Average Weather in September in Helena Valley Northwest Montana, United States
Daily high temperatures decrease by 13°F, from 78°F to 65°F, rarely falling below 50°F or exceeding 89°F.
Daily low temperatures decrease by 9°F, from 49°F to 40°F, rarely falling below 30°F or exceeding 57°F.
For reference, on July 27, the hottest day of the year, temperatures in Helena Valley Northwest typically range from 55°F to 85°F, while on January 1, the coldest day of the year, they range from 13°F to 30°F.
Average High and Low Temperature in September
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on September. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature in September
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
Refahiye, Turkey (6,194 miles away); Brrnakot’, Armenia (6,334 miles); and Tagob, Tajikistan (6,532 miles) are the far-away foreign places with temperatures most similar to Helena Valley Northwest (view comparison).
The month of September in Helena Valley Northwest experiences increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 33% to 42%.
The clearest day of the month is September 1, with clear, mostly clear, or partly cloudy conditions 67% of the time.
For reference, on March 18, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 64%, while on July 23, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 77%.
Cloud Cover Categories in September
0% clear 20% mostly clear 40% partly cloudy 60% mostly cloudy 80% overcast 100%
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Helena Valley Northwest, the chance of a wet day over the course of September is essentially constant, remaining around 15% throughout.
For reference, the year's highest daily chance of a wet day is 36% on June 3, and its lowest chance is 8% on January 22.
Over the course of September in Helena Valley Northwest, the chance of a day with only rain decreases from 15% to 13%, the chance of a day with mixed snow and rain remains an essentially constant 1% throughout, and the chance of a day with only snow remains an essentially constant 0% throughout.
Probability of Precipitation in September
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day rainfall during September in Helena Valley Northwest is essentially constant, remaining about 0.9 inches throughout, and rarely exceeding 2.1 inches or falling below 0.2 inches.
The highest average 31-day accumulation is 1.0 inches on September 7.
Average Monthly Rainfall in September
Over the course of September in Helena Valley Northwest, the length of the day is rapidly decreasing. From the start to the end of the month, the length of the day decreases by 1 hour, 35 minutes, implying an average daily decrease of 3 minutes, 17 seconds, and weekly decrease of 22 minutes, 58 seconds.
The shortest day of the month is September 30, with 11 hours, 45 minutes of daylight and the longest day is September 1, with 13 hours, 20 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in Helena Valley Northwest is 6:47 AM on September 1 and the latest sunrise is 38 minutes later at 7:25 AM on September 30.
The latest sunset is 8:07 PM on September 1 and the earliest sunset is 58 minutes earlier at 7:10 PM on September 30.
Daylight saving time is observed in Helena Valley Northwest during 2019, but it neither starts nor ends during September, so the entire month is in daylight saving time.
For reference, on June 21, the longest day of the year, the Sun rises at 5:34 AM and sets 15 hours, 52 minutes later, at 9:25 PM, while on December 22, the shortest day of the year, it rises at 8:10 AM and sets 8 hours, 33 minutes later, at 4:43 PM.
Sunrise & Sunset with Twilight in September
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
The chance that a given day will be muggy in Helena Valley Northwest is essentially constant during September, remaining around 0% throughout.
For reference, on June 12, the muggiest day of the year, there are muggy conditions 0% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% of the time.
Humidity Comfort Levels in September
dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed in Helena Valley Northwest is gradually increasing during September, increasing from 5.5 miles per hour to 6.2 miles per hour over the course of the month.
For reference, on January 17, the windiest day of the year, the daily average wind speed is 7.9 miles per hour, while on August 5, the calmest day of the year, the daily average wind speed is 5.0 miles per hour.
Average Wind Speed in September
Wind Direction in September
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
The growing season in Helena Valley Northwest typically lasts for 4.5 months (137 days), from around May 12 to around September 27, rarely starting before April 24 or after June 2, and rarely ending before September 8 or after October 14.
During September in Helena Valley Northwest, the chance that a given day is within the growing season is very rapidly decreasing falling from 96% to 40% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in September
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
The average accumulated growing degree days in Helena Valley Northwest are increasing during September, increasing by 248°F, from 1,652°F to 1,901°F, over the course of the month.
Growing Degree Days in September
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy in Helena Valley Northwest is rapidly decreasing during September, falling by 1.7 kWh, from 5.8 kWh to 4.1 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in September
For the purposes of this report, the geographical coordinates of Helena Valley Northwest are 46.729 deg latitude, -112.063 deg longitude, and 4,072 ft elevation.
The topography within 2 miles of Helena Valley Northwest contains significant variations in elevation, with a maximum elevation change of 715 feet and an average elevation above sea level of 4,016 feet. Within 10 miles contains significant variations in elevation (3,041 feet). Within 50 miles contains large variations in elevation (6,368 feet).
The area within 2 miles of Helena Valley Northwest is covered by grassland (81%) and shrubs (18%), within 10 miles by shrubs (42%) and grassland (39%), and within 50 miles by grassland (29%) and trees (27%).
This report illustrates the typical weather in Helena Valley Northwest year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
There are 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Helena Valley Northwest.
For each station, the records are corrected for the elevation difference between that station and Helena Valley Northwest according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
The estimated value at Helena Valley Northwest is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Helena Valley Northwest and a given station.
The stations contributing to this reconstruction are: Helena Regional Airport (90%, 16 kilometers, southeast); Great Falls International Airport (7%, 98 kilometers, northeast); and Missoula International Airport (3.3%, 156 kilometers, west).
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for aiports and weather stations are provided by AskGeo.com .
Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.
The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.
We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.
We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.