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July heatwave outlook

Storm Chris the disrupter?


08/07/2018

Early July continues where June left off

The provisional UK mean temperatures for June was 14.8C, which is 1.8C above the 1981-2010 average and the third warmest since 1910. The warmth wasn't restricted or focused on the south east of England as is often the case and the highest temperature of 33C was recorded at Porthmadog, Gwynedd in North Wales. It was a very dry month with rainfall 48% of the average over the UK as a whole and no higher than 10% in parts of the south. 

July started with more hot and dry weather. The monthly forecast update issued in late June suggested a warm month but with a chance of unsettled spells developing during the second half. Is that still expected?

Sunflowers

Storm Chris the big disrupter?

In the short term a lot of dry weather is expected but temperatures dip. As we head towards the middle of July things become more uncertain as ex-Tropical Storm Chris makes its way across the Atlantic and possibly has an impact on the UK's weather.

Ensemble models suggest the possibility of more a changeable picture leading to an increasing risk of rain, especially in the north west. Nonetheless a significant number of individual runs in the ensemble show pressure remaining high over much of the UK and drier conditions continuing at least in the south. There is also the possibility of Storm Chris acting as a "turbocharger" for the heat by becoming a slow moving area to the west of the UK and pushing very warm air originating in the tropics northwards across the UK. 



The GEFS postage stamp plot below shows Mean Surface Level Pressure (MSLP) and 500hPA height forecasts from each model run on Thursday 19th July.

1) Most of them show higher pressure over the southern half of the UK 

2) Quite a few have low pressure to the west or northwest, influencing the UK's weather to a varying extent

GEFS06z 850hPa temperatures and rainfall, July 2017

The GEFS postage stamp plot below is for the same time and shows maximum forecast temperatures. 

1) The warmest conditions are mostly in southern and central regions

2) In the south most stamps show above average temperatures and a number keep it very warm or hot

3) In the north temperatures are broadly close to the seasonal average

Conclusion

In the short term the heatwave conditions are expected to ease but it remains mostly dry. Temperatures could then climb again but during the middle of July ex-Tropical Storm Chris could disrupt weather patterns across the North Atlantic. That may lead to more changeable weather returning southeastwards across the UK. However, there is also a possibility of Storm Chris injecting more warm air into the flow over the UK and in turn that could send temperatures even higher, at least for a time.

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