U.S. Frost Dates by Weather Station

NOAA station USC00115768 · Illinois

Frost Dates for Monmouth, IL

NOAA weather station Monmouth (USC00115768), Illinois · elevation 720 ft (220 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 10only 1 spring in 10 still has a frost later
  • Typical last frostApril 2250% chance of a frost after it
  • Typical first fall frostOctober 1550% chance of a frost before it
  • Frost-free season174 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Monmouth, ILSpring: a frost after April 9 in 9 years out of 10, after April 22 in half of the years, after May 10 in 1 year out of 10. Fall: the first frost comes before September 29 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: 174 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 22First frost ~Oct 15Frost season at Monmouth, ILSpring: a frost after April 9 in 9 years out of 10, after April 22 in half of the years, after May 10 in 1 year out of 10. Fall: the first frost comes before September 29 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: 174 days.JFMAMJJASONDLast frost ~Apr 22First 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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 10September 29
50% (typical)April 22October 15
90% (likely)April 9October 27
Frost-free season (32°F, 50% chance)
174 days
Hard frost (28°F), last in spring, 50% chance after
April 11
Hard frost (28°F), first in fall, 50% chance before
October 25

The last 15 years (2006–2020 normals)

Typical last frost April 22 (same as the 30-year normal), typical first fall frost October 19 (4 days later). Frost-free season 179 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 Monmouth: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 10, 20% May 3, 30% April 30, 40% April 26, 50% April 22, 60% April 19, 70% April 17, 80% April 13, 90% April 9. Observed last spring frost in recent years: 2021 April 22, 2022 April 27, 2023 April 24, 2024 April 21, 2025 April 16, 2026 April 20.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 10Apr 22Apr 92021: April 222022: April 272023: April 242024: April 212025: April 162026: April 20Fall at Monmouth: chance of a frost before this dateFall, chance of a frost before the date: 10% September 29, 20% October 5, 30% October 9, 40% October 12, 50% October 15, 60% October 18, 70% October 20, 80% October 24, 90% October 27. Observed first fall frost in recent years: 2021 November 1, 2022 October 14, 2023 October 14, 2024 October 7.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 29Oct 15Oct 272021: November 12022: October 142023: October 142024: October 7
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
2021April 22November 1
2022April 27October 14
2023April 24October 14
2024April 21October 7
2025April 16incomplete record
2026April 20season not over yet

What these dates mean

The cautious fall date in Monmouth for finishing the harvest is September 29, when there is only a 10 % chance that a light frost has arrived.

For a gardener, the 50 % light-frost dates—April 22 in spring and October 15 in fall—mark the point where the probability of a light frost is equal. The median hard-frost dates are April 11 and October 25; a hard frost (28 °F) can damage vegetation more severely than a light frost (32 °F).

A cautious planting schedule therefore uses the May 10 light-frost date (10 % chance of frost later) for setting out tender crops in spring, and relies on the September 29 date (10 % chance of frost earlier) to complete tender-crop harvest 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 10" 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 Monmouth, ILNumbered nearest stations: 1 — Galesburg, IL — 13 mi; 2 — Aledo, IL — 18 mi; 3 — near Oquawka, IL — 21 mi; 4 — near Burlington, IA — 28 mi; 5 — Burlington (Airport), IA — 28 mi.IA25 mi · 40 km1. Galesburg, IL — 13 mi12. Aledo, IL — 18 mi23. near Oquawka, IL — 21 mi34. near Burlington, IA — 28 mi45. Burlington (Airport), IA — 28 mi5
  1. Galesburg, IL 13 mi (21 km)
  2. Aledo, IL 18 mi (30 km)
  3. near Oquawka, IL 21 mi (33 km)
  4. near Burlington, IA 28 mi (44 km)
  5. Burlington (Airport), IA 28 mi (45 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: 40.9442, -90.6381.