U.S. Frost Dates by Weather Station

NOAA station USC00476718 · Wisconsin

Frost Dates for Portage, WI

NOAA weather station Portage WWTP (USC00476718), Wisconsin · elevation 785 ft (239 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 1050% chance of a frost before it
  • Frost-free season164 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Portage, WISpring: 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 26 in 1 year out of 10, before October 10 in half of the years, before October 23 in 9 years out of 10. Typical frost-free season: 164 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 28First frost ~Oct 10Frost season at Portage, WISpring: 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 26 in 1 year out of 10, before October 10 in half of the years, before October 23 in 9 years out of 10. Typical frost-free season: 164 days.JFMAMJJASONDLast frost ~Apr 28First frost ~Oct 10
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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 12September 26
50% (typical)April 28October 10
90% (likely)April 14October 23
Frost-free season (32°F, 50% chance)
164 days
Hard frost (28°F), last in spring, 50% chance after
April 16
Hard frost (28°F), first in fall, 50% chance before
October 22

The last 15 years (2006–2020 normals)

Typical last frost April 27 (1 day earlier than the 30-year normal), typical first fall frost October 13 (3 days later). Frost-free season 168 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 Portage: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 12, 20% May 7, 30% May 3, 40% April 30, 50% April 28, 60% April 25, 70% April 22, 80% April 19, 90% April 14. Observed last spring frost in recent years: 2021 May 8, 2022 April 28, 2023 April 26, 2024 April 26, 2025 April 20, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 12Apr 28Apr 142021: May 82022: April 282023: April 262024: April 262025: April 202026: May 7Fall at Portage: chance of a frost before this dateFall, chance of a frost before the date: 10% September 26, 20% October 1, 30% October 3, 40% October 7, 50% October 10, 60% October 12, 70% October 14, 80% October 18, 90% October 23. Observed first fall frost in recent years: 2021 October 22, 2022 October 8, 2023 October 17, 2024 October 16, 2025 October 24.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 26Oct 10Oct 232021: October 222022: October 82023: October 172024: 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 8October 22
2022April 28October 8
2023April 26October 17
2024April 26October 16
2025April 20October 24
2026May 7season not over yet

What these dates mean

May 12 is the cautious spring date in Portage, offering only a 10% chance of a light frost later and allowing a gardener to set out tender crops with low risk. The median 32°F date of April 28 marks the point where a light frost is as likely as not, so a gardener should expect about a 50% chance of frost on either side of that day. The 50% hard-frost date of April 16 signals when a 28°F freeze is equally probable, and a hard frost can damage plants more severely than a light frost.

September 26 serves as the cautious fall date, with only a 10% chance that a light frost has already arrived, so harvesting tender crops can be scheduled up to that day with confidence. October 10 is the median 32°F date, indicating even odds for a light frost, while the 50% hard-frost date of October 22 shows when a hard freeze is as likely as not. A hard frost at 28°F can cause more severe plant injury than a light frost at 32°F.

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 26" 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 Portage, WINumbered nearest stations: 1 — Arlington, WI — 16 mi; 2 — Wisconsin Dells, WI — 17 mi; 3 — Montello, WI — 18 mi; 4 — Madison (Madison-Dane Airport), WI — 27 mi; 5 — Beaver Dam, WI — 30 mi.25 mi · 40 km1. Arlington, WI — 16 mi12. Wisconsin Dells, WI — 17 mi23. Montello, WI — 18 mi34. Madison (Madison-Dane Airport), WI — 27 mi45. Beaver Dam, WI — 30 mi5
  1. Arlington, WI 16 mi (26 km)
  2. Wisconsin Dells, WI 17 mi (28 km)
  3. Montello, WI 18 mi (29 km)
  4. Madison (Madison-Dane Airport), WI 27 mi (44 km)
  5. Beaver Dam, WI 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: 43.5306, -89.4369.