A Statistical Preview of Denver's September Weather

 

       ...A STATISTICAL PREVIEW OF DENVER'S SEPTEMBER WEATHER...

 

CLIMATOLOGICALLY, AFTER FAIRLY STABLE TEMPERATURE VARIATION DURING THE WARMEST MONTHS OF JULY AND AUGUST, DAILY TEMPERATURE SPREADS ACROSS THE MONTH OF SEPTEMBER BEGIN TO WIDEN. AS PACIFIC DISTURBANCES BEGIN TO PASS ACROSS THE NORTHERN U.S. ROCKIES IN SEPTEMBER, DRYER AND COOLER AIR FROM THE NORTHERN HIGH PLAINS AND CANADIAN REGION PERIODICALLY PUSH ONTO THE HIGH PLAINS OF EASTERN COLORADO.

 

DENVER'S MONTHLY MEAN TEMPERATURE FOR SEPTEMBER IS 63.4 DEGREES (1981-2010 AVERAGES) AND IS DENVER'S FOURTH WARMEST MONTH OF THE YEAR BEHIND JULY, AUGUST AND JUNE RESPECTIVELY, THE MONTH STARTS WITH A NORMAL HIGH OF 84 DEGREES AND ENDS WITH A NORMAL HIGH OF 72 DEGREES. FOR LOW TEMPERATURES, THE MONTH BEGINS WITH A NORMAL OF 54 DEGREES AND FINISHES WITH A NORMAL OF 42.

 

THE WARMEST TEMPERATURE EVER RECORDED FOR THE MONTH OF SEPTEMBER, SINCE RECORDS BEGAN IN 1872, WAS 97 DEGREES AND OCCURRED SIX TIMES. THE FIRST OCCURRENCE WAS ON THE 5TH DAY OF 1899, THEN FOLLOWED BY THE 4TH DAY OF 1960, THE 4TH OF 1995 AND ON THE 1ST OF 1995. 97 DEGREES WAS ALSO RECORDED BOTH ON THE 5TH AND THE 6TH LAST SEPTEMBER. THE COLDEST TEMPERATURE EVER RECORDED IN DENVER FOR AUGUST WAS 17 DEGREES AND OCCURRED ON THE 29TH DAY OF 1985.

 

SEPTEMBER: DENVER'S TOP 5 WARMEST (MEAN TEMPERATURES):

 

68.3 DEGREES 1948

68.2         1931, 1981

68.0         1998

67.7         1933

67.6         1947

 

SEPTEMBER: DENVER'S TOP 5 COLDEST (MEAN TEMPERATURES):

 

54.8 DEGREES 1912

55.7         1965

56.3         1961

57.5         1971

58.1         1918

 

AS THE NORTH AMERICAN MONSOON SEASON GRADUALLY WEAKENS THROUGH LATE SUMMER, SEPTEMBER BEGINS TO RECEIVE A PORTION OF IT'S PRECIPITATION FROM QUICK MOVING PACIFIC TROUGHS OF LOW PRESSURE WHICH SWING ACROSS THE STATE AS THE SUMMER RIDGE OF HIGH PRESSURE WEAKENS. PRECIPITATION IN SEPTEMBER CAN STILL COME IN THE FORM OF HEAVY RAINFALL FROM LATE SEASON THUNDERSTORMS AND MONSOONAL FLOW. AN EXAMPLE OF THIS WAS THE TORRENTIAL RAINFALL AND ENSUING CATASTROPHIC FLOODING THAT OCCURRED IN SEPTEMBER OF 2013.

 

ALONG WITH JUNE, JULY AND AUGUST THE POSSIBILITY OF SEVERE WEATHER STILL EXISTS FOR THE MONTH OF SEPTEMBER, HOWEVER THE CHANCES GREATLY DIMINISH AFTER THE FIRST HALF OF THE MONTH.

 

SEPTEMBER: DENVER'S WETTEST

 

5.61 INCHES 2013

4.67        1961

3.78        1909

3.70        1902

3.42        1938

2.89        1875, 1918, 1936

 

SEPTEMBER: DENVER'S DRIEST

 

   T INCHES 1892, 1944

0.01        1956, 1992

0.02        1879, 1921

0.05        1893

0.06        1882, 1920, 2010

 

SEPTEMBER IS THE FIRST MONTH OF THE SNOW SEASON IN DENVER. SNOW IN SEPTEMBER OFTEN ARRIVES AT AN INCONVENIENT TIME AS THE TREES ALONG THE FRONT RANGE OF COLORADO TYPICALLY STILL HOLD LEAVES. THE FIRST SNOWFALL ARRIVAL IS ASSOCIATED WITH THE PASSAGE OF THE SEASONS FIRST STRONG PUSH OF COLD AIR FROM CANADA. IN THE 131 YEAR HISTORY OF DENVER’S RECORD KEEPING FOR SNOWFALL, SNOW HAS OCCURRED 45 TIMES. OF THE 45 OCCURRENCES, 28 YEARS HAD MEASURABLE OF 0.1” OR HIGHER. THE OTHER 17 YEARS RECEIVED A TRACE.

 

SEPTEMBER: DENVER'S SNOWIEST

 

17.2 INCHES 1971

16.5        1936

12.9        1959

11.4        1895

 8.7        1985

 

SEPTEMBER: DENVER`S LEAST SNOWIEST

 

Numerous years with

0.0" snowfall (85).

 

 

                    ...THE OUTLOOK FOR SEPTEMBER 2014...

 

AFTER A COOL AND UNSETTLED AUGUST WITH ABOVE NORMAL PRECIPITATION, THE SHORT RANGE FORECAST MODELS INDICATE A PROGRESSIVE PATTERN OF WEAK RIDGES AND TROUGHS OVER THE WESTERN AND CENTRAL UNITED STATES FOR THE FIRST 10 DAYS OF SEPTEMBER 2014. THE CLIMATE PREDICTION CENTER'S LONG RANGE OUTLOOK, BEYOND EARLY TO MID SEPTEMBER, INDICATE NEAR NORMAL TEMPERATURES WITH A BIAS TOWARD ABOVE NORMAL PRECIPITATION.

 

                     ...SEPTEMBER STATISTICS...

 

...TEMPERATURES (1981-2010 NORMALS)...

 

AVERAGE HIGH........................... 78.5

AVERAGE LOW............................ 48.3

MONTHLY MEAN........................... 63.4

DAYS WITH HIGH 90 OR ABOVE.............   3 (3.4)

DAYS WITH HIGH 32 OR BELOW.............   0 (0.0)

DAYS WITH LOW 32 OR BELOW..............   1 (0.8)

DAYS WITH LOWS ZERO OR BELOW...........   0 (0.0)

 

...PRECIPITATION...

 

MONTHLY MEAN...........................   0.96 INCHES

DAYS WITH MEASURABLE PRECIPITATION.....   7 (6.5)

AVERAGE SNOWFALL.......................   1.3 INCHES

 

...MISCELLANEOUS SEPTEMBER AVERAGES...

 

HEATING DEGREE DAYS.................... 125

COOLING DEGREE DAYS.................... 76

WIND SPEED (MPH).......................   8.0*

WIND DIRECTION......................... SOUTH*

DAYS WITH THUNDERSTORMS................   8* (7.9)

DAYS WITH DENSE FOG....................   1* (0.6)

PERCENT OF SUNSHINE.................... 74*

(NOTE: SUNSHINE DATA ARE NO LONGER COLLECTED AS OF OCTOBER 1, 2009)

 

* = 1971-2002 NORMALS

 

...SEPTEMBER EXTREMES...

 

RECORD HIGH............................ 97 DEGREES ON 9/1/1995

                                                       9/4/1995

                                                       9/4/1960

                                                       9/5/1899

RECORD LOW............................. 17 DEGREES ON 9/29/1985

WARMEST JULY........................... 68.3 DEGREES IN 2011

MAXIMUM COOLING DEGREE DAYS............ 143 IN 1999

MINIMUM COOLING DEGREE DAYS............ 20 IN 1986, 2006

COLDEST................................ 54.8 DEGREES IN 1912

MAXIMUM HEATING DEGREE DAYS............ 345 IN 1912

MINIMUM HEATING DEGREE DAYS............ 19 IN 1981

WETTEST................................   4.67 INCHES IN 1961

DRIEST................................. TRACE IN 1892, 1944

SNOWIEST............................... 17.2 INCHES 1971

LEAST SNOWY............................   0.0 INCHES NUMEROUS YEARS

                                                     (86)

 

WFO Boulder



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