Good afternoon and welcome to our long range forecast. This is posted either once or twice a week covering from 4 days ahead to 10 days ahead. In this update covering from Thursday 20th May until Saturday 29th May
Please note there is not a lot of accuracy for this kind of time frame. Anything beyond 4/5 days out becomes really difficult and subject to change so we will be looking at trends. Mainly take with a large vat of salt. We will be using the main two computer models for this. The ECMWF ( European Centre for medium range weather forecasts) which is based in Reading and the GFS (Global forecasting system) which is the American based global model.
This comes from the GFS 06z run and ECMWF 00Z run. I will refer to EC to save on typing.
Thursday 20th May
Some model agreements for next Thursday. The EC brings in a deep area of low pressure across the UK. it will bring blustery winds and rain. For us in the Western Isles we will see overcast conditions with spells of rain. Brisk winds. The GFS also bring a low pressure across the UK but brings it further North then then EC and would bring more in the way of rain across the Western isles which would be heavy at times with brisk winds. As you can see some uncertainty on the exact track of the low pressure system.
Friday 21st May
Again model differences for next Friday. The EC moves the low pressure away from the UK. Behind it we will see a showery airflow. For us in the Western Isles we are going to see a mixture of sunshine and showers. These heavy at times with the risk of hail. The GFS stalls out the low pressure system so we would see spells of rain which would be heavy at times with occasional sunny spells and blustery winds.
Saturday 22nd May
Some further model differences the EC suggest that a cold airflow coming down from the North and we will see a day of sunshine and showers and these will be heavy at times. Some decent spells of sunshine in between. A shallow area of low pressure sits across North East England. Now the GFS still has the area of low pressure mentioned above where low pressure sits close to the Western isles which would bring further rounds of rain which would be heavy at times. But this would move away leading to a dry and sunny end to the day.
Sunday 23rd May
Some good model agreement for next Sunday. The EC has a showery start to the day but this clears away as high pressure builds in and we would see a fine and dry afternoon and evening with plenty of sunshine and it would feel warmer. The GFS also brings in high pressure across the islands with sunshine and cloud but brisk winds as well.
Monday 24th May
Some similarities for model agreement for next Monday the EC brings brisk winds and spells of rain across the Western isles the rest of the UK is mainly under high pressure. The GFS on the other hand has a fine and dry start with sunshine and cloud across the Western Isles. Before rain moves in the Uist and Barra later in the day. The next deep area of low pressure develops out in the Atlantic.
Tuesday 25th May
The EC has high pressure mainly in charge of the weather across the UK with some rain and scattered showers. A mixture of sunshine and cloud. A brisk South Westerly airflow sets up so it will be milder. But breezy. The GFS keeps the area of low pressure close to Ireland. Rain and showers moving across the Western Isles with brisk winds. Feeling mild.
26th, 27th, 28th and 29th May
Now this is the really long range. So take it with a large amount of salt and is the silly range. The first couple of days would see rain and showers with blustery winds with high pressure building in for the final two days with some decent and prolonged sunshine and light winds.
Featured photo of Stornoway taken by Jess O’Loughlin.