By Brian Gaze
Computer models are increasingly predicting a period of stormy weather between 27th - 29th September. An area of low pressure is expected to deepen in the Atlantic and approach the UK from the southwest.
Details about its track and intensity remain uncertain, but there is potential for the first named storm of the new season. If that occurs, it will be named Storm Agnes. Heavy rain and disruptive winds pose a risk in parts of the UK.
Currently, computer models are still showing a range of different outcomes. The intensity and track of the low-pressure area are being modeled differently by the simulations. The strongest winds can be expected in quite a small area on its southern flank. Therefore, a shift of 100 miles further north or south will make a significant difference in terms of where the worst conditions will be.
The charts below illustrate some of the possibilities. Firstly, the ECM 00Z update from 23rd September shows the low-pressure system tracking across central Britain, with the strongest winds in southern counties.
Conversely, the Canadian global model positions the storm further north and west. A band of rain is sweeping southeastwards, but if this model is correct, the strong winds will be experienced in the west and northwest of the UK.
Another scenario is offered by the UK Met Office Global model. It also predicts the initial storm tracking further north, hence, the strongest winds are expected in the north and west. However, it suggests another deep low-pressure system quickly following the initial one, bringing wet and windy conditions to much of England and Wales on Thursday night and throughout Friday.
Finally, the American Global Forecast System (GFS) has the track further south. The wettest and windiest conditions affect England and Wales. Scotland and Northern Ireland miss the initial wet and windy conditions, but a second area of low pressure is forecasted to move in from the Atlantic on Thursday, bringing rain and strong winds.
Ensemble models, which are run many times to help account for uncertainty, are also showing a spread of possible outcomes. This emphasises the point about uncertainty, with the details still to be firmed up during the coming days.
As outlined in the previous section, the track and intensity of the low pressure are still uncertain. With that said, gales or severe gales are being shown on an increasing number of computer model runs.
The chart below from the GFS model shows winds gusts of between 50mph and 60mph on Wednesday in much of the south. It's a much calmer picture further north.
The Met Global model, which has the system tracking further north, shows the strongest winds in parts of the north and west. Gusts of 50mph to 65mph affect inland areas, but values of 70mph or even a little higher are indicated in coastal areas of Wales and western England.
As well as strong winds, heavy outbreaks of rain could also lead to disruption. At the present time things are uncertain, with some computer models showing a less windy and wet outcome. However, the advice at this stage is to stay up to date with the short range forecasts through the coming days. There is a real possibility that storm Agnes could arrive during the middle part of next week.
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