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August 2012 Forecast Verification

Thundery wintry showers


Here was the August 2012 forecast:
[quote]August will complete a full hat-trick of cool unsettled summer months, although the first third of the month will continue on from late July in the sense of offering more in the way of "summery" weather.

A slow-moving low pressure area will continue to drift eastwards across the British Isles between the 2nd and 6th of the month, but the weather will be rather brighter and warmer than we've seen during most recent low pressure spells. Frontal activity will be limited and thus the emphasis will be on sunshine and showers. Many places will be mostly dry on the 3rd with just one or two showers, but showers will be heavy and thundery over a wide area of the country on the 4th and 5th. On the 6th the heavy thundery showers will be concentrated in eastern England while other regions will have well-scattered showers and more frequent sunshine. Temperatures, at 18-22C, will be about average for the time of year.

Between the 7th and 10th August high pressure is expected to temporarily build from the south which will promise a brief dry sunny interlude, particularly for eastern and southern Britain. Ireland and western and northern Scotland will be prone to patches of cloud and light rain, but elsewhere it will be generally dry and sunny and the southern half of England will see highs between 25 and 28C. Around the 10th/11th, however, a new depression will head in off the North Atlantic and bring some rain with it, and as the fronts push against relatively warm continental air, thunderstorms are likely to break out over central and eastern parts of England.

For the middle part of the month (12th-20th) our weather will be dominated by low pressure, with a dominant high pressure system in the mid-Atlantic increasingly sending cold northerly airstreams our way. The emphasis is likely to be on sunshine and showers with thunder and near-average temperatures until midmonth. Towards the 20th, however, we will most likely see an increased emphasis on frontal rain, extensive cloud cover and below-average daytime temperatures.

The last third of the month is likely to see the high pressure in the mid-Atlantic ridge towards the British Isles and then flatten towards the Azores towards the month's end allowing westerlies in. Thus, between the 21st and 25th the weather may turn progressively more settled from the west, though rather cool with frequent northerly winds. South-western areas are most likely to experience long dry sunny periods, with northern and eastern areas most likely to be prone to cloud and cool daytime temperatures. Towards the month's end, as the westerlies set in, it will turn dull and wet in northern and western regions whereas eastern and southern England will have a brief interlude of warm dry sunny weather before unsettled westerlies establish at the end of the month.

August 2012 will be another fairly cool summer month and I am predicting a Central England Temperature of 15.9C. Mean temperatures will generally be 0.5 to 1.0C below the 1981-2010 average in all regions, with the departure from normal slightly greater by day than by night.

Rainfall in most regions will be 20-50% above normal, but locally totals will be swollen by thundery downpours in the first half. Generally speaking, central and eastern England will have the largest rainfall excesses while northern and eastern Scotland will have near average rainfall.

Sunshine will come out 10-20% below normal in most regions, with sunshine likely to be a little above normal in the first half of the month, but below normal sunshine will resume in the second half. There will be some regional variation though; Wales and south-west England are most likely to have a significant shortage (around 20-30%) whereas Northern Ireland and parts of southern England and northern Scotland may have close to average sunshine for August as a whole.

[b]Synoptic pattern and UK weather[/b]
I think I identified the pattern for the first third of the month very well, but didn't score so well on the patterns for the remaining two-thirds. For the middle third of August, the low pressure was centred further west than I expected, and consequently the weather was rather warmer (though still showery, bright in eastern areas, with occasional longer spells of rain).
I had envisaged a brief ridge of high pressure from the west around the 21st-25th before westerlies came in towards the month's end, but instead the ridge didn't materialise and we went straight into a westerly type.

[b]Temperature, sunshine and rainfall anomalies[/b]
Judging by the stats over at Philip Eden's stats (Climate-uk) it appears that I got the sunshine anomalies spot-on. For rainfall anomalies, I got the UK-wide average close to right (as most regions were indeed 20-50% above average) but fell down on predicting the regional distribution, as central and eastern England actually ended up among the driest regions, not among the wettest as predicted. Temperatures were about 0.5-1.0C higher than I had predicted due to the southerlies in the middle third of the month, but the first and last third of the month were about as warm as predicted.


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