A wet week to come

  • Figure 1 - raindrops on a window pane (photo credit: webneel.com)

    Figure 2 - Cumulative radar showing estimated rainfall for the 24 hours to 1800 GMT on 13/11/15

    Figure 3 - Total column water vapour across the UK

  • A wet week to come
    14.11.2015 15:56

    The first half of autumn tended towards dry and settled weather with blocking areas of high pressure keeping rain-bearing weather fronts and showery activity at bay. And with the jet stream positioned to the north of the British Isles, low pressure systems from the North Atlantic were steered away from our shores.

    There has been a big change as we’ve moved into November with an unusually powerful jet stream drifting southwards and putting us in the firing line for some wild and wet autumn weather. Named by the UK Met Office as Storm Abigail, severe winds were not the only feature to affect the UK on Thursday and Friday. In the 24 hours to 1800 GMT on Friday 13th, an estimated 107mm of rainfall fell across the hills of western Scotland (see figure 2).

    There will be little let-up in the soaking conditions as we head into the second half of November with plenty more rain on the way. Overnight and into Sunday 15th, the main focus of the rain will be on the northern half of the UK with some high totals expected across the hills and mountains of western Scotland, north-west England and north Wales. In fact, some forecast models suggest that these areas could see in excess of 100mm of rainfall during the 48 hours to Sunday night, with up to 200mm possible in a few spots. There are a number of factors responsible for producing such copious rainfall totals (see figure 3). Firstly, the rain is being produced from a slow-moving warm front, allowing the rain to fall continuously for several hours. Secondly, there is a large amount of warm air advection associated with this frontal system, akin to a ‘conveyor belt’ which will further enhance the rainfall. Lastly, the areas likely to experience the highest totals will do so because of a feature known as ‘orographic enhancement’. This means that when strong winds are associated with a spell of rain, many upland areas exposed to the wind (in this case western hills and mountains exposed to the strong south-westerly flow) will record substantially higher totals than similar locations at lower levels and/or sheltered from the wind.

    Into Monday, and parts of Scotland will remain wet with squally showers and gusts of up to 70mph in some exposed locations. Meanwhile, southern Britain will see a new low pressure system drift in from the west bringing a soggy evening rush hour for many. Following a brief drier interlude on Wednesday, another deep low pressure system will arrive into northern Britain bringing another spell of rain. Once again, due to the strong westerly airflow and track of the frontal systems, the aforementioned upland parts of northern and western Britain are expected to see the highest rainfall totals, further adding to an already soaking week in these areas.

    You can keep up to date on the rain in your local area by downloading our #RainToday app for your tablet or smartphone. The app can alert you when rain is on its way and our predictive radar shows where the rain is forecast to be up to an hour ahead!

    By: John Lee