Weekend weather analysis

For tuition purposes meteorologist Libby Greenhalgh looks back at what happened weatherwise last weekend

Saturday produced a strong southerly breeze in the morning of between 18 and 22 knots, slightly less than forecast. But what was most significant from the day was that it was around midday that the change occurred, as was forecast.

A low pressure centre pushed into the southwest of the British Isles around midday and the associated trough feature (depicted as a simple black line on a synoptic chart) brought along the rain and the change in conditions that were forecast. While winds were expected to drop to as little as 15 knots, the conditions of the day were much less with almost complete calm by 1300, when the trough had passed and the skies began to clear.

The formation of low pressure systems, over Spain and France, were forecast to be the key to the weather for the weekend and proved to be so. The variable conditions were not only produced by some large cloud structures impressing changes in wind strength and direction (as those taking part in the Hamble Winter Series on Sunday morning had direct experience of) but also the change in dominance of synoptic pressure systems.

The cyclonic low pressure system that produced the strong winds on Saturday morning was continually filling and slowly advancing northwards away from the Solent. While a weak low pressure centre developed over Europe producing a fight for dominance that produced the very still and misty conditions of Monday.

For Sunday the forecast of 10-15 knots and generally dry was not met well with rain and about 6 knots from a variable direction between 0900 and 1030. This was due to that nasty cloud that we tried to scamper out from to our racing area.

By 1100 there had been several large shifts but the wind began to settle towards the southwest at an average of 8 knots. By the afternoon a significant cloud feature had developed over the Isle of Wight that brought more shifty and gusty conditions with the rain showers and a more southerly breeze. By mid-afternoon the wind had settled to around 190deg and increased towards the forecasted values of 10-15 knots.

A key point from this weekend would have to be the effect of the cloud feature which was on the local area. As the cloud feature was not part of a frontal system it is hard to forecast, however there are up-to-date satellite images that you can pick up in the morning to help you identify the extent of cloud features. Go to www.sat.dundee.ac.uk which is free for anyone to use and pick the latest image in channel 4 (infra-red).

Outlook

This morning sees the start of the mid-week change as another low pressure and associated frontal systems makes itself felt across the UK bringing more wind (up to 25 knots in the west) and rain. With temperatures not nearly as chilly as Monday at a mild 10degC during the day.

By the weekend the weather could be dominated by a high pressure system ridging over the southern UK. With northern England, Scotland and the west of Wales and Ireland looking set for a generally wet weekend with winds in region of 20-25 knots from south west direction on Saturday and a westerly direction on Sunday.

Southern England will be experiencing the fight of the high pressure centred off the coast of Portugal and the low pressure system centred near Iceland. This will provide us with light north westerly wind in the region of 4-8 knots on Saturday which by Sunday will remain light but have veered to a westerly direction.

Have a look at the synoptic chart here and key through them day by day - note the development of the high pressure off Portugal.

Libby will be back later in the week with more about the weather.

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