U.S. Frost Dates by Weather Station

NOAA station USC00470045 · Wisconsin

Frost Dates near Janesville, WI

NOAA weather station Afton - Janesville WWTP (USC00470045), Wisconsin · elevation 762 ft (232 m) · hardiness zone 5b*

Janesville is the nearest town to the station (3.9 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter May 11only 1 spring in 10 still has a frost later
  • Typical last frostApril 2650% chance of a frost after it
  • Typical first fall frostOctober 1550% chance of a frost before it
  • Frost-free season170 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season near Janesville, WISpring: a frost after April 13 in 9 years out of 10, after April 26 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before October 1 in 1 year out of 10, before October 15 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 170 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 26First frost ~Oct 15Frost season near Janesville, WISpring: a frost after April 13 in 9 years out of 10, after April 26 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before October 1 in 1 year out of 10, before October 15 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 170 days.JFMAMJJASONDLast frost ~Apr 26First frost ~Oct 15
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 11October 1
50% (typical)April 26October 15
90% (likely)April 13October 27
Frost-free season (32°F, 50% chance)
170 days
Hard frost (28°F), last in spring, 50% chance after
April 15
Hard frost (28°F), first in fall, 50% chance before
October 26

The last 15 years (2006–2020 normals)

Typical last frost April 27 (1 day later than the 30-year normal), typical first fall frost October 17 (2 days later). Frost-free season 174 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 near Janesville: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 11, 20% May 5, 30% May 1, 40% April 29, 50% April 26, 60% April 23, 70% April 20, 80% April 17, 90% April 13. Observed last spring frost in recent years: 2021 May 11, 2022 April 28, 2023 April 27, 2024 April 25, 2025 April 17, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 11Apr 26Apr 132021: May 112022: April 282023: April 272024: April 252025: April 172026: May 7Fall near Janesville: chance of a frost before this dateFall, chance of a frost before the date: 10% October 1, 20% October 6, 30% October 10, 40% October 12, 50% October 15, 60% October 17, 70% October 20, 80% October 23, 90% October 27. Observed first fall frost in recent years: 2021 October 23, 2022 October 8, 2023 October 11, 2024 October 16, 2025 October 24.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Oct 1Oct 15Oct 272021: October 232022: October 82023: October 112024: 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 11October 23
2022April 28October 8
2023April 27October 11
2024April 25October 16
2025April 17October 24
2026May 7season not over yet

What these dates mean

Setting out tender crops on the early spring date of April 13 carries a 90% chance that a light frost will still occur later, so planting then is a gamble in the Janesville area.

The 50% dates—April 26 for spring and October 15 for fall—represent the point where a light frost is equally likely to have happened or not. Gardeners can use those dates as a neutral reference, while the cautious spring date of May 11 (10% chance of a later frost) guides when to start tender crops, and the cautious fall date of October 1 (10% chance of an earlier frost) tells when to finish harvesting them.

A hard frost at 28 °F is more damaging than a light frost at 32 °F, and the median hard-frost dates are April 15 in spring and October 26 in fall.

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 October 1" 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 Janesville, WINumbered nearest stations: 1 — Beloit, WI — 9 mi; 2 — Brodhead, WI — 16 mi; 3 — Fort Atkinson, WI — 21 mi; 4 — Whitewater, WI — 23 mi; 5 — Rockford, IL — 31 mi.WIIA25 mi · 40 km1. Beloit, WI — 9 mi12. Brodhead, WI — 16 mi23. Fort Atkinson, WI — 21 mi34. Whitewater, WI — 23 mi45. Rockford, IL — 31 mi5
  1. Beloit, WI 9 mi (15 km)
  2. Brodhead, WI 16 mi (26 km)
  3. Fort Atkinson, WI 21 mi (34 km)
  4. Whitewater, WI 23 mi (36 km)
  5. Rockford, IL 31 mi (50 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: 42.6375, -89.0642.