'What is going on with weather predictions?'

There have been warnings about ice on the road. Picture: Getty Images/iStockphoto

There have been warnings about ice on the road. Picture: Getty Images/iStockphoto - Credit: Getty Images/iStockphoto

Last month, the newspapers had fun at the expense of meteorologists from differing organisations, who were seemingly offering diverse predictions for our approaching winter weather.  

What is going on?  

DTN, with the BBC weather contract, forecasts “cold, dry with low winds”, and for Devon, Cornwall and Dorset, “cold, often dry, with wet spells”.  

The Met Office predicts “above average temperatures with normal precipitation”.  

DTN suggests “winter is likely to feature a weak polar vortex which could bring cold Arctic air over the UK”.  

There appears to be a discrepancy here. But we need to understand what these agencies are saying. 

The Met Office forecasts warmer temperatures on average, while DTN suggests a “winter featuring a weak polar vortex which could bring cold Arctic air over the UK”.  

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The clue to their discrepancies; one describes overall temperatures and the other (DTN) that we will have extremes.  

So, both predictions could agree – above average temperatures with cold and warm outbreaks.  

The Falkland Islands in the Southern Hemisphere are the same latitude as Pembrokeshire, 80 miles north of Sidmouth. Sidmouth and the Falklands have 200 hours of sunshine mid-summer.  

However, the Falklands record high temperature is 27C (79F), with an average high temperature 17C (63F) and an average daily temperature of 13C. Sunbathing could lead to both sunburn and hypothermia! The penguins of course love it, but they have thick coats combatting chill factor from biting winds of 16mph average speed. Few trees withstand this relentless wind.  

The Southern Hemisphere (SH) wind circulation differs from the Northern Hemisphere (NH). The SH has no Gulf stream nor Atlantic Meridional Overturning Circulation (AMOC) warming. These both provide Sidmouth and the UK with a warm temperate climate.  

Without the warm water of the Gulf Stream and the AMOC, our climate would be very different, as occurred about 12,000 years ago (Younger-Dryas) when meltwater flowed into the North Atlantic, cutting off the AMOC.

The AMOC state currently concerns palaeontologists due to the rapid melting of Arctic/Greenland ice sheet. Discussion on this is for next month’s article.  

Alan Gadian

Professor Alan Gadian discusses weather forecasting in Sidmouth and beyond - Credit: Alan Gadian

Northern Hemisphere (NH) has large landmasses and less sea. Importantly, the jet stream currently “meanders” more (the jet stream is a fast stream of air 5-10 miles above us in the atmosphere and created by the temperature difference between the Equator and Poles).  

In the SH there is a “strong circum-polar vortex” resulting in high West-East winds over the Falklands.  

In the NH the meanders cause stationary blocks of air (blocking anticyclones). The meanders transport streams of warm moist air moving South-North and cold Arctic air North-South (e.g. Storm Arwen, November, over the East of the UK).  

Warm northwards air melts Arctic/Greenland ice; cold southward arctic air brings snow and ice, even to Florida. A warming planet produces more of these meanders as it tries to compensate for hotter subtropics.  

Planning authorities like EDDC and Sidmouth Council need to take heed of meteorologists, plan for more extreme events, restrict covering land with impermeable surfaces, build places for extreme rainfall (retention ponds), not build on flood plains, fortify sea defences and allow land to flood. 

Future Sidmouth Christmas lights need be more strongly fastened down! These winter weather predictions may be typical for future years.  

So back to the introduction, as a dynamical meteorologist I think the models could essentially be consistent. The Met Office prediction of warmer overall winter temperatures could well be correct.  

However, there will be more extremes and strong Northerly and Southerly jet-stream meanders and flows bringing very cold Arctic air south and warm sub-tropical air North from time to time.  

On average for winter, the East of the country could be drier and the West be wetter. If the AMOC and the Gulf Stream weakens in future decades then our mid-latitude temperate climate could disappear, as was the case 12,000 years ago; but more on that next month.