Preview of Thornton's September Weather - Summer Ends and Fall Arrives

As temperatures start to drop, September reminds us that summer is at an end and fall is now here. Sunshine is predominant though as the month actually has the highest percentage of sun out of any month. Sunny days and clear, cool nights are the standard weather pattern for the month.

Normal highs on the first of the month are 82 degrees with a low of 53. By the end of the month we see those high temperatures drop to an average of 73 and the lows get to a chilly 42. Overall the month averages 62.4 degrees.

In some years summer hangs on as late as the end of September - right to the last day of the month. September 30, 1980 saw a record high temperature of 90 degrees. 97 degrees is the hottest temperature ever recorded for the month - that happening on four occasions, the most recently on September 1st and 5th in 1995. Interestingly enough, all but four days of the month have recorded record highs in the 90s! The ones that haven't - the 21st, 22nd, 24th and 28th - have record temperatures pretty close of either 88 or 89 degrees. The chance for thunderstorms still exists in September but they are infrequent and severe ones are pretty rare.

All that sounds great for lovers of warm weather but winter too can make appearances in September - in fact, it can appear early on. The earliest measurable snow happened on September 3, 1961 when 4 inches of the white stuff fell at the old Stapleton International Airport and the mercury dropped to 33 degrees.

The snowiest September on record occurred in 1971 which 17.2 inches of snow was recorded. Freezing has occurred in Denver as early as September 8, 1962 with a low of 31 degrees. Winter really hit home in 1985 when on September 29th, 9 inches of snow fell and Denver had a low of 17 degrees and a high of 29.

Having said that though, in the 129 years since 1882, 85 Septembers have passed without a trace of snow. At the current time, Denver has gone ten Septembers in a row without snow being recorded with the last being in September 2000 when 0.2 inch was recorded.

In conclusion, September can bring just about anything but the odds do favor a beautiful month.

Looking back at September 2010 – A warm and dry month

September 2010 was a warmer and drier than normal month making the top 10 list of Septembers in both categories. 

High temperature records crumbled during the month and September 2010 finished with an overall average temperature of 67.0 degrees.  This secured it a spot in the books as the seventh warmest September on record.  Temperatures ranged from a high of 96 on the 19th down to a low of 40 on the 3rd. 

High temperature records including the 96 degree mark on the 19th which broke the old record for the date of 93 degrees set in 1980.  It also was the hottest temperature ever recorded that late in the season, passing the 95 degree mark on the 17th. 

Other records including 94 degrees on the 20th which broke the old record of 92 set in 1956.  The 26th saw 90 degrees thus tying the record for the date last set in 1892.  The 92 degrees recorded on the 28th easily broke the old record of 89 set in 1994.

Thornton was quite a bit cooler than DIA.  Our average temperature across September 2010 was 64.7 degrees.  Our highest mark was 93.6 degrees on the 4th and the lowest was on the 19th when the mercury fell to 40.7. 

In all, there were eight days with 90 degree or higher temperatures which was six more than normal.  25 days saw temperatures 0f 80 degrees or higher, the highest number of 80 degree days ever recorded during the month of September. 

In terms of precipitation, a paltry 0.06 inch fell into the rain bucket at Denver International Airport.  This was 1.08 inches below normal and put September 2010 into a three way tie for the fifth driest September on record.  Only one thunderstorm was observed at DIA, three fewer than normal.

While Thornton received more precipitation than the airport, it wasn't by much.  We recorded only 0.09 inch, the vast majority of which (0.07 inch) was recorded on the 22nd. 

Click here to view Thornton's September 2010 climate report.

Outlook for September 2011

Long range climate outlooks from the National Weather Service indicate below normal precipitation and temperatures at or above normal are expected this year. Climatological normals should be used for guidance (below).

For More Information

Temperature Normals and Extremes for September

Monthly Temperature, Rainfall and Snowfall Extremes for September

SEPTEMBER AVERAGE STATISTICS * Normals & Means, 1981 - 2010
TEMPERATURE
AVERAGE HIGH	78.5
AVERAGE LOW	48.3
MONTHLY MEAN	63.4
DAYS WITH HIGH 90 OR ABOVE	3
DAYS WITH HIGH 32 OR BELOW	0 * (LESS THAN 1)
DAYS WITH LOW 32 OR BELOW	1
DAYS WITH LOWS ZERO OR BELOW	0
PRECIPITATION
MONTHLY MEAN	1.14 INCHES
DAYS WITH MEASURABLE PRECIPITATION	6
AVERAGE SNOWFALL	2.1 INCHES
DAYS WITH 1.0 INCH OR MORE SNOWFALL	0 * (LESS THAN 1)
MISCELLANEOUS SEPTEMBER AVERAGES
HEATING DEGREE DAYS	136
COOLING DEGREES DAYS	57
WIND SPEED (MPH)	7.9 MPH
WIND DIRECTION	SOUTH
DAYS WITH THUNDERSTORMS	4
DAYS WITH DENSE FOG	1
PERCENT OF SUNSHINE POSSIBLE	74 (SUNNIEST MONTH)
SEPTEMBER EXTREMES
RECORD HIGH	97 ON 9/4/1995, 9/1/1995, 9/4/1960, 9/5/1899
RECORD LOW	17 ON 9/29/1985
WARMEST	68.3 IN 1948
COLDEST	54.8 IN 1912
WETTEST	4.67 INCHES IN 1961
MAXIMUM 24 HOUR MOISTURE	2.7 INCHES IN 1902 (20TH - 21ST)
DRIEST	TRACE IN 1944 AND 1892
SNOWIEST (Could include hail)	17.2 INCHES IN 1936
MAXIMUM 24 HOUR SNOWFALL	15.0 INCHES IN 1936 (27TH - 28TH)
LEAST SNOWIEST	0.0 (82 YEARS WITH NO SNOW REPORTED)

* Historical weather statistics gathered from the National Weather Service's Denver / Boulder forecast office data archives.