U.S. Frost Dates by Weather Station

NOAA station USC00139067 · Iowa

Frost Dates for Williamsburg, IA

NOAA weather station Williamsburg 1 E (USC00139067), Iowa · elevation 780 ft (238 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 12only 1 spring in 10 still has a frost later
  • Typical last frostApril 2850% chance of a frost after it
  • Typical first fall frostOctober 950% chance of a frost before it
  • Frost-free season164 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Williamsburg, IASpring: a frost after April 14 in 9 years out of 10, after April 28 in half of the years, after May 12 in 1 year out of 10. Fall: the first frost comes before September 24 in 1 year out of 10, before October 9 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 164 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 28First frost ~Oct 9Frost season at Williamsburg, IASpring: a frost after April 14 in 9 years out of 10, after April 28 in half of the years, after May 12 in 1 year out of 10. Fall: the first frost comes before September 24 in 1 year out of 10, before October 9 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 164 days.JFMAMJJASONDLast frost ~Apr 28First frost ~Oct 9
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 12September 24
50% (typical)April 28October 9
90% (likely)April 14October 21
Frost-free season (32°F, 50% chance)
164 days
Hard frost (28°F), last in spring, 50% chance after
April 18
Hard frost (28°F), first in fall, 50% chance before
October 19

The last 15 years (2006–2020 normals)

Typical last frost April 29 (1 day later than the 30-year normal), typical first fall frost October 11 (2 days later). Frost-free season 166 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 Williamsburg: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 12, 20% May 7, 30% May 3, 40% May 1, 50% April 28, 60% April 25, 70% April 22, 80% April 18, 90% April 14. Observed last spring frost in recent years: 2021 May 12, 2022 April 20, 2023 May 3, 2024 April 21, 2025 April 13, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 12Apr 28Apr 142021: May 122022: April 202023: May 32024: April 212025: April 132026: May 7Fall at Williamsburg: chance of a frost before this dateFall, chance of a frost before the date: 10% September 24, 20% September 29, 30% October 2, 40% October 6, 50% October 9, 60% October 11, 70% October 14, 80% October 17, 90% October 21. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 10, 2024 October 15, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 24Oct 9Oct 212021: October 222022: September 282023: 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
2021May 12October 22
2022April 20September 28
2023May 3October 10
2024April 21October 15
2025April 13October 23
2026May 7season not over yet

What these dates mean

May 12 is the cautious spring date for Williamsburg, Iowa, with only a 10 % chance that a light frost will occur later; this lets a gardener plan to set out tender crops with minimal frost risk. The 50% spring date, April 28, marks the day on which a light frost is equally likely, while the 90 % early date, April 14, would be a gamble for early planting.

In the fall, September 24 serves as the cautious date, offering only a 10 % chance that a light frost has already arrived, so it is the day to finish harvesting tender crops safely. The 50% fall date, October 9, indicates an even chance of a light frost, and the later October 21 date carries a 90 % chance, making it risky for late crops. The 50% hard-frost dates, April 18 in spring and October 19 in fall, show when a 28 °F temperature has an even probability. A hard frost at 28 °F can damage plants more severely than a light frost at 32 °F because it reaches temperatures that freeze plant tissue.

How to read the probabilities

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

  • Spring: "10% after May 12" 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 24" 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 Williamsburg, IANumbered nearest stations: 1 — Belle Plaine, IA — 21 mi; 2 — Cedar Rapids, IA — 21 mi; 3 — Sigourney, IA — 26 mi; 4 — Iowa City, IA — 26 mi; 5 — Washington, IA — 30 mi.IL25 mi · 40 km1. Belle Plaine, IA — 21 mi12. Cedar Rapids, IA — 21 mi23. Sigourney, IA — 26 mi34. Iowa City, IA — 26 mi45. Washington, IA — 30 mi5
  1. Belle Plaine, IA 21 mi (33 km)
  2. Cedar Rapids, IA 21 mi (34 km)
  3. Sigourney, IA 26 mi (41 km)
  4. Iowa City, IA 26 mi (41 km)
  5. Washington, IA 30 mi (49 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: 41.665, -91.9978.