U.S. Frost Dates by Weather Station

NOAA station USC00396947 · South Dakota

Frost Dates near Blackhawk, SD

NOAA weather station Rapid City 4 NW (USC00396947), South Dakota · elevation 3479 ft (1060 m) · hardiness zone 5b*

Blackhawk is the nearest town to the station (2.4 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter May 25only 1 spring in 10 still has a frost later
  • Typical last frostMay 1050% chance of a frost after it
  • Typical first fall frostOctober 550% chance of a frost before it
  • Frost-free season147 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season near Blackhawk, SDSpring: a frost after April 27 in 9 years out of 10, after May 10 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 5 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 147 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 10First frost ~Oct 5Frost season near Blackhawk, SDSpring: a frost after April 27 in 9 years out of 10, after May 10 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 5 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 147 days.JFMAMJJASONDLast frost ~May 10First frost ~Oct 5
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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 25September 20
50% (typical)May 10October 5
90% (likely)April 27October 18
Frost-free season (32°F, 50% chance)
147 days
Hard frost (28°F), last in spring, 50% chance after
April 28
Hard frost (28°F), first in fall, 50% chance before
October 15

The last 15 years (2006–2020 normals)

Typical last frost May 8 (2 days earlier than the 30-year normal), typical first fall frost October 5. Frost-free season 149 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 near Blackhawk: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 25, 20% May 19, 30% May 15, 40% May 12, 50% May 10, 60% May 7, 70% May 4, 80% May 1, 90% April 27. Observed last spring frost in recent years: 2023 May 3, 2024 May 3, 2025 April 25, 2026 May 8.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 25May 10Apr 272023: May 32024: May 32025: April 252026: May 8Fall near Blackhawk: chance of a frost before this dateFall, chance of a frost before the date: 10% September 20, 20% September 26, 30% September 29, 40% October 2, 50% October 5, 60% October 8, 70% October 11, 80% October 15, 90% October 18. Observed first fall frost in recent years: 2021 October 13, 2022 October 7, 2023 October 9, 2024 October 21, 2025 October 13.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 20Oct 5Oct 182021: October 132022: October 72023: October 92024: October 212025: October 13
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
2021incomplete recordOctober 13
2022incomplete recordOctober 7
2023May 3October 9
2024May 3October 21
2025April 25October 13
2026May 8season not over yet

What these dates mean

September 20 is the cautious fall date in the Blackhawk area for finishing the harvest. It marks a 10% chance that a light frost has already arrived, so tender crops are most likely still safe. In spring, the cautious date for setting out tender crops is May 25, when only a 10% chance of a later light frost remains. The 50% date is May 10, indicating an even odds point for a light frost. For fall, the cautious date is September 20, while the 50% date is October 5. The interval between the two 32°F 50% dates, May 10 and October 5, spans 147 days in the 25-year record. Hard-frost median dates are April 28 in spring and October 15 in fall; a hard frost at 28 °F can damage plants more severely 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 25" 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 20" 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 Blackhawk, SDNumbered nearest stations: 1 — near Colonial Pine Hills, SD — 11 mi; 2 — Rapid City (Airport), SD — 13 mi; 3 — Hill City, SD — 19 mi; 4 — Mount Rushmore National Memorial, SD — 19 mi; 5 — near Sturgis, SD — 22 mi.WY25 mi · 40 km1. near Colonial Pine Hills, SD — 11 mi12. Rapid City (Airport), SD — 13 mi23. Hill City, SD — 19 mi34. Mount Rushmore National Memorial, SD — 19 mi45. near Sturgis, SD — 22 mi5
  1. near Colonial Pine Hills, SD 11 mi (17 km)
  2. Rapid City (Airport), SD 13 mi (20 km)
  3. Hill City, SD 19 mi (30 km)
  4. Mount Rushmore National Memorial, SD 19 mi (30 km)
  5. near Sturgis, SD 22 mi (36 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: 44.1206, -103.2842.