3 Jul 2012

Why the risk of flash flooding remains

Yesterday saw confirmation of what most of us expected. June was wet – very wet. In fact it was the wettest June on record for the UK, with double the average rainfall.

Northern Ireland and Wales also had their wettest June on record, with 235 per cent and 238 per cent of their average June rainfall respectively.

The wet weather has seen some parts of the UK hit by flooding, with the most recent case experienced in the Midlands and northern England at the end of last week.

I’ve just had a look at the weather charts for the rest of this week and the potential to see further flash flooding remains. Whilst it’s impossible to say exactly where could be affected, some places are at greater risk than others.

What’s bringing the risk of further flash floods?

Most of the time, low pressure systems move through giving a spell of rain, followed by drier weather. However, during the next few days an area of low pressure will move over the UK and get stuck.

When low pressure is present, air is more prone to rising in the atmosphere and as a result water vapour condenses to produce cloud and rain.

An additional cause of the air rising at this time of year is the strong sun heating the land. This enhances the upward motion of the air, causing it to rise further and faster. As a result, clouds are bigger, with cumulonimbus developing that are capable of producing thunderstorms and intense downpours.

The effects of these downpours can be insignificant if they move through on the wind, but in the middle of an area of low pressure there is little or no wind. This means that thunderstorms can affect the same place for a long period of time, with torrential rain potentially causing flash flooding.

Which areas are at greatest risk?

Whilst there is some uncertainty at the moment, it looks like the spine of the UK is at greatest risk from slow-moving intense downpours for the rest of the week as the wind becomes lighter from Wednesday onwards.

The latest forecast information for Wednesday to Friday suggests that the Midlands, northern England, north Wales and inland parts of Scotland and Northern Ireland are most prone to seeing some intense rain at times.

Despite these areas being most at risk, other parts of the UK are not immune from heavy showers.

What’s the outlook for July?

There’s no doubt that July has picked up where June left off. Looking a bit further ahead towards mid-July, there are no signs of any prolonged settled spells of sunny and very warm weather.

The jet stream looks likely to remain to the south of the UK which means that low pressure will dominate our weather for a little while yet with rain at times.

Whilst the latest Met Office 30-day trend forecast for the second half of July suggests that it could turn a little bit drier and warmer, uncertainty remains high, so this could easily change.

In the meantime, you can get the latest five day forecast on the Channel 4 Weather website and you can send me your weather reports and pictures at anytime on Twitter – @liamdutton

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