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September 2012 Forecast Verification

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Thundery wintry showers


[quote]High pressure will be dominant over central and northern Europe during the first half of September, which will bring plenty of warm dry sunny weather to most of southern and central Britain, although Scotland and Ireland will always be more prone to banks of cloud. During the second half of the month, low pressure will take up residence to the west of the British Isles giving us a changeable but rather warm south-westerly type.

Dry, sunny weather will dominate over most of England and Wales on the first three days but with more cloud over Scotland and Ireland. A weakening belt of cloud and drizzle will head south-eastwards over England and Wales on the 4th, with brighter weather following from the north-west, and bar a few isolated showers over western and northern Scotland, it will become dry again.

Between the 5th and 9th September, the weather over Wales, together with central, southern and eastern England, will be consistently sunny and dry, with generally warm daytime temperatures, though not exceptionally so. Most places will see highs between 21 and 24C. Scotland, Ireland and Cumbria will be prone to more cloud and some light rain, particularly on the 6th and 7th, although these areas will be dry and sunny on the 5th.

High pressure will start to pull away between the 10th and 15th which will allow a changeable south-westerly type to establish over north-western Britain. This will bring some rain belts interspersed with brighter showery weather, with the majority of the showers confined to northern and western Scotland. For most of central, southern and eastern England, though, there will be long dry sunny periods and rainfall amounts will be very small. Temperatures will continue mostly above normal, although daytime temperatures may drop a little below normal at times over western Scotland and Northern Ireland.

Around the 16th/17th we can expect a large depression, containing the remains of a tropical hurricane, to approach the British Isles, and this will herald the shift towards more unsettled conditions. A bout of wet and windy weather is expected, with gales possible in western areas. After that, the winds will be mostly southerly or south-westerly and this will bring belts of rain interspersed with brighter showery weather, with some of the showers heavy and thundery, particularly in the west and south. Temperatures will continue rather above normal except over western Scotland and Ireland. During the last week of September lowest pressure will transfer north-eastwards and give us a more "traditional" westerly type, with temperatures returning to normal, and the majority of the rain will affect north-western parts of the country, with small amounts over central, eastern and southern England.

September 2012 will be a warm month. I am predicting a Central England Temperature of 15.4C, with temperatures generally ranging from 1.5 to 2.0C above the 1981-2010 average in eastern England to 0.5-1.0C above in Ireland and western and northern Scotland.

Rainfall totals will be slightly above normal over western Scotland and Northern Ireland, but elsewhere it will be a generally dry month with rainfall shortages of 20-40%. Shortages of over 50% are expected quite widely over the eastern half of England and also in south-east Scotland.

Sunshine totals will be 10-20% below average in western and north-western Scotland, but near average over most other parts of Scotland, together with Ireland and Cumbria. Elsewhere it will be a sunny month with excesses of 10 to 30% in most places. Sunshine is likely to be 40-50% above at some places across a belt of eastern England from Northumberland down to East Anglia.

I was pleased with how the June, July and August forecasts went, but my September forecast wasn't one of my better ones. The first 10 days of the month went much as predicted, as did the change towards less settled conditions in the middle third of the month, but the weather turned out somewhat cooler than I expected after midmonth and we did not get a large depression around the 16th/17th. That prediction had been based on the likelihood of an ex-hurricane approaching the British Isles, but in reality its remnants passed by harmlessly to the north of Scotland. A second ex-hurricane then swung across the country on the 24th-26th and gave those exceptionally large rainfall totals in northern England.

It goes to show how much of a difference these depressions containing remnants of tropical storms can make to the UK's weather, and they can certainly provide forecasting headaches especially at long range.

As a result of the above, temperatures were generally a degree or two down on the values that I had forecast. Rainfall totals were looking set to be similar to what I had predicted before the big depression on the 24th/26th raised totals above average over many parts of the country, while sunshine totals were quite similar, maybe a little higher than I expected in N and W Scotland and lower in eastern England.

The saying, "you win some, you lose some" certainly applies to long-range forecasting!
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