Late May bank holiday forecast hints at early summer warmth

This year the late May bank holiday falls on Monday 25th May, bringing with it the final long weekend before summer and the start of the school half term break for many. It is a time of year which can bring sunshine and very warm weather, but the UK’s late May climate is often highly changeable. Some years deliver early summer warmth and lengthy sunny spells, while others bring outbreaks of rain, blustery winds and temperatures closer to spring than summer.

Arctic air has kept May cool to date

The first half of May has been cooler than normal. Provisionally, up to the 14th, the Central England Temperature (CET) stands at 10.7°C, which is 0.4°C below the 1961–90 norm and 1.3°C below the 1991–2020 average.

These cooler conditions are the result of winds frequently blowing from a northerly quadrant, drawing Arctic air southwards across the UK and keeping warmer air bottled up well to the south. However, there are signs of a significant change taking place over the coming days, as this cold air mass is replaced by a much warmer one.

How the weekend is shaping up across the UK

With the bank holiday still around a week away, pinpointing exact day by day details remains difficult. Weather models are continuing to firm up on the pressure patterns across the North Atlantic region. However, the overall trend suggests pressure will rise across the UK, increasing the likelihood of more settled and warmer conditions.

The box and whisker chart above shows the ECMWF AIFS ensemble pressure forecast spread for York, starting on Saturday 16th May and running through to Saturday 30th May.

The vertical axis represents pressure and the horizontal axis represents time. Therefore, the signal for pressure to rise is shown by the boxes being plotted at higher levels. This becomes clear from Wednesday 20th May onwards. Another thing these plots help to establish is the level of forecast confidence. The height of the box and whiskers represents the range of outcomes in the ensemble model. Basically, the taller they are, the wider the range of outcomes shown by the individual ensemble runs and the lower the level of forecast confidence.

Plots for locations farther north and south are quite similar. That suggests northern locations could be just as likely to have a period of fine and settled weather as southern ones. At this time of year the northern half of the UK can experience some of its most settled weather, with high pressure building farther north rather than primarily influencing southern Britain.

Will it become very warm?

High pressure at this time of year increases the likelihood of warmth. The long days, strong sunshine and sinking, warming air at least provide the ingredients for real warmth or even an early taste of heat. Indeed, computer models are showing temperatures rising in the coming days. However, given recent conditions that in itself does not mean a great deal, but the chart below helps to put some flesh on the bones.

It follows similar principles to the one above, but this time shows forecast afternoon temperatures for London. The message here is that they are likely to rise significantly from Wednesday 20th May, with the ensemble median reaching 25°C by Friday. The skew looks fairly neutral, with comparable spreads of runs below and above this value. 

As we head through the bank holiday weekend, the median initially rises further on Saturday before dipping on Sunday and Bank Holiday Monday. However, it is not until Wednesday 27th May that the downward trend really becomes apparent.

In other words it suggests there is a good chance of warm conditions persisting through the bank holiday weekend and into the half term period, with a chance of it becoming very warm for a time. Temperatures in the upper 20s°C are possible and even 30°C cannot be ruled out at this stage. Plots for locations farther north exhibit similar trends, albeit with generally lower values, though still pleasantly warm for many.

Bank holiday looks hopeful, but uncertainty increases for half term

As ever it is important to caveat weather predictions this far ahead by noting there is still plenty of time for things to change. Nonetheless, at the moment, prospects for the bank holiday weekend look encouraging if you are hoping for warm conditions with dry periods.

The warmth could extend northwards across much of the UK and may last into the start of the half term period. With that said, there is support from computer models for more generally changeable conditions to return through the middle of the half term week.