U.S. Frost Dates by Weather Station

NOAA station USC00477226 · Wisconsin

Frost Dates for River Falls, WI

NOAA weather station River Falls (USC00477226), Wisconsin · elevation 940 ft (287 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 26only 1 spring in 10 still has a frost later
  • Typical last frostMay 1150% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season143 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at River Falls, WISpring: a frost after April 26 in 9 years out of 10, after May 11 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 19 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 143 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 11First frost ~Oct 1Frost season at River Falls, WISpring: a frost after April 26 in 9 years out of 10, after May 11 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 19 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 143 days.JFMAMJJASONDLast frost ~May 11First frost ~Oct 1
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.Today

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 26September 19
50% (typical)May 11October 1
90% (likely)April 26October 15
Frost-free season (32°F, 50% chance)
143 days
Hard frost (28°F), last in spring, 50% chance after
May 1
Hard frost (28°F), first in fall, 50% chance before
October 11

The last 15 years (2006–2020 normals)

Typical last frost May 9 (2 days earlier than the 30-year normal), typical first fall frost October 5 (4 days later). Frost-free season 148 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 River Falls: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 26, 20% May 21, 30% May 17, 40% May 14, 50% May 11, 60% May 8, 70% May 4, 80% May 1, 90% April 26. Observed last spring frost in recent years: 2021 May 13, 2022 May 5, 2023 May 4, 2024 April 25, 2025 May 1, 2026 May 10.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 26May 11Apr 262021: May 132022: May 52023: May 42024: April 252025: May 12026: May 10Fall at River Falls: chance of a frost before this dateFall, chance of a frost before the date: 10% September 19, 20% September 23, 30% September 26, 40% September 29, 50% October 1, 60% October 4, 70% October 7, 80% October 10, 90% October 15. Observed first fall frost in recent years: 2021 October 17, 2022 September 28, 2023 October 12, 2025 October 8.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 19Oct 1Oct 152021: October 172022: September 282023: October 122025: October 8
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 13October 17
2022May 5September 28
2023May 4October 12
2024April 25incomplete record
2025May 1October 8
2026May 10season not over yet

What these dates mean

Setting out tender crops on April 26 in River Falls carries a 90% chance of a light frost later than that day, making it a gamble for any delicate seedlings. The cautious spring plan waits until May 26, when only a 10% chance of a light frost remains.

The 50% light-frost date of May 11 marks the point where a light frost is equally likely before or after, while the 50% date of October 1 plays the same role in fall. The 50% hard-frost dates are May 1 in spring and October 11 in fall, each indicating an even chance of a 28 °F freeze. For a low-risk autumn harvest, the cautious fall date is September 19, with only a 10% chance that a light frost has already arrived.

A hard frost, recorded at 28 °F, is more damaging than a light 32 °F frost because it can injure plant tissue that survives milder freezes.

How to read the probabilities

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

  • Spring: "10% after May 26" 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 19" 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 River Falls, WINumbered nearest stations: 1 — Ellsworth, WI — 11 mi; 2 — near Hastings, MN — 14 mi; 3 — near Red Wing, MN — 17 mi; 4 — Rosemount, MN — 26 mi; 5 — near Roseville, MN — 29 mi.MNWI25 mi · 40 km1. Ellsworth, WI — 11 mi12. near Hastings, MN — 14 mi23. near Red Wing, MN — 17 mi34. Rosemount, MN — 26 mi45. near Roseville, MN — 29 mi5
  1. Ellsworth, WI 11 mi (18 km)
  2. near Hastings, MN 14 mi (23 km)
  3. near Red Wing, MN 17 mi (27 km)
  4. Rosemount, MN 26 mi (41 km)
  5. near Roseville, MN 29 mi (47 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: 44.8544, -92.6122.