Mid-August washout: explained

  • Parts of the south-east were deluged with heavy rain today. Photo: Daniel Leal-Olivas/PA Wire/Press Association Images.

    Synoptic chart for tonight (0000UTC 14/08/2015) showing fronts extending north across the UK.

    Thousands of lightning strikes were recorded earlier in the south-east, with further thunderstorms brewing over northern France.

  • Mid-August washout: explained
    13.08.2015 14:39


    Whilst Scotland and Northern Ireland have enjoyed a fine summer’s day, conditions have been a lot more unsettled further south in the UK with torrential rain and lightning. High rainfall totals have been reported as pulses of heavy rain have been edging northwards over the southern half of England and Wales, with further rain to come tonight along with active thunderstorms.

    When preparing the forecast several days ago, it was apparent on the weather charts that an unseasonal spell of weather was likely to evolve through Thursday and into Friday this week due to a complex synoptic set developing to the south of the UK. The initial component was a surge of hot and humid air originating from Iberia, spreading north through France into the English Channel, and this is often the ingredient for the more severe summertime thunderstorms seen in the UK. However, it was an accompanying low pressure system developing in the Bay of Biscay that caught the eye of meteorologists. Low pressure in basic terms is an area of rising air in the atmosphere that encourages the formation of precipitation, and this in conjunction with the hot air mass meant that the likelihood of heavy rain and thunderstorms was on the cards.

    The first signs of trouble were apparent during the early hours of Thursday morning as thunderstorms crept north over the English Channel into southern coastal counties of England. Areas of heavy rain continued to edge north as the morning progressed, with particularly heavy bursts in south-east England where more than 3000 lightning strikes were detected in a few hours, reflective on the huge amounts of energy being lifted and released in the atmosphere. Frittenden in Kent recorded an impressive hourly total of 14.4mm of rain this morning with surface flooding an additional hazard also being reported in the region. Conditions are not likely to improve through the coming night as a more defined frontal zone develops over the UK (see synoptic chart to the left) allowing heavy rain to become more widespread. Unusually for summer and in contrast to recent weeks, western Scotland will probably be the driest part of Britain through this current unsettled spell.

    With rain only gradually clearing east through Friday, high totals can be expected quite widely with some parts potentially accumulating more than 50mm through a 48 hour period. With similar totals being the monthly average for August across parts of the UK, it is reflective of the variable conditions we have experienced so far this summer. The good news is that once the rain has cleared on Friday much more settled conditions are to be expected through the weekend and into the early part of next week. Whilst temperatures won’t be overly impressive, there will be some sunshine around and it will be a considerable improvement to the current wet weather.

    By: Nick Prebble