# Boulder, Colorado Weather and Climate

## Relationship to El Niņo/La Niņa

Go to:1997-98 El Nino discussion Precipitation Snowfall Maximum Temperature Minimum Temperature

What is the weather in Boulder in relationship to El Niņo? In order to examine this question, I looked at Boulder monthly mean temperature and precipitation for a set of 11 El Niņo years and a set of 11 La Niņa years (as defined by COAPS) from 1949-1996. This excludes the current El Niņo. For each of the 11 years, I computed the mean value of that variable and the number of years that that variable was above and below the mean. The former gives an idea of the strength of the "signal". The latter indicates the possibility that only a few years are dominating the average response. Note that any set of 11 years will differ from the mean; whether the difference is statistically significant or not is another question. Also, a different or smaller set of El Niņo years will most likely not give the exact same result. For example, see a precipitation comparison for Denver during El Nino winters (choose Denver) from the Desert Research Institute.

## To interpret tables

For each of the months listed, the variable of interest is averaged for the 11 El Niņo years (column 2) and 11 La Niņa years (column 5), respectively. If a month is missing data for a year, less years are used than the 11 possible. The total mean (1949-1996) is calculated from all non-missing years and is in the last column. Then for the El Niņo average, the number of the 11 years that are below (column 2) and above (column 3) this mean is calculated. This gives an idea of the odds that an El Niņo year will be below/above the mean. The same is done for La Niņa years. For example, for January precipitation, the average during El Niņo is .45 inches; during La Niņa years it is .72 inches and the total mean is .68 inches. Of the 11 El Niņo years, 8 had below normal precipitation and 3 had above normal. For La Niņa, 5 were below normal and 6 were above.

Years listed are for the spring at the end of an El Niņo/La Niņa. That is, 1983 for a warm year is the El Niņo starting in autumn 1982 and ending in spring 1983.

Warm Years
1952,1958,1964,1966,1970,1973,1977,1983,1987,1992,1988
Cold years
1950,1955,1956,1957,1965,1968,1971,1972,1974,1976,1989

## Precipitation

MonthEl Niņo average below abovetotLa Niņa averagebelowabovetotmean
Jan 0.45 8 311 0.72 5 6 11 0.68
Feb 0.68 8 311 1.01 3 8 11 0.77
Mar 2.52 3 811 1.15 9 2 11 1.73
Apr 2.21 5 611 2.74 6 5 11 2.30
May 2.88 6 511 2.79 8 3 11 3.26
Jun 1.78 8 311 1.89 7 4 11 2.19
Jul 1.73 7 411 1.72 8 3 11 1.82
Aug 1.21 7 411 1.36 5 6 11 1.47

## Precipitation: Different set of El Nino years

To see how using a different set of years might affect the results, I used the same years as the Desert Research Institute for their analysis of El Nino and precipitation the west. Partly the different results are caused by using the earlier El Ninos but partly they are caused by using different events (which depend on exactly how an El Nino is defined). These differences give an indication of the sensitivity of the results to the technique.

mean 1940-1996
years used: 1941,1942,1966,1973,1978,1983,1988,1995

MonthEl Niņo average below abovetotmean
Jan 0.72 4 48 0.75
Feb 0.86 4 48 0.76
Mar 1.91 5 38 1.77
Apr 3.78 2 68 2.46
May 4.55 3 58 3.21
Jun 1.96 5 38 2.15
Jul 1.37 6 28 1.79
Aug 1.30 5 38 1.54
Note that for this set of years, April and May are noticeably wetter. Jun-Aug are still relatively dry.

## Snowfall

MonthEl Niņo average below abovetotLa Niņa averagebelowabovetotmean
Jan 8.25 7 411 9.40 6 5 11 10.78
Feb 9.22 7 411 14.00 5 6 11 10.50
Mar 22.63 3 811 15.04 7 4 11 17.11
Apr 12.62 5 611 11.28 7 4 11 10.94
May 0.93 9 211 1.61 9 2 11 1.57

## Maximum Temperature

MonthEl Niņo average below abovetotLa Niņa averagebelowabovetotmean
Jan 45.28 4 610 45.64 3 8 11 45.02
Feb 49.30 4 610 47.74 6 5 11 48.45
Mar 51.36 7 411 54.79 4 7 11 53.64
Apr 61.96 6 511 62.25 6 5 11 62.65
May 72.98 3 811 71.21 6 5 11 71.66
Jun 82.35 5 611 82.55 4 7 11 82.01
Jul 87.57 5 611 87.45 5 6 11 87.40
Aug 85.69 5 611 85.03 7 3 10 85.64

## Minimum Temperature

MonthEl Niņo average below abovetotLa Niņa averagebelowabovetotmean
Jan 20.64 4 610 20.87 6 5 11 20.01
Feb 24.01 4 5 9 23.16 5 6 11 23.51
Mar 26.78 9 211 27.61 5 6 11 27.86
Apr 35.13 6 511 35.67 5 6 11 35.75
May 45.94 3 811 44.31 7 4 11 44.67
Jun 53.78 4 711 53.60 4 7 11 53.23
Jul 58.84 6 511 58.82 6 5 11 58.71
Aug 57.37 5 611 57.50 4 6 10 57.42

## Discussion of the 1997-1998 El Niņo in Boulder

For precipitation and snowfall, values from the current El Niņo "match" the Boulder El Niņo signal for 8 out of the 10 categories. Red indicates the 1998 values that match the sign of the average El Niņo signal in Boulder.

For temperature, the signal for January and February is warmer than normal for both max and min temperatures. For both March and April it was colder than normal. For May, it was again warmer than normal. This was matched in 8 of the 10 categories, Feb maximum and May minimum temperatures being the exception. For every month, the sign of the composite was matched in at least 3 out the 4 possible categories.

Month98 precipmean98 snowmean
Jan1.07.6810.410.78
Feb0.23.771.710.5
Mar3.411.7342.617.11
Apr4.562.3019.410.94
May1.843.260.01.57
Month98 Max Tmean98 Min Tmean
Jan48.645.0224.420.01
Feb48.048.4524.823.51
Mar52.153.6425.227.86
Apr59.162.6533.935.75
May74.271.6643.344.6