U.S. Frost Dates by Weather Station

NOAA station USC00450844 · Washington

Frost Dates for Boundary Dam, WA

NOAA weather station Boundary Dam (USC00450844), Washington · elevation 1837 ft (560 m) · hardiness zone 6a*

  • Lower-risk planting dateafter May 18only 1 spring in 10 still has a frost later
  • Typical last frostMay 450% chance of a frost after it
  • Typical first fall frostOctober 450% chance of a frost before it
  • Frost-free season153 daystypical year, 32°F
  • Hardiness zone6acalculated*

The year at a glance

Frost season at Boundary Dam, WASpring: a frost after April 20 in 9 years out of 10, after May 4 in half of the years, after May 18 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 4 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 153 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 4First frost ~Oct 4Frost season at Boundary Dam, WASpring: a frost after April 20 in 9 years out of 10, after May 4 in half of the years, after May 18 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 4 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 153 days.JFMAMJJASONDLast frost ~May 4First frost ~Oct 4
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 18September 21
50% (typical)May 4October 4
90% (likely)April 20October 20
Frost-free season (32°F, 50% chance)
153 days
Hard frost (28°F), last in spring, 50% chance after
April 17
Hard frost (28°F), first in fall, 50% chance before
October 19

The last 15 years (2006–2020 normals)

Typical last frost May 4 (same as the 30-year normal), typical first fall frost October 5 (1 day later). Frost-free season 153 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 Boundary Dam: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 18, 20% May 13, 30% May 9, 40% May 6, 50% May 4, 60% May 1, 70% April 30, 80% April 26, 90% April 20. Observed last spring frost in recent years: 2021 May 20, 2022 May 14, 2023 April 23, 2024 May 3, 2025 May 24, 2026 April 28.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 18May 4Apr 202021: May 202022: May 142023: April 232024: May 32025: May 242026: April 28Fall at Boundary Dam: chance of a frost before this dateFall, chance of a frost before the date: 10% September 21, 20% September 26, 30% September 29, 40% October 1, 50% October 4, 60% October 7, 70% October 10, 80% October 14, 90% October 20. Observed first fall frost in recent years: 2021 October 10, 2022 October 18, 2023 October 26, 2024 September 21, 2025 September 29.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 21Oct 4Oct 202021: October 102022: October 182023: October 262024: September 212025: September 29
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 20October 10
2022May 14October 18
2023April 23October 26
2024May 3September 21
2025May 24September 29
2026April 28season not over yet

What these dates mean

May 18 is the cautious spring light-frost date, with only a 10 % chance of a frost later, so gardeners can plan to set out tender crops on that day with minimal risk.

The 50 % date for light frost in spring is May 4, and the 50 % date for light frost in fall is October 4; at each of these dates there is an even chance of a frost occurring later (spring) or earlier (fall), marking the midpoint of the frost-free window. The 50 % dates for hard frost are April 17 and October 19, indicating when a 28 °F frost is equally likely to have happened or not.

September 21 is the cautious fall light-frost date, with only a 10 % chance that a frost has arrived earlier, guiding the latest time to finish harvesting tender crops. A hard frost of 28 °F can damage plants that survive a 32 °F light frost, so its timing is critical for winter protection.

How to read the probabilities

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

  • Spring: "10% after May 18" 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 21" 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 Boundary Dam, WANumbered nearest stations: 1 — Northport, WA — 23 mi; 2 — Priest River, ID — 50 mi; 3 — Bonners Ferry, ID — 52 mi; 4 — Chewelah, WA — 53 mi; 5 — Republic, WA — 67 mi.WAID25 mi · 40 km1. Northport, WA — 23 mi12. Priest River, ID — 50 mi23. Bonners Ferry, ID — 52 mi34. Chewelah, WA — 53 mi45. Republic, WA — 67 mi5
  1. Northport, WA 23 mi (36 km)
  2. Priest River, ID 50 mi (81 km)
  3. Bonners Ferry, ID 52 mi (83 km)
  4. Chewelah, WA 53 mi (85 km)
  5. Republic, WA 67 mi (108 km)

Sources

* Hardiness zone 6a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.9947, -117.3544.