U.S. Frost Dates by Weather Station

NOAA station USC00137726 · Iowa

Frost Dates for Sioux Rapids, IA

NOAA weather station Sioux Rapids 4 E (USC00137726), Iowa · elevation 1420 ft (433 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 11only 1 spring in 10 still has a frost later
  • Typical last frostApril 3050% chance of a frost after it
  • Typical first fall frostOctober 550% chance of a frost before it
  • Frost-free season159 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Sioux Rapids, IASpring: a frost after April 14 in 9 years out of 10, after April 30 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 5 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 159 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 30First frost ~Oct 5Frost season at Sioux Rapids, IASpring: a frost after April 14 in 9 years out of 10, after April 30 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 5 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 159 days.JFMAMJJASONDLast frost ~Apr 30First frost ~Oct 5
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 11September 22
50% (typical)April 30October 5
90% (likely)April 14October 21
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 April 29 (1 day earlier than the 30-year normal), typical first fall frost October 11 (6 days later). Frost-free season 163 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 Sioux Rapids: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 11, 20% May 7, 30% May 4, 40% May 2, 50% April 30, 60% April 27, 70% April 23, 80% April 19, 90% April 14. Observed last spring frost in recent years: 2021 May 29, 2022 May 5, 2023 May 4, 2024 April 23, 2025 April 20, 2026 May 12.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1Jun 1May 11Apr 30Apr 142021: May 292022: May 52023: May 42024: April 232025: April 202026: May 12Fall at Sioux Rapids: chance of a frost before this dateFall, chance of a frost before the date: 10% September 22, 20% September 27, 30% September 30, 40% October 2, 50% October 5, 60% October 9, 70% October 12, 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 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 22Oct 5Oct 212021: October 222022: September 282023: October 102024: October 152025: 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 29October 22
2022May 5September 28
2023May 4October 10
2024April 23October 15
2025April 20October 22
2026May 12season not over yet

What these dates mean

The cautious fall date for finishing the harvest in Sioux Rapids is September 22, when there is only a 10 % chance that a light frost has arrived. In spring, the cautious date for setting out tender crops is May 11, also carrying a 10 % chance of a later frost, while planting on April 14 brings a 90 % chance of frost and is a gamble.

For gardeners, the 50 % dates are reference points: April 30 marks the median moment in spring when the odds of a light frost are even, and October 5 does the same in fall. These dates are not frost dates themselves but indicate when the probability of a frost switches from less than to greater than 50 %.

Hard frost, recorded at 28 °F, is more damaging than a light frost at 32 °F; the median hard-frost dates are April 18 in spring and October 18 in fall, underscoring that a drop to 28 °F can harm plants that survive a 32 °F event.

How to read the probabilities

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

  • Spring: "10% after May 11" 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 Sioux Rapids, IANumbered nearest stations: 1 — Storm Lake, IA — 18 mi; 2 — Spencer, IA — 19 mi; 3 — Pocahontas, IA — 23 mi; 4 — Cherokee, IA — 26 mi; 5 — Primghar, IA — 31 mi.MN25 mi · 40 km1. Storm Lake, IA — 18 mi12. Spencer, IA — 19 mi23. Pocahontas, IA — 23 mi34. Cherokee, IA — 26 mi45. Primghar, IA — 31 mi5
  1. Storm Lake, IA 18 mi (29 km)
  2. Spencer, IA 19 mi (31 km)
  3. Pocahontas, IA 23 mi (38 km)
  4. Cherokee, IA 26 mi (41 km)
  5. Primghar, IA 31 mi (51 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: 42.8931, -95.0653.