U.S. Frost Dates by Weather Station

NOAA station USC00475364 · Wisconsin

Frost Dates for Merrill, WI

NOAA weather station Merrill (USC00475364), Wisconsin · elevation 1250 ft (381 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 27only 1 spring in 10 still has a frost later
  • Typical last frostMay 1150% chance of a frost after it
  • Typical first fall frostSeptember 3050% chance of a frost before it
  • Frost-free season139 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Merrill, WISpring: a frost after April 29 in 9 years out of 10, after May 11 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before September 30 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 139 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 11First frost ~Sep 30Frost season at Merrill, WISpring: a frost after April 29 in 9 years out of 10, after May 11 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before September 30 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 139 days.JFMAMJJASONDLast frost ~May 11First frost ~Sep 30
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 27September 17
50% (typical)May 11September 30
90% (likely)April 29October 12
Frost-free season (32°F, 50% chance)
139 days
Hard frost (28°F), last in spring, 50% chance after
May 2
Hard frost (28°F), first in fall, 50% chance before
October 10

The last 15 years (2006–2020 normals)

Typical last frost May 10 (1 day earlier than the 30-year normal), typical first fall frost October 3 (3 days later). Frost-free season 143 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 Merrill: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 27, 20% May 21, 30% May 18, 40% May 15, 50% May 11, 60% May 9, 70% May 6, 80% May 3, 90% April 29. Observed last spring frost in recent years: 2021 May 29, 2022 May 23, 2023 May 3, 2024 April 25, 2025 May 24, 2026 May 14.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 27May 11Apr 292021: May 292022: May 232023: May 32024: April 252025: May 242026: May 14Fall at Merrill: chance of a frost before this dateFall, chance of a frost before the date: 10% September 17, 20% September 21, 30% September 25, 40% September 27, 50% September 30, 60% October 2, 70% October 5, 80% October 8, 90% October 12. Observed first fall frost in recent years: 2021 October 17, 2022 September 28, 2023 October 16, 2024 October 7, 2025 October 8.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 17Sep 30Oct 122021: October 172022: September 282023: October 162024: October 72025: 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 29October 17
2022May 23September 28
2023May 3October 16
2024April 25October 7
2025May 24October 8
2026May 14season not over yet

What these dates mean

Setting out tender crops on the early spring date, April 29, carries a 90 % chance of a frost later, making it a gamble for Merrill growers. A more cautious plan waits until the May 27 date, which has only a 10 % chance of a later frost. In the fall, the cautious date to finish harvesting tender crops is September 17, when there is only a 10 % chance a frost has already arrived.

The 50% light-frost dates are May 11 in spring and September 30 in fall; each marks a point where the odds of a frost are even, with a 50 % chance of a frost later than May 11 and a 50 % chance of a frost earlier than September 30. Hard-frost 50% dates are May 2 and October 10, indicating the median timing for a 28 °F event. Because a hard frost at 28 °F can damage plant tissue that survives a light frost at 32 °F, it is a more critical threshold for tender crops.

How to read the probabilities

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

  • Spring: "10% after May 27" 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 17" 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 Merrill, WINumbered nearest stations: 1 — near Wausau, WI — 18 mi; 2 — Medford, WI — 33 mi; 3 — Rosholt, WI — 36 mi; 4 — Marshfield, WI — 44 mi; 5 — Stevens Point, WI — 46 mi.25 mi · 40 km1. near Wausau, WI — 18 mi12. Medford, WI — 33 mi23. Rosholt, WI — 36 mi34. Marshfield, WI — 44 mi45. Stevens Point, WI — 46 mi5
  1. near Wausau, WI 18 mi (28 km)
  2. Medford, WI 33 mi (54 km)
  3. Rosholt, WI 36 mi (58 km)
  4. Marshfield, WI 44 mi (70 km)
  5. Stevens Point, WI 46 mi (74 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: 45.1786, -89.6617.