St. Mary, MT climate
St. Mary has cold winters and cool summers. A typical July afternoon reaches 75°, a January night drops to 13°.
NOAA 1991-2020 climate normals Updated once a decadeWhat a year in St. Mary feels like
Winter
Spring
Summer
Fall
St. Mary weather by month
Average highs, lows, rain and snow for every month, from NOAA’s 1991–2020 normals.
| Month | High | Low | Rain + melt | 30-yr range |
|---|---|---|---|---|
| January | 31° | 13° | 1.9" | 11° – 30° |
| February | 33° | 13° | 1.8" | 1° – 33° |
| March | 40° | 19° | 2.1" | 16° – 37° |
| April | 48° | 27° | 2.1" | 32° – 44° |
| May | 58° | 34° | 2.7" | 41° – 51° |
| June | 65° | 41° | 3.7" | 50° – 59° |
| July | 75° | 45° | 1.4" | 51° – 68° |
| August | 75° | 43° | 1.4" | 54° – 63° |
| September | 65° | 37° | 1.8" | 47° – 56° |
| October | 50° | 30° | 2.1" | 34° – 47° |
| November | 38° | 22° | 2.2" | 20° – 39° |
| December | 31° | 15° | 1.9" | 12° – 30° |
| Year | 51° | 28° | 25.2" |
“Wet days” have at least 0.01" of rain or melted snow. “Snow on ground” counts days with 1" or more. The 30-year range is the coldest and warmest that month’s average temperature got between 1991 and 2020.
Rain and snow in St. Mary
St. Mary gets 25 inches of rain and melted snow a year. June is the wettest month and July the driest.
About 82 frost-free days a year
In half of years the last 32° night comes by June 12 and the first one by September 5. That leaves about 82 frost-free days for gardens, shorter than 100% of U.S. towns.
Heating and cooling a home in St. Mary
Engineers use “degree days” to estimate how hard a furnace or air conditioner works. In St. Mary, 99% of that work is heating and 1% is cooling, so the furnace matters far more than the air conditioner.
Degree days measure how far each day’s average temperature falls below or rises above 65°, added up over a year. Source: NOAA station normals.
How St. Mary compares
Compared with every U.S. town and city on TownTapestry, each counted once.
Warmer than it used to be
The last 30 years in St. Mary averaged 1.9° warmer than the 1900s. The biggest change is in January (+5.1°). The shift is larger than in 92% of U.S. towns.
Change by month1991–2020 average minus 1901–2000 average, at St. Mary’s spot on NOAA’s grid
How much a month can swing
A normal is an average, and real years wander around it. February swings the most: between 1991 and 2020 its average temperature ranged from 1° to 33°. September is the most reliable, staying within 8°.
Common questions about St. Mary’s climate
What is the coldest month in St. Mary?
January, with a typical high of 31° and a low of 13°.
What is the hottest month in St. Mary?
July, with a typical high of 75° and a low of 45°. About 7 days a year reach 90°.
When is the first frost in St. Mary?
In half of years the first 32° night comes by September 5, and the last spring frost by June 12.
What is the rainiest month in St. Mary?
June, with 3.7 inches of rain and melted snow. The year totals 25 inches.
Temperatures and rainfall come from NOAA’s gridded climate normals for St. Mary itself (about a 3-mile square). Snow, frost dates and day counts come from NOAA’s weather station at St Mary, MT (1 mi, frost, heat and degree days). Normals are 30-year averages, not a forecast. Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000.