The hot season is June to September, with an average daily high temperature above 75°F. The hottest day of the year is July 30, with an average high of 86°F and low of 59°F. The cold season is November to March, with an average daily high temperature below 35°F. The coldest day of the year is January 3, with an average low of 4°F and high of 22°F. Forecasts. NOAA Weather Forecast for Fort Union Trading Post NHS.