USDA Hardiness Zone 4b
Frost Dates for 59404 — Great Falls, Montana
NOAA 1991–2020 normals from Great Falls International Airport, MT (3,664 ft), 3 miles southwest.
Updated
In Great Falls, the last spring frost (32°F) is around May 19 in half of years and has passed by June 2 in nine years out of ten. The first fall frost comes around September 24, as early as September 9 one year in ten, leaving a typical freeze-free season of 126 days (NOAA, Great Falls International Airport, MT).
- Last spring frost
- May 19
- First fall frost
- September 24
- Growing season
- 126 days
- Hardiness zone
- 4b
50% probability
50% probability
freeze-free, 50%
-25 to -20°F (-31.7 to -28.9°C) coldest night
- First snow
- October 5
- Last snow
- May 11
- Winters with snow
- 100%
- Snow per year
- 66.1"
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Great Falls
Half of springs at Great Falls International Airport, MT see their last 32°F night by May 19; only one in ten is past it by May 7, and one in ten still has one after June 2. Planting tender crops after May 25 leaves a 30% risk of a frost on them, after May 29 20%, and after June 2 10%. The 14 days between the even-odds date and the one-in-ten date is narrower than at 60% of the 68 Montana stations behind this site's ZIP pages.
In fall the first 32°F night comes by September 24 in half of years, as early as September 9 one year in ten, and holds off past October 5 one year in ten. A hard freeze of 28°F follows 9 days later, around October 3; in spring the one-in-ten date for 28°F is May 18, 15 days before the 32°F one. The freeze-free season lasts 126 days in a typical year, shorter than at 50% of the 68 Montana stations; it reaches 143 days one year in ten and falls short of 107 one year in ten. On average 168 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 17 days longer, to about June 5, and return around September 12. Severe cold of 16°F or below comes every winter, usually between November 1 and April 5.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 22 | May 27 | May 30 | Jun 2 | Jun 5 | Jun 8 | Jun 12 | Jun 17 | Jun 25 |
| 32°F | May 7 | May 11 | May 14 | May 16 | May 19 | May 22 | May 25 | May 29 | Jun 2 |
| 28°F | Apr 23 | Apr 29 | May 1 | May 3 | May 5 | May 8 | May 10 | May 13 | May 18 |
| 24°F | Apr 6 | Apr 12 | Apr 17 | Apr 21 | Apr 24 | Apr 27 | Apr 30 | May 3 | May 8 |
| 20°F | Mar 27 | Apr 2 | Apr 5 | Apr 9 | Apr 13 | Apr 17 | Apr 21 | Apr 26 | May 1 |
| 16°F | Mar 16 | Mar 23 | Mar 30 | Apr 2 | Apr 5 | Apr 8 | Apr 13 | Apr 17 | Apr 25 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 27 | Sep 2 | Sep 6 | Sep 10 | Sep 12 | Sep 15 | Sep 18 | Sep 22 | Sep 26 |
| 32°F | Sep 9 | Sep 15 | Sep 19 | Sep 22 | Sep 24 | Sep 27 | Sep 29 | Oct 1 | Oct 5 |
| 28°F | Sep 20 | Sep 26 | Sep 29 | Oct 1 | Oct 3 | Oct 6 | Oct 9 | Oct 13 | Oct 18 |
| 24°F | Sep 30 | Oct 3 | Oct 6 | Oct 10 | Oct 13 | Oct 16 | Oct 20 | Oct 24 | Oct 29 |
| 20°F | Oct 4 | Oct 10 | Oct 14 | Oct 18 | Oct 23 | Oct 26 | Oct 30 | Nov 2 | Nov 8 |
| 16°F | Oct 11 | Oct 17 | Oct 23 | Oct 28 | Nov 1 | Nov 3 | Nov 6 | Nov 11 | Nov 18 |
Freeze probabilities 1991–2020 at Great Falls International Airport, MT, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 59404 →Compare Great Falls with another place →
Weather by month in Great Falls
July is the warmest month at Great Falls International Airport, MT, averaging 84°F by day and 51°F at night; January is the coldest at 35°F and 15°F. A freezing night is all but certain in every month from October to May (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, June still brings one in 13% of years.
Warmth for growing adds up to 1,848 growing degree days a year (base 50°F), more than at 50% of the 68 Montana stations. 24% of them (448) have built up by the end of June, and July adds the most, 553. Precipitation totals 14.7 inches a year, most in June (2.7") and least in December (0.5"); May to September bring 60% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 35 | 15 | 0.6 | 100% | 27 | 1 |
| Feb | 37 | 15 | 0.6 | 100% | 25 | 2 |
| Mar | 46 | 22 | 0.7 | 100% | 25 | 7 |
| Apr | 55 | 30 | 1.7 | 100% | 18 | 25 |
| May | 65 | 38 | 2.4 | 98% | 6 | 124 |
| Jun | 73 | 46 | 2.7 | 13% | <1 | 288 |
| Jul | 84 | 51 | 1.2 | 0% | 0 | 553 |
| Aug | 83 | 50 | 1.2 | 2% | <1 | 522 |
| Sep | 72 | 43 | 1.3 | 75% | 2 | 251 |
| Oct | 57 | 33 | 1.1 | 100% | 15 | 64 |
| Nov | 44 | 24 | 0.7 | 100% | 23 | 11 |
| Dec | 36 | 16 | 0.5 | 100% | 27 | 0 |
Monthly normals 1991–2020 at Great Falls International Airport, MT (3,664 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Great Falls 16st, MT, 4 miles southeast of Great Falls and 159 feet lower, has a last frost 13 days earlier (May 6), a first frost 7 days later (October 1), a season 21 days longer.
When it snows in Great Falls, Montana
The first measurable snow (0.1 inch or more) in Great Falls usually arrives around October 5, and the last of the season around May 11. Those are the middle dates across the winters of 1991–2020 at Great Falls International Airport, MT, 3.3 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 8 days after the last snow.
Planting calendar for 59404
| When | What to do |
|---|---|
| March 10 – March 24 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 24 – April 7 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 21 – May 5 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 19 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 2 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 10 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| September 24 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 59404 is 3 miles from Great Falls International Airport, MT, which stands at 3,664 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59404 in zone 4b, the band where the coldest night of the year averaged -25 to -20°F (-31.7 to -28.9°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.