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October 2012 forecast verification

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Thundery wintry showers


Here was my forecast for October 2012:
[quote]October 2012 will start off changeable with low pressure dominating, but high pressure over the Azores and mid-Atlantic will often influence our weather, particularly around the middle of the month.

After a showery spell during the first five days of October, with some longer outbreaks of rain on the 2nd, the weather will dry up from the west on the 5th/6th as a ridge of high pressure pushes in from the west. The night of the 6th/7th is expected to be cold, with a widespread ground frost inland. There is then some uncertainty over the track of a southerly-tracking low, but I expect wet and windy weather to sweep in from the south-west between the 8th and 10th which is likely to produce high rainfall totals in some places, with the Midlands, Wales, Northern Ireland and northern England most at risk from the heaviest and most persistent rain. Northern Scotland will stay mostly dry, while southern England will be brighter with showers once the initial rain belt moves through, where it will also become warm with highs of 17-20C in places.

High pressure sat in the mid-Atlantic will settle the weather down towards midmonth but it will still allow weak Atlantic weather systems to penetrate around its north-eastern flank. Thus, the period 11th-16th is likely to be mostly dry, particularly in southern areas, but sunshine will be variable rather than plentiful, and some belts of light rain will crop up from time to time, particularly over Scotland, Ireland and northern England.

Long-term teleconnection signals keep pressure high in the mid-Atlantic around midmonth and this high pressure will most likely start to extend its influence further east in the third week of the month, which will promise a few days of dry, brighter weather with some cool nights and widespread ground frosts.

The high pressure is then expected to pull out into the North Sea during the last third of October which will allow Atlantic weather systems to increasingly influence the weather, particularly over western areas, where some belts of heavy rain and strong to gale force winds are expected at times. Eastern England will hold onto the driest weather during this period. With southerly winds expected, temperatures will rise above the seasonal average.

October 2012 will have close to average temperatures for the most part, but the last third of the month will raise the mean slightly above average. I envisage a Central England Temperature of 11.1C, with mean temperatures set to range from 0 to 0.5C below the 1981-2010 average over much of Ireland and western Scotland to 0.5-1.0C above in eastern England.

Rainfall totals will be above normal over much of Wales, the Midlands and northern England with excesses of 20-50% likely, bolstered by a wet spell between the 8th and 10th. Conversely it will be a dry month in East Anglia and the south-east, and also in northern Scotland, with rainfall shortages of 20-40% expected. Other regions of the UK will have close to average rainfall.

Sunshine totals for October 2012 will be near or slightly above the long-term average, with an excess of around 10% taking the UK as a whole. Southern England and eastern Scotland are likely to be sunniest relative to normal with excesses of around 30%.

Forecast issued by Ian Simpson (aka TWS) on the 3rd October 2012.[/quote]

This wasn't one of my more accurate forecasts. The first week went as expected but the wet weather on the 8th-10th in reality passed away to the south with only the far south seeing any rain at all from it, and then an Atlantic-driven regime took over midmonth with a flattening of the expected mid-Atlantic high.

The weather temporarily moved into line with my forecast around the 19th-25th with an anticyclonic interlude followed by warm southerlies, but an abrupt change saw an unusually cold northerly outbreak on the 26th/27th with snow for some, and the month ended changeable with below-average temperatures. As a result, temperatures were over a degree down on what I had predicted, while sunshine totals were mostly lower over the southern two-thirds of England although similar to what I had predicted over Scotland and northern England. Rainfall totals weren't far off what I forecast.

Overall, though, after a series of forecasts which I felt were quite accurate, September and October's forecasts were rather wide of the mark and illustrate how hard long-range forecasting really is.
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