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 Post subject: Foothill and Denver Snowfall Climatology
Post Posted: Feb 10, 2013 9:43 pm 
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Joined: Feb 19, 2007 6:52 pm
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Location: Conifer Mountain
Note - edited Sep 3 2013 to include ENSO relationship to snowfall climatology.

As many people have begun to make statements regarding our current dry start to the season, I thought it would be interesting to do some digging and find snowfall records to determine how many dry and wet snowfall years we have had locally, as well as in Denver since they have records that go back much further and Denver snowfall has a very strong positive correlation with foothill snowfall. So I used the Conifer Mountain almanac here on Pinceam that has records back to 1993, and the Denver NWS office snowfall statistics that go back 130 years. As I prefer to base weather statements on data versus perception or memory, which has been proven to be very unreliable. Please also note that snowfall data is only part of the overall drought equation, as rainfall records would need to be included to determine the overall moisture for a given year. However, there are 2 critical fire seasons here in CO, the first is from March into June which is highly dependent on snowfall, and the second is from September into November which is dependent on monsoon rainfall. Large fires during the monsoon season (July-August) are quite rare in the foothills. Here is what I have found.

Conifer Mountain (21 year history) - The average snowfall from 1993 to 2013 is 173 inches. There have been 11 years with below averave snowfall, and 10 years with above average snowfall. There has only been one preiod with consecutive below average years, which was 1999 - 2002. There have been 3 times with consecutive above average years, 1993-1995, 1996-1999, and 2006-2008. The average snowfall since 2006 is 189 inches, so a higher average than the entire period for the past 7 seasons. Highest seasonal snowfall was in 280 inches in 2006-2007, and the lowest snowfall was 99 inches in 2005-2006, and second driest was 102 inches in 2001-2002. Including a look at ENSO (El Nino Southern Oscillation) and the ONI (Oceanic Nino Index) can provide some seasonal insight into our coming snowfall. There are 3 phases of ENSO; Cool phase is when the ONI is below -0.5 and referred to as La Nina, Warm Phase when the ONI is above +0.5 and referred to as El Nino, and Neutral when the ONI is between -0.5 and +0.5, no reference other than El Nada (joking). The ENSO phase is determined by the sea surface temperature anomoly in the central to eastern equatorial Pacific ocean. Over the past 21 seasons, there have been 8 Cold ENSO phases, 6 Warm ENSO phases, and 7 Nuetral ENSO phases. There is a strong correlation to foothill snowfall and the ENSO phase. During Warm phases (EL Nino), the average snowfall is 226 inches, so 53 inches above our 21 year average. In all 6 Warm episodes, snowfall was above our 21 year average, ranging from 194 inches to 280 inches, so a very strong correlation. During the 8 Cold episodes (La Nina), the average snowfall is 152 inches. 2 years had above average snowfall, with 6 years being below average ranging from 99 inches to 209 inches. During 7 Nuetral ENSO phases, the average snowfall is 152 inches also. 2 years had above average snowfall, with 5 years below average ranging from 102 inches to 180 inches. So, when the ENSO phase is Warm, we have 100% probability of above average snowfall. When the ENSO is Cold or Nuetral, we have a 26% probability of above average snowfall, and a 74% probability of below average snowfall.

Denver (130 year history) - The average snowfall in Denver is 57.5 inches. There have been 74 seasons below average, and 56 seasons above average. There have been 7 periods with 3 consecutive below average snow seasons, which are 1882-1890, 1899-1906, 1933-1936, 1962-1967, 1974-1978, 1998-2000, and 2003-2006. There have been 9 periods with 3 consecutive above average seasons, which are 1911-1914, 1925-1931, 1936-1939, 1946-1949, 1950-1953, 1956-1962, 1971-1974, 1985-1988, 1990-1995. Highest Denver snowfall was 118 inches in 1908-1909. Lowest snowfall was 21 inches in 1888-1889, and second driest was 23 inches in 2010-2011. Adding ENSO phases to Denver snowfall since 1950 (wnen ENSO began being measured, looks a little difefrent than for the foothills. Over the 62 seasons since 1950-1951, there have been 21 Cold ENSO phases, 19 Warm ENSO phases, and 22 Nuetral ENSO phases. For Cold phases (La Nina), the average snowfall is 56.3 inches, or 1.2 inches below the long term average with 14 seasons below average and 7 above average. For Warm ENSO phases (El Nino), the average snowfall is 63.8 inches, or 6.3 inches above average with 13 seasons above average and 6 below average. During Nuetral ENSO phases, the average snowfall is 57.2 inches, or 0.3 inches below average with 13 seasons above average and 9 below average. Hence, with ENSO Cold phase, Denver has a 67% probability of below average snowfall, and a 33% probability of above average snowfall. During ENSO Warm phases, there is a 68% probability of above average snowfall, and a 32% probability of below average snowfall. During ENSO Nuetral phases, there is a 59% probability of above average snowfall, and a 41% probability of below average snowfall.

In general, the above average periods (consecutive seasons) seem more prevalent and persist longer than the below average periods. However, the most prevalent climatological condition is for seasons to be near average. Let me be clear here, we cannot make direct assumptions from such a small data set. With only 130 years of actual weather observations, we simply do not have sufficient data to make long term climate statements. The weather 1,000 or 1,000,000 years ago likely was very different than the past 100 years. The droughts of the late 1800's and early 1900's seemed pretty bad, but not many people living out here then. In general the 1950's were pretty wet, and the 1980's to early 1990's were pretty wet.

I have done my own analysis from Denver snowfall history by decade with corresponding ENSO patterns during the decade. Note how dry Denver was during the 1880-1900 period, and then relatively snowy during the 1950's and then during 1970-1990, and then quite dry from 2000 to present.

Decade Avg Snowfall Max Snowfall Min Snowfall ENSO Patterns
1880-1889 32.0 51.9 21.3 2W, 2C, 5N
1890-1899 53.9 72.0 39.2 2W, 2C, 6N
1900-1909 54.0 118.7 39.6 3W, 4C, 3N
1910 - 1919 61.6 98.9 31.5 3W, 2C, 5N
1920 - 1929 62.6 85.4 32.3 2W, 2C, 6N
1930 - 1939 59.1 81.5 45.3 1W, 1C, 8N
1940 - 1949 57.9 90.5 28.6 1W, 3C, 6N
1950 - 1959 68.8 99.3 41.5 4W, 4C, 2N
1960 - 1969 54.8 80.6 33.3 4W, 1C, 5N
1970 - 1979 66.8 94.9 34.6 2W, 5C, 3N
1980 - 1989 60.0 81.6 26.7 3W, 3C, 4N
1990 - 1999 58.2 79.0 44.7 3W, 3C, 4N
2000 - 2009 47.5 72.6 30.2 3W, 3C, 4N
2010 - 2018 46.6 78.4 21.8 1W, 3C, 4N

"Climatology is what you expect, Weather is what you get" - R. Heinlein.

"Sunshine is delicious, rain is refreshing, wind braces us up, snow is exhilarating; there is really no such thing as bad weather, only different kinds of good weather" - John Ruskin

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