U.S. Frost Dates by Weather Station

NOAA station USC00137664 · Iowa

Frost Dates for Sibley, IA

NOAA weather station Sibley (USC00137664), Iowa · elevation 1529 ft (466 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 17only 1 spring in 10 still has a frost later
  • Typical last frostMay 550% chance of a frost after it
  • Typical first fall frostOctober 350% chance of a frost before it
  • Frost-free season152 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Sibley, IASpring: a frost after April 22 in 9 years out of 10, after May 5 in half of the years, after May 17 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 3 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 152 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 5First frost ~Oct 3Frost season at Sibley, IASpring: a frost after April 22 in 9 years out of 10, after May 5 in half of the years, after May 17 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 3 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 152 days.JFMAMJJASONDLast frost ~May 5First frost ~Oct 3
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 17September 21
50% (typical)May 5October 3
90% (likely)April 22October 17
Frost-free season (32°F, 50% chance)
152 days
Hard frost (28°F), last in spring, 50% chance after
April 25
Hard frost (28°F), first in fall, 50% chance before
October 13

The last 15 years (2006–2020 normals)

Typical last frost May 5 (same as the 30-year normal), typical first fall frost October 5 (2 days later). Frost-free season 153 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 Sibley: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 17, 20% May 13, 30% May 9, 40% May 6, 50% May 5, 60% May 2, 70% April 30, 80% April 27, 90% April 22. Observed last spring frost in recent years: 2021 May 7, 2022 May 22, 2023 May 3, 2024 April 22, 2025 April 21, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%May 1May 17May 5Apr 222021: May 72022: May 222023: May 32024: April 222025: April 212026: May 7Fall at Sibley: chance of a frost before this dateFall, chance of a frost before the date: 10% September 21, 20% September 25, 30% September 28, 40% October 1, 50% October 3, 60% October 6, 70% October 9, 80% October 12, 90% October 17. Observed first fall frost in recent years: 2021 October 16, 2022 September 28, 2023 October 7, 2024 October 14, 2025 October 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 21Oct 3Oct 172021: October 162022: September 282023: October 72024: October 142025: October 22
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 7October 16
2022May 22September 28
2023May 3October 7
2024April 22October 14
2025April 21October 22
2026May 7season not over yet

What these dates mean

The 152 days between the 50% dates in Sibley, Iowa, outline the period during which the chance of a light frost is at most 50 %. The spring 50% date is May 5 and the fall 50% date is October 3; these median dates are not frost thresholds but mark the point of even odds.

The cautious spring date for setting out tender crops is May 17, when only a 10 % chance of a later frost remains, while planting on April 22 carries a 90 % chance and is a gamble. In the fall, the cautious date to finish harvesting tender crops is September 21, with a 10 % chance of an earlier frost, and waiting until October 17 involves a 90 % chance and is a gamble.

The 50% hard-frost dates are April 25 and October 13; a hard frost at 28 °F can injure plants that would survive a light frost at 32 °F, making it the more critical temperature threshold. The record spans 26 years.

How to read the probabilities

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

  • Spring: "10% after May 17" 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 21" 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 Sibley, IANumbered nearest stations: 1 — Sanborn, IA — 16 mi; 2 — Sheldon, IA — 16 mi; 3 — Worthington, MN — 19 mi; 4 — Rock Rapids, IA — 21 mi; 5 — Lake Park, IA — 21 mi.IAMNSD25 mi · 40 km1. Sanborn, IA — 16 mi12. Sheldon, IA — 16 mi23. Worthington, MN — 19 mi34. Rock Rapids, IA — 21 mi45. Lake Park, IA — 21 mi5
  1. Sanborn, IA 16 mi (26 km)
  2. Sheldon, IA 16 mi (26 km)
  3. Worthington, MN 19 mi (30 km)
  4. Rock Rapids, IA 21 mi (34 km)
  5. Lake Park, IA 21 mi (35 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.4047, -95.7483.