A Statistical Preview of Denver's September Weather

        ...A STATISTICAL PREVIEW OF DENVER'S SEPTEMBER WEATHER...
 
AUGUST 2012 STAYED CONSISTENT WITH THE PREVIOUS FEW MONTHS OF ABOVE NORMAL TEMPERATURES AS A DOMINANT DRY AND STABLE RIDGE OF HIGH PRESSURE REMAINED FASTENED OVER THE SOUTHERN AND CENTRAL UNITED STATES.
 
AFTER THE RECORD HOT MONTHS OF JUNE AND JULY, AUGUST 2012 FINISHED THE MONTH AS THE 5TH WARMEST AND 4TH DRIEST IN DENVER'S 141 YEAR HISTORY.
 
CLIMATOLOGICALLY, AFTER FAIRLY STABLE TEMPERATURE VARIATION DURING THE WARMEST MONTHS OF JULY AND AUGUST, TEMPERATURE SPREADS ACROSS THE MONTH OF SEPTEMBER BEGIN TO WIDEN AS DRYER AND COOLER AIR FROM THE NORTHERN PLAINS AND CANADIAN REGION PERIODICALLY WORK THEIR WAY INTO THE HIGH PLAINS OF
EASTERN COLORADO BEHIND PASSING STORM SYSTEMS TO THE NORTH.
 
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 DURING THE MONTH OF SEPTEMBER, SINCE RECORDS BEGAN IN 1872, WAS 97 DEGREES AND OCCURRED FOUR 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. THE COLDEST TEMPERATURE EVER RECORDED IN DENVER FOR SEPTEMBER 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 RIDGE OF HIGH PRESSURE WEAKENS. PRECIPITATION IN SEPTEMBER CAN STILL COME IN THE FORM OF HEAVY RAINFALL FROM LATE SEASON THUNDERSTORMS.
 
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
 
4.67 INCHES 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 140 YEAR HISTORY OF DENVER’S WEATHER, 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 2012...
 

AFTER RECORD BREAKING HEAT IN JUNE, JULY AND AUGUST ALONG WITH THE 6TH CONSECUTIVE MONTH OF BELOW NORMAL PRECIPITATION IN DENVER, THE MID TO LONG RANGE FORECAST MODELS CONTINUE TO INDICATE A BIAS TOWARD THE STABLE RIDGE OF HIGH PRESSURE CONTINUING TO DOMINATE THE CENTRAL UNITED STATES FROM THE 10TH THROUGH THE END OF SEPTEMBER, 2012. THE CLIMATE PREDICTION CENTER'S LONG RANGE OUTLOOKS FOR TEMPERATURE AND PRECIPITATION INDICATE A BIAS TOWARD ABOVE NORMAL TEMPERATURES WITH PRECIPITATION PROBABILITIES HOLDING NEAR TO BELOW SEASONAL NORMALS.

 
                     ...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
                                                     (85)
 
FREDIN


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