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A Winter's Tale

September CET Watch

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In response to Millhouse post, perhaps I should have clarified what I meant by the word 'extremes', whilst September can deliver very warm weather even hot weather, in terms of weather it tends to be the most 'benign' month in terms of feel.. any very warm weather isn't normally associated with the intense humidity and sun strength of the summer months and generally feels much more pleasant. Unlike the Oct-March period, atlantic weather systems whilst they can pack a punch in terms of rain and can bring gales in Sept, again such conditions don't feel as unpleasant as they would do in the Oct-Mar period. In terms of cold, Sept is less likely to deliver frost than May and low temps are very hard to achieve, more likely to see simgle digit maxima in May than September.

In response to northerly blast post, I'm too sure about the theory that warm spells in Sept usually mean less chance of a cold winter, though it is interesting to note out of the years you mentioned, only winter 05/06 ended up average or below. The warm spells in sept 99, 04, 06 and 11 indeed were followed by mild or very mild winters.. Going further back though didn't september 78 bring alot of warm weather, indeed autumn 78 was very mild and we ended up with the third coldest winter of the 20th century.

Although September 1978 was slightly warmer than average, it wasn't extraordinarily so; with a CET of 14.2 it was only around 0.6*C above average, so it is stretching it a bit to call that a significantly warm September. I also do not think that Sept 1978 had any notable hot spells of the likes of mid Sept 2003, early Sept 2004 / 2005 / 2006 / late Sept 2011 etc. In many ways Sept 1978 was very similar to Sept 2009, and the overall CETs for Septembers 1978 and 2009 were identical. You are correct about autumn 1978 being a very warm one, indeed October and November that year were further above the average than September was, with some notable warm spells for so late in the year, and there was no sign of any cold spells until around November 24th / 25th.

Winter 2005-06, although respectable by 1990s and 2000s standards, it was close to the long term average and certainly not a particularly cold one by any means - certainly nothing like 2009-10 for example.

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I am certainly of the opinion that if it gets too warm during September, or has notable heatwaves, then it is unlikely to be followed by a particularly cold winter and certainly increases the chance of the following winter being milder than average, although after an average to cool September this theory has no evidence to work the opposite way round. Whilst it is not uncommon to see a mild winter after a cool September, it is very rare to see a cold winter after a much warmer than average September. I do think that the warm September / mild winter theory has more truth in it than the warm / dry October / cold winter theory that many on this forum have talked about in the past.

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I am certainly of the opinion that if it gets too warm during September, or has notable heatwaves, then it is unlikely to be followed by a particularly cold winter and certainly increases the chance of the following winter being milder than average, although after an average to cool September this theory has no evidence to work the opposite way round. Whilst it is not uncommon to see a mild winter after a cool September, it is very rare to see a cold winter after a much warmer than average September. I do think that the warm September / mild winter theory has more truth in it than the warm / dry October / cold winter theory that many on this forum have talked about in the past.

Back it up with the percentages over the

Although September 1978 was slightly warmer than average, it wasn't extraordinarily so; with a CET of 14.2 it was only around 0.6*C above average, so it is stretching it a bit to call that a significantly warm September. I also do not think that Sept 1978 had any notable hot spells of the likes of mid Sept 2003, early Sept 2004 / 2005 / 2006 / late Sept 2011 etc. In many ways Sept 1978 was very similar to Sept 2009, and the overall CETs for Septembers 1978 and 2009 were identical. You are correct about autumn 1978 being a very warm one, indeed October and November that year were further above the average than September was, with some notable warm spells for so late in the year, and there was no sign of any cold spells until around November 24th / 25th.

Winter 2005-06, although respectable by 1990s and 2000s standards, it was close to the long term average and certainly not a particularly cold one by any means - certainly nothing like 2009-10 for example.

The thing with winter 2006 is that rather than being cold, it simply contained no warmth and was very dry and frosty.

I would quite like a repeat if there were more snow.

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I am certainly of the opinion that if it gets too warm during September, or has notable heatwaves, then it is unlikely to be followed by a particularly cold winter and certainly increases the chance of the following winter being milder than average, although after an average to cool September this theory has no evidence to work the opposite way round. Whilst it is not uncommon to see a mild winter after a cool September, it is very rare to see a cold winter after a much warmer than average September. I do think that the warm September / mild winter theory has more truth in it than the warm / dry October / cold winter theory that many on this forum have talked about in the past.

I generally agree with the warm September theory reducing the chances of a cold winter to follow. This certainly seems to have been the case during the last 15 years anyway.

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Just where is the evidence for the 'warm September' hypothesis?

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I've scanned the Hadley CET series for Septembers with a CET of 15.0 and above, and while I may have missed the odd one, here's a list of the winters following Septembers with CETs of 15C and above. My assessments of mild/cold relate to the 1659-2011 averages (Dec 4.2, Jan 3.3 and Feb 3.9 if I remember rightly) rather than 1981-2010 which had anomalously warm Januarys and Februarys.

1678/79- very cold

1708/09- cold

1729/30- mild

1730/31- fairly cold

1731/32- mild

1752/53- average

1779/80- very cold

1780/81- cold

1795/96- very mild

1825/26- average

1865/66- mild

1895/96- average

1898/99- mild

1929/30- average

1949/50- fairly mild

1958/59- fairly cold

1961/62- fairly cold

1999/00- mild

2005/06- average

2006/07- very mild

2011/12- mild

I don't see much evidence of a correlation here. There may have been a slight relationship with mild winters since 1795, but it is unlikely to be large enough to be statistically significant, and there were many warm Septembers followed by cold winters in the late 1600s and 1700s.

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I generally agree with the warm September theory reducing the chances of a cold winter to follow. This certainly seems to have been the case during the last 15 years anyway.

Why would it? There is a two month interval between the two. So why would a four week period have an effect on a thirteen week period of which the start of it is nine weeks from the end of the four week period?

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Although September 1978 was slightly warmer than average, it wasn't extraordinarily so; with a CET of 14.2 it was only around 0.6*C above average, so it is stretching it a bit to call that a significantly warm September. I also do not think that Sept 1978 had any notable hot spells of the likes of mid Sept 2003, early Sept 2004 / 2005 / 2006 / late Sept 2011 etc. In many ways Sept 1978 was very similar to Sept 2009, and the overall CETs for Septembers 1978 and 2009 were identical. You are correct about autumn 1978 being a very warm one, indeed October and November that year were further above the average than September was, with some notable warm spells for so late in the year, and there was no sign of any cold spells until around November 24th / 25th.

Winter 2005-06, although respectable by 1990s and 2000s standards, it was close to the long term average and certainly not a particularly cold one by any means - certainly nothing like 2009-10 for example.

Thanks for the response, I wasn't sure how mild Sept 78 was, and you are right it wasn't that mild. Looking at the stats, warm spells in septembers over the last 30 years or so has been met with mild winters in the main.. but this could just be a statistical quirk..

Autumn 78 was uncannily similiar to Autumn 09, both saw a mild september followed by much warmer than average conditions in October and for a time November, although it became very cold at the tail end of Nov 78 unlike 09 which was very mild almost throughout. The following winters were also very similiar though the cold took a bit longer to take hold in december 09 than in 78.

Back to the coming September, I foresee a warm start followed by a very unsettled and quite cool second half, however, the warmth at the beginning will mean a milder than average month - my estimate is 14.5 degrees.

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I'll guess at 14.7C, please...

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What are the coldest and warmest Septembers? I am thinking this could be a significant warm one!

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What are the coldest and warmest Septembers? I am thinking this could be a significant warm one!

Would have to be something exceptional to beat the 16,8c set in 2006.

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15.4C, a very warm first week/ten days, followed by more unsettled weather mid-month and mixed to finish with some warm periods. above average overall.

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I'm going for 15C well above average.

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16.3C, let's go. Mr sunshine, show me what you've got!

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My prediction is 14.2 C.

Here are the September CET stats (I have been on holiday, so later than usual) ...

22.6 ... warmest day (2nd, 1906)

16.8 ... warmest (2006)

16.6 ... second warmest (1729)

16.3 ... third warmest (tied 1865 and 1949)

14.5 ... average 2001-2011

14.0 ... average 1981-2010

13.7 ... average 1971-2000

13.6 ... average 1961-1990

13.5 ... average 1701-1800, 1901-2000

13.3 ... average 1659-2011

13.1 ... average 1801-1900

12.6 ... average 1659-1700

11.3 ... coldest since 1952 and tied 14th coldest (1986)

10.7 ... coldest recent month and sixth coldest (1952)

10.6 ... fifth coldest (1703)

10.5 ... coldest (four tied, 1674,1675,1694,1807)

4.9 ... coldest day (28th, 1824)

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