Ex-hurricane causes an uncertain forecast this weekend

As a weather forecaster, every once in a while I stare at the weather charts and just know that a particular forecast is going to be very difficult to get right.

Whilst this alone can be frustrating, it’s even more annoying when it happens over a weekend, because it’s when most people have time off and are outside doing things, for which the weather can potentially have a big impact.

So here’s the headache for later this weekend. An ex-hurricane, now Tropical Storm Nadine, is lurking to the south of the Azores. Nadine was formerly a category one hurricane, but has now weakened and is drifting slowly around the eastern Atlantic.

What is certain is that Nadine itself won’t affect the UK, but what could happen is some of the energy and moisture associated with the storm could break away and spawn a new area of low pressure that will potentially arrive on our shores later this weekend.

Image copyright 2012 EUMETSAT – shows Tropical Storm Nadine (circled in red).

This is where the huge level of uncertainty lies. Weather computer models don’t tend to handle the remnants of former hurricanes or tropical storms very well when they arrive at our latitude.

It’s often because of the huge amount of moisture and potential energy that they contain. If this is engaged by the jet stream, then they can explode to life spawning deep areas of low pressure bringing strong winds and heavy rain.

On the other hand, if the jet stream doesn’t engage with this moisture and potential energy, then low pressure is less likely to form and the end result can be very different.

How does the jet stream cause low pressure to form?

When the jet stream moves from west to east, as well as travelling in a straight line, it also bends around. In cases where it dips and curves in a broad U-bend shape, something called a trough forms.

As the ribbon of air approaches the base of a trough, it slows down slightly as it goes around the bend, then accelerates once it has passed through it.

It’s the acceleration of the jet stream after passing through the U-bend that is a key factor because air high up in the atmosphere is sucked upwards and removed faster that it can be replaced at the surface.

As a result of this process, air has a tendency to rise and pressure falls (forming an area of low pressure), resulting in cloud, wind and rain.

So how is this relevant for this weekend’s forecast?

At the moment, there looks to be a good possibility that a trough in the jet stream will interact with some moisture and energy tied in with the forward side of Tropical Storm Nadine.

In the past few days, the weather computer models have disagreed on the eventual outcome, with some developing an area of low pressure that moves across us and others not.

However, today there seems to be more agreement amongst the various models which favour this interaction taking place, with an area of low pressure forming and moving over the UK later this weekend.

How much uncertainty is there?

Whilst the area of low pressure looks likely to form, I think the uncertainty lies in its track, how developed it will be, and when it will arrive.

The latest information suggests that it may arrive across southern parts of the UK later on Sunday, before moving further northwards overnight into Monday.

This would bring a spell of rain and brisk winds, although the intensity of the rain and exact strength of the winds will depend on how developed the low pressure system ends up being.

Despite Sunday’s forecast looking uncertain, it’s worth highlighting that most places on Saturday will have a fine day with sunny spells after a chilly start.

As ever, do stay in touch with the latest forecast on the Channel 4 Weather website and I’ll be tweeting regular updates on Twitter – @liamdutton