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Is a May heatwave coming?

Goldilocks weather more likely


18/05/2018

Roasting end to May?

There has been much discussion in recent days about the possibility of a 2 week heatwave in Britain and a record breaking hot late May Bank Holiday. Is that likely?

The latest forecast data from ensemble models points towards a prolonged period of warm weather in the southern half for the UK but the north looks cooler at times. The 16 day plot below shows the London 2m maximum and minimum temperature forecasts from each of the GEFS 6z ensemble model runs on Friday 18th May.

GEFS 6z maximum temperatures

Things to note from the plot:

1) None of the runs show maximum temperatures climbing above 27C (81F).

2) A significant number of runs show minimum temperatures falling to single figures.

3) The average of all the runs has daily temperatures frequently peaking at around 20C (68F). However the mean can be unrepresentative so not too much stock should be put in it.

What that means

The points and data above suggest there is little sign of "blow torch" (temperatures approaching or exceeding 30C) heat in the short to medium term. Nonetheless the days are expected to be warm or very warm for much of the period, but some chilly nights are possible. It is worth remembering the GEFS has a tendency to underestimate maximum temperatures by a couple of degrees.

Therefore the rest of May could bring a lot of Goldilocks weather, "neither too hot nor too cold, but just the right temperature" to the southern half of the UK at least.

There are potential flies in the ointment. Computer models are indicating that low pressure areas centred over France could push northwards next week. That would lead to an increased risk of heavy showers and thunderstorms in southern and central Britain. It also probably remains more changeable in the north west as weather fronts push in from the Atlantic.

Summary

At this stage a "heatwave" doesn't look likely during the rest of May but it wouldn't take much change in the synoptic set-up across western Europe and the North Atlantic region for that to change. The most probable outcome is for warm days during much of the period in the south and the Goldilocks principle could apply. A good deal of dry and sunny weather is likely but there is a risk of heavy showers in the south and more persistent rain in the north west at times.

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