Storm Isha to batter UK

A deep area of low pressure named Storm Isha by the UK Met Office is forecast to bring disruptive winds to the UK during Sunday night and into Monday. Heavy outbreaks of rain add to the potential problems.

Storm Isha is forecast to track just to the north of Scotland, with very strong winds on its southern flank affecting the whole of the UK. The general pattern shown by the various computer models is broadly consistent, but there are differences in the details. 

The Met Office have issued amber warnings for strong winds and yellow warnings for heavy rain. 

What do the forecast models show?

Guidance is still varying, but large parts of the UK are likely to record gusts of between 50mph and 70mph on Sunday night and Monday morning. The charts below from the ICON model show the worst conditions affecting the north and west.

At midnight on Sunday peak gusts are close to 85mph in Wales, north western England and exposed parts of Northern Ireland.

Moving forwards to 03:00 on Monday, the strongest winds are now in northern England and parts of Scotland. Once again, gusts in exposed locations are close to 85mph, but more generally around 70mph to 80mph.  

The charts from the ICON model suggest gusts of between 50mph to 65mph in the densely populated southern counties. That is strong enough to cause problems, for example with fallen trees and roof tiles being blown off. 

Gusts of over 70mph in the south?

In contrast, the Arpege model is indicating something a little different and more concerning for the south. At 21:00 on Sunday gusts of close to or over 70mph are blowing across southern and central regions. The earlier timing and higher gust speeds increase the risk of disruption and damage.  

Moving forwards to the early hours of Monday, the strongest winds are now impacting Scotland and northern England.

Other forecast models are showing variations on the above. In terms of accuracy, the differences between models is often small, which can make it difficult to be confident about a particular outcome. The point to emphasise is that they show possible outcomes, so be prepared for the worst.

Bigger impact than Storm Henk?

There are differences with other recent storms, for example Henk. It brought similar wind speeds, but only to a relatively small area and it moved through very quickly. In contrast, Storm Isha's impact will be felt across all of the UK and the very strong winds will last for several hours. On the plus side, the worst conditions are likely to be happening through Sunday night when most people will be indoors.  

Heavy rain in the west

Heavy outbreaks of rain will add to the problems in places. The largest rain totals are forecast to be in the north and west, as is usually the case when areas of low pressure are pushing in from the Atlantic.

The chart below shows rain totals in mm by 10:00am on Monday. The wettest conditions are in western Scotland, north western England and Wales. Over 100mm of rain is indicated in places, but forecast totals from computer models can be fairly inaccurate, even at relatively short range. Also, they often don't pick up local variations very well. Therefore, the chart is used only to illustrate the general distribution.

Temperatures climb

As well as wet and windy, it will become a lot milder. The Arctic air mass, which led to temperatures struggling to climb much above freezing even during the days, is being replaced by an Atlantic one. On Tuesday, values of 13C are possible in much of the United Kingdom. That would be about 10°C higher than the same day last week, and it is very possible that the Central England Temperature (CET) for January will finish above the 30 year norm despite the cold spell. 

Storm Jocelyn next week?

Winds gradually ease during Monday as Storm Isha pulls away. However, another area of low pressure is expected to steam in from the Atlantic during Tuesday. Although it currently doesn't look likely to be quite as intense, it will probably bring another period of very wet and windy weather. There is a possibility of it becoming a named storm; if so, it will be Jocelyn.

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