Keystone, SD weather in September
In September, Keystone sees highs around 69°F and lows around 40°F: mild, with some rain. There are about 7 days with rain or snow. Snowfall averages about 0.8 in. Great for outdoor plans.
- Normal high
- 69°F
- Normal low
- 40°F
- Wet days
- 7
- 1.6 in total
- Snowfall
- 0.8 in
- normal snowfall
How September changes from start to finish
| September 1 | September 30 | |
|---|---|---|
| Normal high | 75°F | 63°F |
| Normal low | 44°F | 35°F |
| Chance of a wet day | 25% | 24% |
| Sunrise / sunset | 6:17 am / 7:30 pm | 6:50 am / 6:37 pm |
| Daylight | 13.2 h | 11.8 h |
Chance of a wet day: average for the first and last 7 days of the month. Times are local clock time (Mountain Time).
A typical September day
- 6 am47°Fdew point 36°F
- 9 am60°Fdew point 38°F
- Noon66°Fdew point 36°F
- 3 pm66°Fdew point 36°F
- 6 pm59°Fdew point 37°F
- 9 pm51°Fdew point 37°F
NOAA hourly normals (local standard time) from Custer Co Airport (15 mi away); average wind 6 mph; clear or few clouds in 58% of daytime hours, mostly cloudy or overcast in 31%.
What kind of days to expect
- Comfortable dayshigh 65-80°F and dew point under 60°F15 of 30
- Hot dayshigh 90°F or more1 of 30
- Muggy daysdaily mean dew point 65°F or more0 of 30
- Wet daysat least 0.01 in of rain or melted snow7 of 30
- Notable rainat least 0.10 in4 of 30
- Freezing nightslow 32°F or below6 of 30
- Snow on the groundsnow depth 1 in or more0 of 30
Average days per September, counted from daily records for each year 1991-2020.
Rain and snow in September
Normal precipitation is 1.6 in; a dry September brings about 0.8 in and a wet one about 2.2 in (25th and 75th percentiles). About 1 day brings half an inch or more. Normal snowfall is 0.8 in.
How September varies from year to year
- Warmest September, 1991-20201998: average high 75°F
- Coolest September1993: average high 62°F
- Wettest September1998: 4.4 in
- Driest September2012: 0.06 in
- Highest reading94°F (September 6, 2020)
- Lowest reading18°F (September 13, 2014)
From the station's daily records with NOAA quality flags removed; most recent date shown when a reading repeats.
September compared with August and October
| August | September | October | |
|---|---|---|---|
| Normal high | 78°F | 69°F | 55°F |
| Normal low | 49°F | 40°F | 29°F |
| Wet days | 9 | 7 | 7 |
| Comfortable days | 15 | 15 | 8 |
| Daylight | 13.9 h | 12.5 h | 11.0 h |
Highs change by -9°F from August and -14°F into October.
Nearby alternatives in September
- Warmer: Rapid City, SD16 mi · high 76°F · 7 wet days
How Keystone ranks in September
- Normal high356th warmest of 378 cities; 4th of 4 in South Dakota
- Wet days156th driest of 378
- Comfortable days86th most of 342
What to pack for Keystone in September
- A warm jacket – lows near 40°F, highs near 69°F
- Layers – afternoons near 69°F but nights near 40°F
Frost in September
The first fall frost typically comes around Sep 18; in 1 year in 10 it comes before Sep 4. All frost dates for Keystone.
Heating and cooling
September has 327 heating degree days and 10 cooling degree days (base 65°F), 4% of the year's total heating and cooling demand.
Questions
How cold is Keystone in September?
The normal high is 69°F and the normal low is 40°F. From 1991 to 2020 the coldest September reading was 18°F (most recently on September 13, 2014).
Does it rain a lot in Keystone in September?
About 7 days in September have at least 0.01 in of precipitation, and the normal total is 1.6 in. The wettest September in 1991-2020 was in 1998 (4.4 in); the driest was in 2012 (0.06 in).
Does it snow in Keystone in September?
Yes. Normal snowfall in September is 0.8 in.
Is September a good time to visit Keystone?
September ranks 3rd of 12 months on our comfort score for Keystone (70 out of 100). It has about 15 comfortable days and about 1 day at 90°F or hotter. The best month is June.
Where the numbers come from
- Temperature, rain, snowfall, snow depth and frost dates: Pactola Dam (USC00396427), 12 mi NNW of the city center, 4,720 ft (385 ft higher than the city). NOAA normals and daily records, 1991-2020.
- Humidity and wind by hour: Custer Co Airport (USW00094032), 15 mi SW of the city center, 5,545 ft (1,210 ft higher than the city). NOAA normals and daily records, 1991-2020.
- Cloud cover: Ellsworth AFB (USW00024006), 24 mi NE of the city center, 3,215 ft (1,120 ft lower than the city). NOAA normals and daily records, 1991-2020.
- Daily humidity and wind: Rapid City Regional Airport (726620-24090), 21 mi ENE of the city center, 3,162 ft (1,173 ft lower than the city). Hourly observations, 1991-2020.