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European Monsoon

Change on the way?


09/06/2018

Overview

As we head through June you may hear the terms "European Monsoon" or the "return of the westerlies" being used. The terms are used to describe periods in June and July when westerly winds become more frequent. In the UK a mobile westerly flow brings in unsettled weather from the Atlantic and is possible all year round. However, there is a tendency for it to become weaker in May before strengthening again in the first half of the summer. 

Muddy festival

By Domharrison (Own work) [Public domain], via Wikimedia Commons

Will the westerlies return this month?

In recent weeks weather patterns have been changing slowly and for much of the time pressure has been high to the north or west of the UK and low to the south. That blocks the flow of weather fronts from the Atlantic and can lead to fine and warm periods. However it is not plain sailing as low pressure areas centred over the Bay of Biscay and France can push northwards and introduce thundery conditions.

There are signs of a change taking places during the middle third of June. Computer models are showing a more typical pattern returning with high pressure centred to the south of the UK. That allows a westerly flow to return to northern areas and perhaps to southern counties if the high pressure becomes centred too far south.

Ensemble model view

Today's 16 day GEFS 06z ensemble computer model shows the transition that is expected. The plot below is for London and the south east. The top half of it shows forecast upper level air temperatures and for the whole period the mean of the individual runs remains above the 30 year average. There are some cooler runs in the set but they are in a minority, so for most of the period the south is likely to be covered by a warmer than average air mass.



In the summer months temperatures at ground level are to a large extent determined by cloud cover and precipitation, therefore a warm air mass does not necessarily equate to warm weather. So is it looking wet? Not really. On the bottom half of the plot some rain spikes are shown but there would probably be a lot of dry periods. 

GEFS 06z London

 

The GEFS plot below is for Glasgow and it covers the same time period as the London one. On the top half of the plot the mean upper air temperature is often close to the 30 year average and sometimes dips below it. On the bottom half of the plot quite a wet picture is shown for much of the period.

GEFS 06z Glasgow

The important point is the Glasgow chart looks cooler and wetter than the London one. That suggests a typical UK summer weather pattern with high pressure to the south and low pressure areas tracking to the northwest. The key factor then becomes the balance between two and how far north the jet stream moves.

In years when the "European Monsoon" brings deluges to all areas the jet stream tends to sit on top of the UK. At the moment the GEFS data is suggesting quite a weak start to the European Monsoon season with the wettest periods being mostly restricted to the northern half of the UK. That could change in the coming weeks but tentative indications at this stage are that it could be relatively benign this year.

Summary

A change is expected to develop during the middle third of June as a more typical weather patterns return. That means the driest and warmest periods are most likely to be in the south with weather fronts from the Atlantic bringing a greater risk of rain to the north of the UK.

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