May is one of the warmer months of the year in Helena, with average highs near 15.2°C and lows around 1.5°C (mean 8.4°C). It is a relatively wet time of year, averaging 74 mm of rainfall and 66% relative humidity across the period.
May is one of the warmer months of the year in Helena. Across 45 years of records, the warmest May in Helena was 2023 — when the month averaged 11.9°C — while the coldest was 2010 at 5.2°C. Typical May days reach 15.2°C with nights near 1.5°C. The long-term May temperature trend is broadly flat.
May is a relatively wet time of year in Helena, averaging 74 mm of rainfall. The wettest May on record was 1981 with 158 mm, while the driest was 2001 at 23 mm.
Relative humidity in Helena during May averages 66%. The most humid May was 2018 at 74%, and the least humid was 2001 at 53%.
May receives about 5.7 kWh/m²/day of solar energy in Helena. The sunniest May was 2001 at 6.7 kWh/m²/day, and the cloudiest was 2010 at 5.
Average wind speed in Helena during May is 1.4 m/s. The windiest Mays were 1984, 2001 and 2024 at 1.6 m/s, while the calmest were 2014 and 2023 at 1.1 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 8.3°C | 8.5°C | 8.4°C | 8.2°C | 8.6°C | +0.3°C |
| Avg High | 15.4°C | 15.4°C | 15.4°C | 14.6°C | 15.2°C | -0.2°C |
| Avg Low | 1.2°C | 1.5°C | 1.4°C | 1.7°C | 1.9°C | +0.7°C |
| Avg Rain | 74 mm | 73 mm | 64 mm | 80 mm | 78 mm | +4 mm |
May temperatures in Helena have risen by about 1.1°C between 1981–1985 and 2021–2025.
May has ranged from a record low of -9.7°C (1983) to a record high of 30.9°C (2003).
Rainfall in May is a relatively wet time of year, averaging 74 mm, with a wettest year of 158 mm.
Daytime highs and overnight lows have shifted differently — highs fell about 0.1°C per decade while lows rose 0.2°C per decade, narrowing the gap between day and night.
A good time to visit Helena
May is a good time to visit Helena, with days averaging 15.2°C and nights around 1.5°C (mean 8.4°C, warmer than Helena's yearly average) and moderate rainfall. July and August are marginally more temperate, but May still suits most travellers.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 17.7°C | 10.5°C | 3.3°C | 29.8°C | -4.4°C | 97 mm | 65% | 1.3 m/s | 6 |
| 2024 | 13.2°C | 7.0°C | 0.8°C | 25.1°C | -5.0°C | 72 mm | 67% | 1.6 m/s | 5.5 |
| 2023 🔥 | 18.5°C | 11.9°C | 5.2°C | 26.0°C | -0.4°C | 72 mm | 69% | 1.1 m/s | 5.6 |
| 2022 | 13.0°C | 6.7°C | 0.3°C | 23.4°C | -5.2°C | 59 mm | 66% | 1.4 m/s | 5.2 |
| 2021 | 13.5°C | 6.9°C | 0.3°C | 25.0°C | -7.9°C | 93 mm | 66% | 1.3 m/s | 5.5 |
| 2020 | 15.4°C | 8.4°C | 1.4°C | 26.1°C | -4.8°C | 74 mm | 69% | 1.4 m/s | 5.9 |
| 2019 | 13.8°C | 7.5°C | 1.2°C | 21.7°C | -6.8°C | 77 mm | 72% | 1.2 m/s | 5.1 |
| 2018 | 17.2°C | 10.9°C | 4.5°C | 23.3°C | -1.2°C | 108 mm | 74% | 1.2 m/s | 5.5 |
| 2017 | 16.5°C | 9.4°C | 2.3°C | 26.2°C | -3.2°C | 64 mm | 65% | 1.3 m/s | 6 |
| 2016 | 14.9°C | 8.3°C | 1.7°C | 23.7°C | -4.4°C | 83 mm | 68% | 1.4 m/s | 5.2 |
| 2015 | 15.1°C | 8.6°C | 2.0°C | 21.7°C | -2.3°C | 74 mm | 69% | 1.3 m/s | 5.5 |
| 2014 | 15.7°C | 9.0°C | 2.3°C | 25.3°C | -5.0°C | 44 mm | 68% | 1.1 m/s | 6.1 |
| 2013 | 15.9°C | 9.3°C | 2.7°C | 26.0°C | -5.1°C | 69 mm | 66% | 1.4 m/s | 5.7 |
| 2012 | 14.0°C | 7.4°C | 0.8°C | 26.2°C | -4.5°C | 73 mm | 64% | 1.4 m/s | 5.8 |
| 2011 | 11.7°C | 5.9°C | 0.1°C | 20.2°C | -4.8°C | 96 mm | 72% | 1.5 m/s | 5.3 |
| 2010 ❄️ | 11.1°C | 5.2°C | -0.7°C | 23.0°C | -6.6°C | 115 mm | 70% | 1.5 m/s | 5 |
| 2009 | 15.6°C | 8.4°C | 1.3°C | 26.6°C | -5.6°C | 44 mm | 65% | 1.5 m/s | 6.4 |
| 2008 | 13.8°C | 7.4°C | 0.9°C | 25.3°C | -5.4°C | 95 mm | 70% | 1.4 m/s | 5.3 |
| 2007 | 16.5°C | 9.2°C | 1.9°C | 26.3°C | -6.4°C | 98 mm | 67% | 1.3 m/s | 6.1 |
| 2006 | 16.7°C | 9.7°C | 2.7°C | 27.5°C | -4.6°C | 48 mm | 62% | 1.3 m/s | 6.3 |
| 2005 | 14.9°C | 7.9°C | 0.9°C | 21.9°C | -7.1°C | 76 mm | 68% | 1.4 m/s | 5.6 |
| 2004 | 13.7°C | 7.5°C | 1.3°C | 22.9°C | -6.0°C | 87 mm | 66% | 1.4 m/s | 5.2 |
| 2003 | 14.7°C | 8.1°C | 1.4°C | 30.9°C | -5.5°C | 54 mm | 68% | 1.3 m/s | 5.6 |
| 2002 | 14.7°C | 7.4°C | 0.0°C | 26.6°C | -6.8°C | 57 mm | 61% | 1.4 m/s | 5.7 |
| 2001 | 17.7°C | 9.8°C | 1.9°C | 28.6°C | -5.1°C | 23 mm | 53% | 1.6 m/s | 6.7 |
| 2000 | 15.2°C | 8.5°C | 1.8°C | 24.5°C | -2.7°C | 59 mm | 63% | 1.4 m/s | 5.5 |
| 1999 | 13.8°C | 7.1°C | 0.4°C | 25.2°C | -5.9°C | 74 mm | 66% | 1.3 m/s | 5.9 |
| 1998 | 16.9°C | 9.4°C | 1.9°C | 23.1°C | -1.7°C | 71 mm | 63% | 1.4 m/s | 5.9 |
| 1997 | 16.0°C | 9.1°C | 2.2°C | 23.6°C | -3.6°C | 83 mm | 69% | 1.3 m/s | 6.2 |
| 1996 | 11.7°C | 5.9°C | 0.1°C | 16.6°C | -4.4°C | 100 mm | 74% | 1.3 m/s | 5.3 |
| 1995 | 13.6°C | 7.0°C | 0.4°C | 22.8°C | -6.0°C | 72 mm | 68% | 1.2 m/s | 6 |
| 1994 | 17.7°C | 10.5°C | 3.3°C | 26.0°C | -1.1°C | 56 mm | 63% | 1.3 m/s | 5.5 |
| 1993 | 18.3°C | 10.7°C | 3.1°C | 26.1°C | -4.6°C | 72 mm | 65% | 1.3 m/s | 5.9 |
| 1992 | 18.9°C | 10.8°C | 2.6°C | 27.5°C | -3.0°C | 40 mm | 53% | 1.4 m/s | 6.4 |
| 1991 | 13.9°C | 7.4°C | 0.9°C | 22.7°C | -4.6°C | 83 mm | 69% | 1.3 m/s | 5.2 |
| 1990 | 13.4°C | 6.8°C | 0.1°C | 23.3°C | -5.3°C | 81 mm | 68% | 1.3 m/s | 5.2 |
| 1989 | 14.5°C | 7.6°C | 0.6°C | 22.9°C | -4.1°C | 64 mm | 66% | 1.5 m/s | 5.3 |
| 1988 | 17.4°C | 9.6°C | 1.8°C | 28.3°C | -5.5°C | 70 mm | 60% | 1.4 m/s | 5.3 |
| 1987 | 18.4°C | 11.1°C | 3.8°C | 28.2°C | -3.8°C | 89 mm | 62% | 1.3 m/s | 5.4 |
| 1986 | 16.0°C | 8.7°C | 1.3°C | 29.5°C | -5.4°C | 49 mm | 63% | 1.4 m/s | 5.8 |
| 1985 | 18.3°C | 10.5°C | 2.8°C | 27.2°C | -5.9°C | 48 mm | 58% | 1.3 m/s | 6 |
| 1984 | 13.6°C | 6.7°C | -0.2°C | 28.0°C | -4.1°C | 51 mm | 64% | 1.6 m/s | 5.9 |
| 1983 | 13.3°C | 6.4°C | -0.4°C | 25.9°C | -9.7°C | 56 mm | 67% | 1.4 m/s | — |
| 1982 | 12.9°C | 6.2°C | -0.5°C | 24.5°C | -6.4°C | 80 mm | 68% | 1.4 m/s | — |
| 1981 🌧️ | 13.9°C | 7.7°C | 1.4°C | 21.9°C | -4.5°C | 158 mm | 73% | 1.4 m/s | — |
Actual recorded May data for each year, 1981–2025 — sourced from NASA POWER.
The average high in May is around 15.2°C with lows near 1.5°C (mean 8.4°C), based on 45 years of NASA POWER data.
May averages 74 mm of rainfall in Helena, a relatively wet time of year. The wettest on record was 158 mm in 1981.
The hottest May reached 30.9°C in 2003. The coldest dipped to -9.7°C in 1983.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average May temperatures have risen by about 1.1°C. Daytime highs moved from 14.4°C to 15.2°C and overnight lows from 0.6°C to 2°C over the same span.
May is one of the warmer months of the year in Helena and a relatively wet time of year, so plan for around 15.2°C daytime highs and 1.5°C at night.
Not quite — May is the 8th coolest month in Helena, averaging 8.4°C.
For May in Helena, pack warm layers and a jacket for chilly mornings, with typical highs of 15.2°C and lows of 1.5°C. Bring an umbrella or light rain jacket too.
Relative humidity in May averages 66% — moderately humid.
Average wind speed in May is 1.4 m/s. The windiest May on record averaged 1.6 m/s (1984).
The wettest May on record was 1981 with 158 mm. On average, May sees 74 mm.
All figures are derived from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis, covering 1981–2025.
Explore how the weather in Helena behaves each May, based on 45 years of NASA POWER climate data from 1981 to 2025. This page breaks down average temperatures, rainfall, humidity, wind and solar radiation for May specifically, along with year-by-year records and how the month has changed over the decades.
