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Changeable first half of July

Published 1st July, 16:42

Changeable weather is set to continue through the coming days, and quite possibly through most of the first half of July. However, there are suggestions of it turning significantly warmer, at least for a time.

Changeable start to July

After the record breaking June things have changed a lot in recent days. Low pressure has become dominant and cooler air flowing around it has moved down from the northwest.

In the next few days it is more of the same, but as can be seen on the ensemble chart below, there are signs of it turning warmer around July 8th. The top half of the plot shows forecast 850hPa temperatures from all of the individual model runs for the next 16 days. Initially they are below the thick black line, the 30 year norm, but from July 7th most climb above it.

GEFS London 850hPa temperatures
GEFS London 850 hPa temperatures and precipitation

By July 8th the forecasts are ranging from about 8°C to almost 20°C, but most are between 10°C and 15°C. These are the values at about 1500 m above sea level, and at the surface it will be a lot warmer. The majority of the runs indicate maximum temperatures in the mid 20°Cs to low 30°Cs. However, most of them show cooler air returning quickly, and there is also uncertainty about how far north and west the warmth would extend. 

The number of precipitation spikes on the bottom half of the plot also starts to increase again. That points towards a changeable theme returning.

What could lead to warm conditions?

An area of low pressure moving in from the Atlantic is the cause. It pulls up very warm from the south ahead of it. As the low pressure pushes eastwards cooler air returns and there is the potential for a thundery breakdown. 

The chart below displays the surface pressure and 850hPa temperature forecasts at 15:00 GMT, 7th July. It shows an area of low pressure just to the west of the UK and warm air ahead of it. At this range the details are very uncertain, but the same general pattern is being forecast by a number of computer models. 

GFS 850hPa temperatures
GEFS 2m perturbation max temperature forecast

One other thing to note is that a few of the computer model runs show hot conditions becoming established for longer over the UK. Those are in a minority at the moment, but they should not be discounted.

Summary

On balance the first half of July is looking changeable, nonetheless, it could turn much warmer around July 8th. If that happens the warmth probably won't last for long as cooler and showery air is likely to push back in from the Atlantic. With that said, a few computer model runs are favouring a longer period of fine and very warm weather.

Latest charts


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