Unsettled weather for the early May bank holiday?

At the time of publication there are still 10 days until the bank holiday. Therefore, it is not possible to be confident about the exact weather conditions, but some likely trends are emerging. In recent days it has often been rather cold, particularly in the east, and during the nights frosts have been widespread. The lower than average temperatures have been caused by high pressure centred in the Atlantic and Arctic air moving southwards across the UK. Through the coming week there are signs of changes taking place. 

Low pressure dominates

Computer models are showing a warmer air mass moving over the UK by the start of May. Upper air temperatures which have been below the 30 years average are forecast to rise above it. Temperatures down at the ground level are also expected to rise and there is the potential for it to become quite warm.

Nonetheless, at this time of the year cloud and rain are increasingly important factors. In the winter months they are often associated with mild conditions, but in late spring and through the summer their effect is usually to keep it cooler than it would otherwise be. That's because the sun is now very strong and even under a chilly air mass temperatures will quickly rise at the surface in sunny periods. Block off the sun and the mercury won't rise.

Currently, the likelihood is that areas of low pressure will remain close to the UK through the Bank Holiday weekend. Therefore, the risk of cloud and showers or even longer spells of rain is quite high, although in sunny spells it will probably feel warm and humid.

The chart below shows forecast pressure patterns for Saturday, 4th May. Low pressure is centred to the southwest of the UK, suggesting showers or longer spells of rain would be pushing across the UK.

There are signs of low pressure systems being displaced further south relative to the average. This indicates the risk of wet conditions in the usually drier parts of the UK being higher than normal. Therefore, even southern and south eastern England could see showers or longer spells of rain. 

Warm and humid in the sun?

Although warmer air is expected to move across the UK in the run up to the bank holiday weekend, the range of temperatures being forecast by computer models is still very wide. This is not just due to the air mass, but also the potential for cloud and rain which, as already discussed, will have a big impact at this time of the year. 

The chart below shows forecast maximum temperatures on Sunday, 5th May from one particular model run. It is showing a very cold day with maximum values in single figures even in much of the south.

The maximum temperature forecast chart below is from another run and is at the opposite end of the scale. In the London area a warm 24C is shown. Even in parts of north eastern Scotland a pleasant 18C is predicted. 

These two scenarios are at the extreme ends of the spectrum and it is more likely that temperatures will be somewhere in-between. With that said, it wouldn't be surprising if it turned warm and humid at times, particularly in the south east. Heavy and thundery showers are a possibility if low pressure is close by and a warm air mass is being pulled up from a south or southwesterly direction.

Keep the umbrella handy

There is still time for things to change for the better, but at this stage the prospects for the bank holiday weekend are looking very mixed. Low pressure is expected to be the dominant feature of the UK's weather so the risk of showers or longer spells of rain is high. Nonetheless, there should be some drier and brighter spells too. Also, there is a chance of reasonable temperatures as a mild southwesterly flow is pulled up across the UK. 

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