U.S. Frost Dates by Weather Station

NOAA station USC00136103 · Iowa

Frost Dates for Northwood, IA

NOAA weather station Northwood (USC00136103), Iowa · elevation 1190 ft (363 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 16only 1 spring in 10 still has a frost later
  • Typical last frostMay 450% chance of a frost after it
  • Typical first fall frostOctober 450% chance of a frost before it
  • Frost-free season152 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Northwood, IASpring: a frost after April 21 in 9 years out of 10, after May 4 in half of the years, after May 16 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 4 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 152 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 4First frost ~Oct 4Frost season at Northwood, IASpring: a frost after April 21 in 9 years out of 10, after May 4 in half of the years, after May 16 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 4 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 152 days.JFMAMJJASONDLast frost ~May 4First 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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 16September 20
50% (typical)May 4October 4
90% (likely)April 21October 17
Frost-free season (32°F, 50% chance)
152 days
Hard frost (28°F), last in spring, 50% chance after
April 24
Hard frost (28°F), first in fall, 50% chance before
October 13

The last 15 years (2006–2020 normals)

Typical last frost May 4 (same as the 30-year normal), typical first fall frost October 8 (4 days later). Frost-free season 155 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 Northwood: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 16, 20% May 12, 30% May 9, 40% May 6, 50% May 4, 60% May 2, 70% April 29, 80% April 25, 90% April 21. Observed last spring frost in recent years: 2021 May 12, 2024 April 26.Spring: last frostChance of a frost after this date10%50%90%May 1May 16May 4Apr 212021: May 122024: April 26Fall at Northwood: chance of a frost before this dateFall, chance of a frost before the date: 10% September 20, 20% September 25, 30% September 28, 40% October 1, 50% October 4, 60% October 6, 70% October 9, 80% October 12, 90% October 17. Observed first fall frost in recent years: 2021 October 22, 2023 October 10, 2024 October 7.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 20Oct 4Oct 172021: October 222023: October 102024: October 7
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
2022no datano data
2023incomplete recordOctober 10
2024April 26October 7
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

The 152 days between the 32°F 50% dates of May 4 and October 4 in Northwood define the core frost-free interval, during which light frosts are statistically less likely than not. For a gardener, the 50% dates are the points where the likelihood of a light frost is exactly even. The hard-frost 28°F 50% dates fall on April 24 and October 13, marking the midpoints of hard-frost risk within the season.

A cautious spring plan uses the May 16 date, which carries only a 10% chance of a light frost later, while a cautious fall plan relies on September 20, the date with only a 10% chance of a light frost earlier. Planting or harvesting tender crops around the 90% dates—April 21 in spring and October 17 in fall—represents a gamble. Hard frost at 28°F can damage plants that survive a light frost at 32°F.

How to read the probabilities

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

  • Spring: "10% after May 16" 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 20" 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 Northwood, IANumbered nearest stations: 1 — Albert Lea, MN — 12 mi; 2 — Mason City, IA — 19 mi; 3 — Austin, MN — 20 mi; 4 — Mason City (Airport), IA — 20 mi; 5 — Forest City, IA — 23 mi.MN25 mi · 40 km1. Albert Lea, MN — 12 mi12. Mason City, IA — 19 mi23. Austin, MN — 20 mi34. Mason City (Airport), IA — 20 mi45. Forest City, IA — 23 mi5
  1. Albert Lea, MN 12 mi (20 km)
  2. Mason City, IA 19 mi (31 km)
  3. Austin, MN 20 mi (31 km)
  4. Mason City (Airport), IA 20 mi (33 km)
  5. Forest City, IA 23 mi (37 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: 43.4386, -93.2253.