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Autumn Forecasting

Watch and listen to the latest World and UK weather broadcasts
Brown leaves on trees show that autumn has begun.
BBC Broadcast Meteorologist Helen Willetts looks at the particular problems weather forecasting in the autumn can bring.


Key Points
  • Autumn is recognised in the meteorological world as the season of transition.
  • The jet stream position determines the frequency with which low pressures or depressions cross the UK.
  • Low pressure systems cause wind and rain.
  • Longer nights encourage fog formation, also bringing the risk of frost and ice.
Also in BBC Weather

Time to Change!


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After the clocks fall back and most of us sink into the gloom of autumn, the prospect of shorter days, lack of sunshine and the usual wind, rain and fog are not always welcome thoughts. For weather forecasters the worries of autumn weather begin much earlier, back in September.

Autumn is recognised in the meteorological world as the season of transition, when we move from the calm, dry and sunny summer days to potentially stormy weather days. This transition occurs due to the migration of the jet stream (strong winds in the upper atmosphere). Its position determines the frequency with which low pressures or depressions cross the UK.

...the jet stream moves northwards steering low pressures across Iceland and Scandinavia.
During the summer the jet stream moves northwards steering low pressures across Iceland and Scandinavia. In the autumn this returns south to our latitudes, steering low pressure systems over us and causing our weather to turn more unsettled. As we know this often equates to wind and rain.

The Autumn months of September, October and November also coincide with the end of the Atlantic Hurricane season and we can occasionally find extra tropical storms influencing our weather bringing excessive rain and wind.

Fog can occur at any time of the year but it starts to cause problems from early autumn. As the sun's influence starts to fade and the nights lengthen, the fog tends to linger into the morning coinciding with the rush hour.

Fog in my opinion is the most dangerous weather element to drive in and I avoid it if at all possible. If a morning is foggy it is almost inevitable that there will be problems on the roads as a consequence.

Whilst the longer nights encourage fog formation they also bring the risk of frost and ice, a particular problem for those forecasters predicting road conditions for the morning rush.

Towards the end of the autumn, snow starts to appear more regularly in the forecast, although that is much more of a winter worry which I will look at later.





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