More September-Winter Talk
In my 'climate corner' piece on Monday, I talked about the connection between September temperatures and December temperatures. In 64% of years since 1970 we have seen temperatures relative to normal flip flop between September and December. If you have a warmer than normal September, 74% of the time you see a below normal December in Philadelphia.
How does this correlate to winter though?
When temperatures are above normal in September, the correlation to a colder than normal winter overall is much weaker. Only 47% of the Septembers since 1970 that were warm went colder for the whole winter (9 of 19) in Philadelphia. This means that 53% of them overall averaged warmer than normal.
Among the NINA years, that correlation is 40% in a very small sample (2 years of 5). This year will, in all likelihood, be at least a weak nina. You may notice the super winter of 1995-1996 included in this data set as one of the years that featured a warm September and a colder than normal winter.
What's interesting though is that a colder-than-normal September will correlate to a warmer than normal winter more often than not. 11 of 18 colder than normal Septembers went warm for the whole winter, about a 61% rate.
Among the NINA winters, only September 2000 featured a winter that was colder than normal overall.
It is harder to correlate weather patterns from September to a whole three month season. Additionally, using just one data point to suggest what could or will happen is a risky proposition but this data can be used with other climate statistics from elsewhere across the country to paint a reasonable idea of what is going on.
More importantly, however, when it comes to setting the table for a winter forecast, looking at what is going on now is important in determining what can happen in 90 days time (December) as there is a more certain correlation (albeit opposite) in weather. Does this mean December will definitely be cold if we stay warm? There are no definites in weather but it could mean there's a better chance we start this winter cold if trends continue.
How does this correlate to winter though?
When temperatures are above normal in September, the correlation to a colder than normal winter overall is much weaker. Only 47% of the Septembers since 1970 that were warm went colder for the whole winter (9 of 19) in Philadelphia. This means that 53% of them overall averaged warmer than normal.
| Year | SEP | SEP DEP | WIN TEMP | DEP | ENSO |
| 1970 | 72.0 | 3.2 | 33.2 | -1.6 | NINA |
| 1971 | 71.6 | 2.8 | 36.4 | 1.6 | NINA |
| 1972 | 69.2 | 0.4 | 36.0 | 1.2 | NINO |
| 1973 | 70.7 | 1.9 | 35.4 | 0.6 | NINA |
| 1977 | 69.9 | 1.1 | 28.4 | -6.4 | NINO |
| 1980 | 72.2 | 3.4 | 31.9 | -2.9 | NEUT |
| 1983 | 69.0 | 0.2 | 32.7 | -2.1 | NEUT |
| 1985 | 69.1 | 0.3 | 32.7 | -2.1 | NEUT |
| 1989 | 69.7 | 0.9 | 35.7 | 0.9 | NEUT |
| 1991 | 69.5 | 0.7 | 37.6 | 2.8 | NINO |
| 1993 | 69.7 | 0.9 | 33.0 | -1.8 | NEUT |
| 1995 | 70.4 | 1.6 | 31.9 | -2.9 | NINA |
| 1998 | 71.8 | 3.0 | 38.3 | 3.5 | NINO |
| 1999 | 69.9 | 1.1 | 36.5 | 1.7 | NINA |
| 2002 | 72.2 | 3.4 | 31.3 | -3.5 | NINO |
| 2003 | 70.0 | 1.2 | 32.8 | -2.0 | NEUT |
| 2004 | 70.4 | 1.6 | 35.2 | 0.4 | NINO |
| 2005 | 73.8 | 5.0 | 36.9 | 2.1 | NEUT |
Among the NINA years, that correlation is 40% in a very small sample (2 years of 5). This year will, in all likelihood, be at least a weak nina. You may notice the super winter of 1995-1996 included in this data set as one of the years that featured a warm September and a colder than normal winter.
| Year | SEP | SEP DEP | WIN TEMP | DEP | ENSO |
| 1970 | 72.0 | 3.2 | 33.2 | -1.6 | NINA |
| 1971 | 71.6 | 2.8 | 36.4 | 1.6 | NINA |
| 1973 | 70.7 | 1.9 | 35.4 | 0.6 | NINA |
| 1995 | 70.4 | 1.6 | 31.9 | -2.9 | NINA |
| 1999 | 69.9 | 1.1 | 36.5 | 1.7 | NINA |
What's interesting though is that a colder-than-normal September will correlate to a warmer than normal winter more often than not. 11 of 18 colder than normal Septembers went warm for the whole winter, about a 61% rate.
| Year | SEP | SEP DEP | WIN TEMP | DEP | ENSO |
| 1974 | 68.1 | -0.7 | 37.5 | 2.7 | NINA |
| 1975 | 66.6 | -2.2 | 35.5 | 0.7 | NINA |
| 1976 | 67.3 | -1.5 | 28.0 | -6.8 | NEUT |
| 1978 | 68.5 | -0.3 | 31.4 | -3.4 | NEUT |
| 1979 | 68.5 | -0.3 | 33.2 | -1.6 | NINO |
| 1981 | 66.8 | -2.0 | 31.2 | -3.6 | NEUT |
| 1982 | 67.6 | -1.2 | 36.5 | 1.7 | NINO |
| 1984 | 64.7 | -4.1 | 34.8 | 0.0 | NINA |
| 1986 | 68.3 | -0.5 | 34.1 | -0.7 | NINO |
| 1988 | 66.7 | -2.1 | 35.6 | 0.8 | NINA |
| 1990 | 68.0 | -0.8 | 39.1 | 4.3 | NEUT |
| 1992 | 68.4 | -0.4 | 36.3 | 1.5 | NINO |
| 1994 | 68.3 | -0.5 | 37.2 | 2.4 | NINO |
| 1996 | 68.7 | -0.1 | 37.6 | 2.8 | NEUT |
| 1997 | 67.1 | -1.7 | 40.4 | 5.6 | NINO |
| 2000 | 66.4 | -2.4 | 33.7 | -1.1 | NINA |
| 2001 | 68.4 | -0.4 | 41.3 | 6.5 | NEUT |
| 2006 | 67.1 | -1.7 | 36.3 | 1.5 | NINO |
Among the NINA winters, only September 2000 featured a winter that was colder than normal overall.
| Year | SEP | SEP DEP | WIN TEMP | DEP | ENSO |
| 1974 | 68.1 | -0.7 | 37.5 | 2.7 | NINA |
| 1975 | 66.6 | -2.2 | 35.5 | 0.7 | NINA |
| 1984 | 64.7 | -4.1 | 34.8 | 0.0 | NINA |
| 1988 | 66.7 | -2.1 | 35.6 | 0.8 | NINA |
| 2000 | 66.4 | -2.4 | 33.7 | -1.1 | NINA |
It is harder to correlate weather patterns from September to a whole three month season. Additionally, using just one data point to suggest what could or will happen is a risky proposition but this data can be used with other climate statistics from elsewhere across the country to paint a reasonable idea of what is going on.
More importantly, however, when it comes to setting the table for a winter forecast, looking at what is going on now is important in determining what can happen in 90 days time (December) as there is a more certain correlation (albeit opposite) in weather. Does this mean December will definitely be cold if we stay warm? There are no definites in weather but it could mean there's a better chance we start this winter cold if trends continue.
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