Weather Retrospective September 2017

John McGinley, Boulder County.  Last month we introduced this new monthly weather column to discuss weather data from the previous month, climate issues, extremes, and other interesting tidbits. The weather review appears in the first issue of the Mountain Ear after the last day of the previous month.


September 2017 took us on a wild ride beginning with 10 days of warm, hazy, smoky conditions from the extensive forest fires in the northwest. By the 10th we saw a return of the monsoon that gradually cleared out the smoke and brought some moisture. Associated wimpy showers and thunderstorms brought a one-week total of 0.23 inches. Mid-month it was looking bleak for significant rain, with only 14% of the monthly precipitation realized. September fires are always a possibility with such dry conditions but the saving grace was cooler than normal temperatures and light winds. By the 23rd a big change in global circulation brought an upper level trough over the western US putting Colorado in the storm track. A series of disturbances brought precipitation every day from the 23rd to the 30th totaling a whopping 2.12 inches. Suddenly a dry month turned into a wet month. The storm on the 24th brought sleet showers and snow officially measuring 0.1 inches at my house… the first day of measurable snow. To summarize, we had something for everyone: heat, smoke, dryness, fire threat, thunderstorms, steady rain, sleet, and snow…. quite a ride.


Precipitation: Total for the month was 2.35 inches, 0.56 inches above normal. Note that 2.12 inches fell in the last week of the month. This included sleet and some snow. Biggest day of rain was 0.87 inches on the 28th. This is the second month in row with above normal precipitation.


Temperature: Average high for the month was 66F with an average low of 42F. The high was right on average, but the low was warmer than normal (probably owing to the frequent cloud cover). This made the month 1.6 degrees above normal. Three days were freezing or below with the extreme of 28F on the 26th. The warmest day was 83F on the 3rd.


Winds: We consider a windy day as 40mph or stronger. This September we had no windy days! Usually we see two days.


Other features: September is a transitional month and this year was no exception. No sooner than we had the equinox on the 21st we saw the jet stream plunge south with the arrival of a large upper level trough, cold air and rain. A series of weather disturbances brought our frequent rainy days as explained above.


Outlook for October: In October we see a major drop in our temperatures with an average high of 54F and an average low of 29F. Winds pick up with six windy days. October is generally dry with only 1.2 inches of precipitation with 10 inches of snow for the month. The long-range outlook based on prediction models shows below normal temperatures and wet for the next 8-10 days with warming and drying through the rest of the month.