U.S. Frost Dates by Weather Station

NOAA station USC00476200 · Wisconsin

Frost Dates for Oconomowoc, WI

NOAA weather station Oconomowoc WWTP (USC00476200), Wisconsin · elevation 858 ft (262 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 17only 1 spring in 10 still has a frost later
  • Typical last frostApril 3050% chance of a frost after it
  • Typical first fall frostOctober 1250% chance of a frost before it
  • Frost-free season164 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Oconomowoc, WISpring: a frost after April 16 in 9 years out of 10, after April 30 in half of the years, after May 17 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 12 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 164 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 30First frost ~Oct 12Frost season at Oconomowoc, WISpring: a frost after April 16 in 9 years out of 10, after April 30 in half of the years, after May 17 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 12 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 164 days.JFMAMJJASONDLast frost ~Apr 30First frost ~Oct 12
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 17September 29
50% (typical)April 30October 12
90% (likely)April 16October 27
Frost-free season (32°F, 50% chance)
164 days
Hard frost (28°F), last in spring, 50% chance after
April 19
Hard frost (28°F), first in fall, 50% chance before
October 24

The last 15 years (2006–2020 normals)

Typical last frost April 30 (same as the 30-year normal), typical first fall frost October 15 (3 days later). Frost-free season 169 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 Oconomowoc: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 17, 20% May 10, 30% May 6, 40% May 3, 50% April 30, 60% April 27, 70% April 24, 80% April 21, 90% April 16. Observed last spring frost in recent years: 2021 May 12, 2022 April 28, 2024 April 25, 2025 April 16, 2026 May 8.Spring: last frostChance of a frost after this date10%50%90%May 1May 17Apr 30Apr 162021: May 122022: April 282024: April 252025: April 162026: May 8Fall at Oconomowoc: chance of a frost before this dateFall, chance of a frost before the date: 10% September 29, 20% October 2, 30% October 6, 40% October 9, 50% October 12, 60% October 14, 70% October 18, 80% October 22, 90% October 27. Observed first fall frost in recent years: 2021 October 22, 2022 October 4, 2024 October 16, 2025 October 24.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 29Oct 12Oct 272021: October 222022: October 42024: October 162025: October 24
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 28October 4
2023incomplete recordincomplete record
2024April 25October 16
2025April 16October 24
2026May 8season not over yet

What these dates mean

The frost-free window in Oconomowoc spans 164 days between the 50% light-frost dates of April 30 and October 12, during which most tender crops can be grown if the risk of frost is managed. For a gardener, the April 30 date marks the point at which there is an even chance of a light frost later in the season; likewise, October 12 is the date with an even chance of a light frost earlier. The cautious spring date is May 17, when only a 10 % chance of a later frost remains, while planting on April 16 carries a 90 % chance of frost and is a gamble. In fall, the cautious date is September 29, with only a 10 % chance that a frost has already arrived, whereas counting on crops through October 27 involves a 90 % chance of frost and is risky. A hard frost, defined by the 28 °F 50% dates of April 19 and October 24, can damage plant tissue that survives a light frost, making it a more serious threat.

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 29" 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 Oconomowoc, WINumbered nearest stations: 1 — Watertown, WI — 13 mi; 2 — Hartford, WI — 17 mi; 3 — Whitewater, WI — 21 mi; 4 — Germantown, WI — 21 mi; 5 — Fort Atkinson, WI — 22 mi.25 mi · 40 km1. Watertown, WI — 13 mi12. Hartford, WI — 17 mi23. Whitewater, WI — 21 mi34. Germantown, WI — 21 mi45. Fort Atkinson, WI — 22 mi5
  1. Watertown, WI 13 mi (21 km)
  2. Hartford, WI 17 mi (27 km)
  3. Whitewater, WI 21 mi (33 km)
  4. Germantown, WI 21 mi (35 km)
  5. Fort Atkinson, WI 22 mi (36 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.1003, -88.5036.