Archive | October, 2012

November 2012 – Thoughts & Analysis

31 Oct

We are clearly progressing through Autumn at quite a rapid pace and with October now behind us, attention turns to November.  November, like October, can continue to provide quite significant extremes of weather.  Overall the risk of frosts and wintry precipitation does increase on average as temperatures continue to fall.  Autumn gales can often become noteworthy through this month with notoriously persistent spells of wet, windy and potentially even stormy weather.

At the time of writing this blog I do believe November is likely to be a month of two halves, which, to an extent, helps summarise and discuss the expected weather.  However, what I don’t believe will happen is for November to be a ‘typical’ wet, windy and mild washout.  At this juncture it is worth to highlight the current state of the stratosphere.  Up to present the seasonal fall in temperature across the polar regions has been sort of irregular with some minor falls to below average, but equally then recovering to nearer average.  This can be seen on the below image for example;

As can be seen of late temperatures at the 30hPa level are essentially near average and the stratospheric vortex is becoming noteworthy at high altitudes with some particularly cold temperatures (<-70C) at and above 30hPa.  Out of interest the 30hPa level is approximately at an altitude 40,000ft. What has happened so far this autumn however is that the particularly cold temperatures at these altitudes haven’t been able to descend to lower levels and affect the weather within the troposphere and bring about the significant development of a polar vortex across polar regions, which is one of the reason why October was most certainly not a particularly stormy or frequently mild, wet and windy month.  Overall, I don’t expect a major polar vortex to develop through November either and as a result this creates a particularly difficult month to discuss to an extent, as it may well lead to quite sudden and extreme changes in the weather rather than a more persistent and dominant weather-type.

So what’s the first half of the month expected to be like:

The above pattern may look rather strange, given the position of the high pressure.  However and quite simply, I expect low pressure to be influential across the UK and other northern areas of Europe during the first half of November, whilst pressure is higher to the west and potentially north-west of the UK.  Clearly the above image unfortunately doesn’t extend far enough west, but I expect quite a meridional pattern with higher pressure to the west or north-west of the UK, whilst low pressure dominates across the UK.

This pattern is likely to lead to predominantly cool, if not rather cold conditions, with temperatures at or slightly below average and with clearly precipitation totals above average.  Both the GFS and the ECMWF ensemble mean charts highlight this scenario well;

Both these forecast charts out to 240hrs (10th) clearly show this expected pattern.  The resultant weather throughout a large part of early November is likely to be characterised by rather cold west or north-westerly winds and potentially even northerly winds at times, but nothing sustained.  Showers and longer spells of rain are likely nationwide with some windy conditions as well.  Precipitation is likely to be wintry especially across northern hills and overnight frosts are also possible as well.  The high pressure to the west or potentially south-west as well, may well build into parts of southern areas of the UK at times, so whilst overall I do expect the first half of November to be predominantly unsettled, some drier conditions are temporarily possible.
The second half of the month is interesting.  As I highlighted earlier in the blog at the present time I don’t expect a sudden development of a polar vortex in and around Greenland, so I don’t expect a particularly zonal or unsettled second half of the month, in fact the opposite, which the below image tries to highlight;

Longer range forecast information, at the moment, is signalling a potential reversal of fortunes with lower pressure perhaps becoming influential across central and southern areas of Europe, whilst higher pressure may well become dominant to the north of the UK in some shape or form.  This kind of particular setup is difficult to predict and as a result I have low confidence at the moment over the broader synoptic developments during the second half of the month.  However, despite this, I do believe that some colder weather may develop at times later in the month as the weather patterns become more blocked.  Clearly to what extent is unknown, it may well lead to a temporary ‘cold snap’ for a few days, or potentially something more prolonged.  Overall I expect temperatures through the second half of November to be average at best, if not trending slightly below nationwide and precipitation totals may become near or below average.
So in summary November looks to be quite a changeable and interesting month.  I don’t expect a mild, wet and windy month from start to finish.  The second half of the month in particular could become very interesting and from the middle of November onwards the final signs and signals should be in place as well to give a better idea as to how early winter may develop.  This could go really go either of two ways, increasingly unsettled, zonal and mild as per late November and December of last year, or potentially remain quite blocked with a higher chance of colder conditions into December. Safe to say though November is unlikely to be a mild month.

I’ll attempt to review the first half of November mid-month and then also take a look at December towards the end of the month as usual.

Regards to all,

M.