In recent weeks, or even months, areas of high pressure have dominated the UK's weather. Their positions have varied leading to fluctuating temperatures. When centred to the west, high pressure generally brings cooler conditions because the air circulates clockwise, drawing it down from the north. Conversely, when high pressure is to the east warmer air from the south moves in.
However, regardless of its exact position the descending and drying air under an area of high pressure makes significant rainfall much less likely. Therefore, the key question is: how much longer will high pressure remain in control of the UK's weather?
Ensemble computer models continue to show a signal for pressure to remain higher than normal as we head into the final third of May. Of course forecast confidence begins to decrease significantly at this range. That said, the consistency between different computer models and successive runs suggests an increased degree of confidence.
The forecast chart below is generated from the ECMWF ensemble model and is valid for Monday 19th May. It shows the average of all the individual runs in the model. While this can mask different possible scenarios, the consistent appearance of high pressure across runs makes it reasonable to conclude that it is likely to remain close to the UK.
As ever, there are important caveats. At this time of year rain in the UK increasingly results from convective shower activity rather than Atlantic weather fronts, which tend to be responsible for wetter conditions in autumn and winter. Showery conditions can lead to highly variable rainfall totals over short distances.
Nonetheless, it’s possible that areas of low pressure could at times approach the northwest and southwest of the UK. These would bring the potential for wetter spells, particularly from the southwest, which could result in heavy and thundery outbreaks of rain. In fact this type of pattern looks likely to bring a more changeable interlude between 11th and 14th May. However, as already noted, rainfall amounts could be highly variable.
There are tentative indications that a more westerly-based regime may begin to return towards the end of May. This would lead to more unsettled and typical conditions, with wet spells becoming more frequent. However, this is still some way off, and there is plenty of time for computer models to shift back towards a more settled outlook.
Another consideration is the climatological tendency for the westerly flow across the Atlantic to reinvigorate in June and early July. This pattern, known as the 'European Monsoon' or the 'return of the westerlies,' has been identified in past years. After fading in spring a more mobile westerly flow can strengthen during the first half of summer. It’s something to keep in mind over the coming weeks.
Nonetheless, the ground is becoming very dry, and a prolonged wet period would be required to reverse that trend. If the dryness persists, the likelihood of recording very high temperatures will increase significantly—especially if a Spanish Plume weather pattern develops. Such a setup can pull very warm air northwards from Africa and southern Europe. The highest temperature recorded in the UK so far this year is 29.3°C, and that could be quickly surpassed if a plume were to materialise.
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