Weather Station

NOAA station USW00014950 · Iowa

Frost Dates near Ottumwa, IA

NOAA weather station Ottumwa Industrial Airport (USW00014950), Iowa · elevation 842 ft (257 m) · hardiness zone 5b*

Ottumwa is the nearest town to the station (6.3 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter May 7only 1 spring in 10 still has a frost later
  • Typical last frostApril 2350% chance of a frost after it
  • Typical first fall frostOctober 1450% chance of a frost before it
  • Frost-free season172 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season near Ottumwa, IASpring: a frost after April 9 in 9 years out of 10, after April 23 in half of the years, after May 7 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 14 in half of the years, before October 26 in 9 years out of 10. Typical frost-free season: 172 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 23First frost ~Oct 14Frost season near Ottumwa, IASpring: a frost after April 9 in 9 years out of 10, after April 23 in half of the years, after May 7 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 14 in half of the years, before October 26 in 9 years out of 10. Typical frost-free season: 172 days.JFMAMJJASONDLast frost ~Apr 23First frost ~Oct 14
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 7September 29
50% (typical)April 23October 14
90% (likely)April 9October 26
Frost-free season (32°F, 50% chance)
172 days
Hard frost (28°F), last in spring, 50% chance after
April 13
Hard frost (28°F), first in fall, 50% chance before
October 24

The last 15 years (2006–2020 normals)

Typical last frost April 25 (2 days later than the 30-year normal), typical first fall frost October 16 (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 near Ottumwa: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 7, 20% May 3, 30% April 29, 40% April 26, 50% April 23, 60% April 20, 70% April 17, 80% April 13, 90% April 9. Observed last spring frost in recent years: 2021 April 25, 2022 April 26, 2023 May 3, 2024 April 21, 2025 April 8, 2026 April 20.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 7Apr 23Apr 92021: April 252022: April 262023: May 32024: April 212025: April 82026: April 20Fall near Ottumwa: chance of a frost before this dateFall, chance of a frost before the date: 10% September 29, 20% October 3, 30% October 7, 40% October 11, 50% October 14, 60% October 17, 70% October 20, 80% October 23, 90% October 26. Observed first fall frost in recent years: 2021 October 31, 2022 October 7, 2023 October 10, 2024 October 15, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 29Oct 14Oct 262021: October 312022: October 72023: October 102024: October 152025: 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
2021April 25October 31
2022April 26October 7
2023May 3October 10
2024April 21October 15
2025April 8October 23
2026April 20season not over yet

What these dates mean

Setting out tender crops on the early spring date of April 9 in the Ottumwa area carries a 90 % chance that a light frost will still occur later, so it is a gamble for growers.

The 50 % light-frost dates—April 23 in spring and October 14 in fall—mark the points where the odds of a frost are even. For a cautious plan the spring date is May 7, which has only a 10 % chance of a frost later, and the fall date is September 29, which has only a 10 % chance that a frost has arrived earlier.

The 50 % hard-frost dates are April 13 in spring and October 24 in fall; hard frosts at 28 °F can damage plants more severely than light frosts at 32 °F. Because the record covers 28 years, those median dates reflect a long-term pattern for the Ottumwa area.

How to read the probabilities

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

  • Spring: "10% after May 7" 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 29" 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 Ottumwa, IANumbered nearest stations: 1 — Albia, IA — 18 mi; 2 — Oskaloosa, IA — 18 mi; 3 — Sigourney, IA — 20 mi; 4 — Bloomfield, IA — 26 mi; 5 — Fairfield, IA — 26 mi.IL25 mi · 40 km1. Albia, IA — 18 mi12. Oskaloosa, IA — 18 mi23. Sigourney, IA — 20 mi34. Bloomfield, IA — 26 mi45. Fairfield, IA — 26 mi5
  1. Albia, IA 18 mi (29 km)
  2. Oskaloosa, IA 18 mi (29 km)
  3. Sigourney, IA 20 mi (31 km)
  4. Bloomfield, IA 26 mi (41 km)
  5. Fairfield, IA 26 mi (42 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: 41.1078, -92.4467.