Autumn is here and so is my Autumn 2021 Weather Forecast.
Seasonal forecasts are experimental and things go wrong. My summer forecast accuracy was mixed at best – so do take this as a guide more than an actual forecast. Hopefully more will be right than wrong.
Was more right than wrong for my summer forecast? Hmmm. In terms of weather patterns, for example high pressure being dominant in August, then I’m happy with it. Alas, I had assumed it would be a sunny high as most high pressure systems are in summer – we’ve been really unlucky with the wind direction and hence all the cloud. So, though I may have predicted high pressure to be dominant – I also predicted it would be mostly sunny too. Is that wrong? Half-right?
Same with June and July, though I again feel that the main patterns were reasonably accurate, I vastly underestimated rainfall in June – I totally didn’t expect weak low pressure after weak low pressure to trundle across and set up over south-east England. I don’t think anyone did.
So don’t take this forecast as gospel.
Please can you also share the forecast in some way. Invite friends on Facebook, retweet on Twitter, e-mail it to your boss. Whatever works. Or don’t share it, I’m not fussed.
Finally thanks to Eve for the photograph. Please get in touch so I can make the charity donation!
La Niña. This is expected to develop again over autumn, which tends to promote unsettled conditions in September and settled conditions later in autumn.
Polar vortex. Every August the polar vortex starts to develop – a weaker vortex is more likely this autumn/winter, similar to last year, this increases the chance of “stuck” weather patterns, and reduces the chances of weeks of low pressure systems going west to east on a strong jetstream, like we can often have in autumn.
Hurricane season. This is a curveball rather than a signal, but I do expect a busy September for Atlantic hurricanes and they can significantly alter expected weather patterns. This does make September especially difficult to forecast.
September is a difficult month to predict. It starts dry, and likely becomes very warm/quite hot, though after a couple of hot days thundery downpours may develop. After around 9th, things get tricky. A changeable two or so weeks is the more likely outcome, with some warm sunny days, some cooler cloudier days, some showery/wet days – probably warmer than normal.
However, if we strike it lucky with an Atlantic hurricane curving up and way north of the UK, it could reinforce the previous change to very warm/quite hot conditions – and we could end up with a mostly very warm and sunny middle of the month. This is an unknown curveball.
Towards the end of the month, a change to more settled conditions (assuming we had a changeable two weeks) is the more likely outcome, with temperatures around normal.
Overall I expect above average temperatures, around average sunshine, slightly below average rainfall.
Confidence level of 40%. Which is unusually low.
There are fairly strong signals of high pressure being close to our east for the start of October, which should translate to warmer and sunnier than normal.
A change to more unsettled conditions is likely after the first 7-10 days, with spells of wind and rain, temperatures around normal.
Low confidence by the end of the month, I’d suggest broadly changeable, some wind and rain, some drier spells. A very slight signal for colder north-westerly/northerly flows being more likely.
Overall I expect slightly above-average temperatures, slightly below-average sunshine, around average rainfall.
Again no real signal for the beginning of November, I’d assume fairly mixed, some rain but nothing too much, more dry conditions with variable amounts of cloud. Probably on the mild side.
There is a stronger signal from around mid-month, maybe a bit earlier, for high pressure to build to our west, allowing colder north-westerly or northerly flows, with some showers at times, and a mixture of sunny and cloudy spells otherwise.
Any showers more likely of rain down here, but wintry showers will be possible in any more potent northerly. Overnight frosts more likely too.
Overall I expect slightly above-average temperatures (though colder than average later in the month), around average sunshine amounts, somewhat below average rainfall.
Summary and Winter Thoughts
So, if I’m reasonably on target, we should have a warmer/milder autumn than normal though with the second half of November more likely to be colder than normal. A bit drier than normal overall, though some wet spells. Sunshine amounts around normal.
Background signals are mixed for winter – nothing new there. La Niña would tend to suggest a wetter winter, assuming it develops to be quite strong as expected (weaker La Niña’s are more supportive of cold weather) – yet there are plenty of background signals that suggest that cold spells are more likely than normal too. Kind of contradictory.
On balance, I’d say a colder than normal start to winter, ie late November and into December is more likely.
A mixed January.
A wet and windy February.
Winter always has the wild card of sudden stratospheric warming events, roughly every other winter. We had one last winter and it did bring some cold northerly weather at times, albeit minimal snow. Impossible to predict the chance of one occurring this far in advance.
That’ll do. I need some dinner.