U.S. Frost Dates by Weather Station

NOAA station USC00132977 · Iowa

Frost Dates for Forest City, IA

NOAA weather station Forest City 2 NNE (USC00132977), Iowa · elevation 1280 ft (390 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 10only 1 spring in 10 still has a frost later
  • Typical last frostApril 3050% chance of a frost after it
  • Typical first fall frostOctober 750% chance of a frost before it
  • Frost-free season159 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Forest City, IASpring: a frost after April 14 in 9 years out of 10, after April 30 in half of the years, after May 10 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 7 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 159 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 30First frost ~Oct 7Frost season at Forest City, IASpring: a frost after April 14 in 9 years out of 10, after April 30 in half of the years, after May 10 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 7 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 159 days.JFMAMJJASONDLast frost ~Apr 30First frost ~Oct 7
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)May 10September 22
50% (typical)April 30October 7
90% (likely)April 14October 20
Frost-free season (32°F, 50% chance)
159 days
Hard frost (28°F), last in spring, 50% chance after
April 18
Hard frost (28°F), first in fall, 50% chance before
October 18

The last 15 years (2006–2020 normals)

Typical last frost May 1 (1 day later than the 30-year normal), typical first fall frost October 12 (5 days later). Frost-free season 164 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 Forest City: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 10, 20% May 7, 30% May 4, 40% May 1, 50% April 30, 60% April 27, 70% April 23, 80% April 19, 90% April 14. Observed last spring frost in recent years: 2021 May 12, 2022 April 28.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 10Apr 30Apr 142021: May 122022: April 28Fall at Forest City: chance of a frost before this dateFall, chance of a frost before the date: 10% September 22, 20% September 28, 30% October 1, 40% October 4, 50% October 7, 60% October 9, 70% October 12, 80% October 15, 90% October 20. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 9.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 22Oct 7Oct 202021: October 222022: September 282023: October 9
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 12October 22
2022April 28September 28
2023incomplete recordOctober 9
2024incomplete recordincomplete record
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

The 159 days between the 32°F 50% dates in Forest City span the interval where a light frost has a 50% chance of occurring, with the spring median on April 30 and the fall median on October 7. These dates are not frost dates themselves but mark the point of even odds for a light frost, helping gardeners gauge the central window of risk.

For a cautious plan, the spring light-frost date with a 10% chance of frost later is May 10, which gardeners can use to set out tender crops, while the fall cautious date with a 10% chance of frost earlier is September 22, marking an endpoint for harvesting tender crops. Hard-frost dates at 28°F are April 18 and October 18, and a hard frost can injure plants that would survive a 32°F light frost, making it a more critical temperature 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 10" 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 22" 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 Forest City, IANumbered nearest stations: 1 — Mason City (Airport), IA — 18 mi; 2 — Northwood, IA — 23 mi; 3 — Mason City, IA — 23 mi; 4 — Albert Lea, MN — 28 mi; 5 — Osage, IA — 41 mi.MN25 mi · 40 km1. Mason City (Airport), IA — 18 mi12. Northwood, IA — 23 mi23. Mason City, IA — 23 mi34. Albert Lea, MN — 28 mi45. Osage, IA — 41 mi5
  1. Mason City (Airport), IA 18 mi (29 km)
  2. Northwood, IA 23 mi (37 km)
  3. Mason City, IA 23 mi (38 km)
  4. Albert Lea, MN 28 mi (45 km)
  5. Osage, IA 41 mi (66 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: 43.2844, -93.6306.