Jump to content

Recommended Posts

Yes, I would leave the back end open but there is nowhere near enough evidence for 'A winter of two halves' type prediction, just wouldn't have the whole of Feb nailed down as mild either that is all, anyone who thinks Dec is going to deliver now is in cloud cuckoo land.

Link to post
Share on other sites
17 minutes ago, Frost HoIIow said:

Cheers Ian. Not what most on here including myself wanted to see but we'll see how it all pans out.

Sadly, and I speak as one who likes hard frost and snow myself, the fundamentals in the large-scale circulation and the pattern of sea- surface temperatures just do not point towards any real spells of wintry weather affecting the UK this coming season. However, if one like gales and heavy rain and lives anywhere north-west of a line from Bristol to the Humber there will be no disappointment! 

It would be nice to be proved wrong with my forecast, but I am (sadly) confident that I will not be. 

  • Like 1
  • Thanks 2
Link to post
Share on other sites
3 hours ago, iapennell said:

Dear Readers,

I have, as promised, got a long-term 2020-2021 forecast for the United Kingdom based on information that I have about global and regional sea-surface temperatures, large scale upper wind patterns and the current phase of the ENSO cycle in the Equatorial Pacific.

Firstly, the Quasi Biennial Oscillation. This is a (roughly) 27- month cycle of wind- fluctuations high up over the Equator - in the Equatorial Stratosphere. Wind levels at  the 30 mb level averaged near 8 metres per second from the West and at the 50 mb level were just over 2 metres per second from the West during October 2020. These Westerlies gradually descend and enter the tropical troposphere about two months after their occurrence at the 30 and 50 mb levels in the Equatorial Stratosphere, then over the next few weeks impact on the entire global circulation- mainly affecting the winter circulation. This transfer of Westerly momentum from the Quasi-Biennial Oscillation circulation- magnified as air moves into higher latitudes would imply stronger higher-latitude Westerlies (of 2 to 3 mph) during  January and February.

Currently there is a Lá Niña type circulation in the Pacific Ocean resulting on sea-surface temperatures about 1 to 2C cooler than the seasonal norm from the West Coast of South America right across to the Western Pacific (Polynesia area). Other things being equal cooler-than-normal waters along the Equator around at least one third of the Earth's circumference ought to result in weaker north-east Trade Winds from the Northern Hemisphere arising from a weaker Hadley Cell. However, the Equatorial Atlantic and Indian Oceans are 1 to 2C warmer than usual, which would lead to greater convection along the Intertropical Convergence Zone (ITCZ), a stronger Hadley Cell, and stronger north-east Trade Winds from the Northern Hemisphere. There is therefore a small net effect of hotter-than-usual conditions beneath the (likely) winter position of the Intertropical Convergence Zone leading to a stronger Hadley Circulation and (therefore) greater transfer of Westerly atmospheric momentum into higher latitudes of the Northern Hemisphere. Moreover, with this setting, relatively cool waters over the eastern and central Equatorial Pacific lead to stronger easterlies near the surface of the Pacific (where winds blow west from the cool waters where surface pressure rises towards hotter seas across the westernmost tropical Pacific). That leads to greater Westerly atmospheric momentum transfer into both the Northern and Southern Hemispheres aloft. 

The North Atlantic is substantially warmer than usual at present, whilst early snowfalls have resulted in Canada cooling down fast. Arctic sea-ice levels remain below normal but are recovering quite quickly from record-low levels seen during October 2020. Arctic ice-cover is recovering fastest in the Canadian Arctic but large areas north of Norway and Arctic Russia remain ice-free to date (if Britain is to have very cold north and east winds, due to blocking patterns further north it's best to have ice-growth north of Norway and northwest Russia): A very cold Canada and Greenland, but a warmer than usual North Atlantic sets the scene for a strong atmospheric temperature and pressure gradient across the far north-west Atlantic, one that would favour deeper North Atlantic depressions and a strong North Atlantic jet-stream heading towards the United Kingdom.

In the shorter term, both the fact of a continuation of an exceptional North Atlantic hurricane season, and fact that the upper winds in the stratosphere high above 60N are predicted to be stronger than usual out to 16 days' hence by all 31 atmospheric modellers (http://weatheriscool.com/) points to strong Westerlies heading towards the UK throughout the following month (and covering the Christmas period). The circulation in the stratosphere above 60N takes two to three weeks to impact on the weather near the surface across northern Europe and North America. 

The Sun is still in a quiet phase at the start of Sunspot Cycle 25, among other things that ought to mean a cooler Sun, a sluggish Hadley Cell and less transfer of heat and Westerly atmospheric momentum into higher latitudes. However, the deep tropics are still warmer than usual and the Westerly QBO in the Equatorial Stratosphere will have other impacts. There are also signs that the Sun is becoming more active in any case (see here: https://www.nasa.gov/feature/goddard/2020/new-sunspots-herald-increased-solar-activity-cycle-sdo and here: https://earthsky.org/space/photos-sunspots-ar2781-solar-cycle-25 ) . If, as seems likely, the Sun is growing active it will be stronger and produce more heat to power the Hadley Circulation -and cause more warmth and Westerly momentum to shift to higher latitudes. Lingering hurricanes and typhoons mean that strong easterly winds to the north of them (and westerly Atmospheric momentum being thrown northwards aloft) will also contribute to a greater transfer of heat and Westerly momentum into higher northern latitudes- leading to stronger Westerlies in higher latitudes in the run-up to (and over) Christmas.

The conclusion of this analysis is a dire one for lovers of snow and hard frost, since all these macro-scale influences on Britain's prospective winter- weather weigh heavily towards Winter 2020-2021 being a mild, wet and (yes) potentially stormy one. This is particularly likely, since the strength of the south-westerlies through November has prevented the advance of continental snow-cover westwards of the Ural Mountains. This, along with the persistent south-west flow will prevent very cold air and snow-cover building up over Scandinavia an north-west Russia (crucial if strong blocking highs are to form in the area). It is, instead, eastern Canada and Greenland that have the intense cold and snow-cover, which will help generate some intense North Atlantic baroclinicity and cyclogenesis.

The North Atlantic, being warmer than usual, will also limit the likelihood of really cold air reaching the UK. Sea surface temperatures around Britain are warmer than normal after a slightly milder than usual autumn overall and (even were there to be a fortnight of very cold northerly or easterly winds, which is very unlikely) sea- surface temperatures around Britain will remain warmer than usual. This will ameliorate the impact of any cold air-streams from north or east. 

For a bit more detail to my prediction December is likely to be dominated by persistent south-westerlies, with plenty of wind and rain. Stormy conditions can be expected during the second half of December across Scotland, Northern England, Northern Ireland and North Wales. There will be snow in the mountains at times, interspersed with mild strong south-westerlies that remove all traces of snow (except perhaps on Ben Nevis and on the Cairngorms). The south of England will be drier, but still quite windy. Temperatures of 15C can be expected on some days in the South and Midlands. Frost at night will be rare except in northernmost mountain areas.

January 2021 will continue the wet- and at times- stormy theme. Expect something like January 2014- it will be cold enough for snow at times in the uplands of Scotland and across Northern Britain, but don't expect widespread snowfall across the lowlands. There will be some frosty nights with clear spells following colder west- northwest winds, but nothing of any severity anywhere. The Midlands and South will get the driest and brightest weather, and with mild south-west winds at times air temperatures will still reach 13C widely.

February 2021 will continue the unsettled theme, but it will not be as wet and stormy as in February 2020. With Canada and Greenland by this time very cold and the north Atlantic approaching its coolest, February will bring the highest chance of any cold weather. Tropical cyclones will be happening by then in the Southern Hemisphere (over the Indian Ocean and western Pacific south of the Equator where the seas will likely be very warm by then); these will by this time impact the general circulation in the Northern Hemisphere in the opposite manner to hurricanes north of the Equator- as the Easterlies high up on the equator-wards side of these Southern Hemisphere tropical cyclones will enter the Northern Hemisphere circulation, this time reducing Westerly Atmospheric Angular Momentum: This will lead to some weakening of the Atlantic Westerlies. Thus, we can expect a week of more of dry fine weather with colder north-westerly winds, which may become easterly as the high-pressure drifts into northern Europe. This will mean widespread night frost for a time and some bright cold days. However, a nationwide severe wintry spell with lots of snow is not at all likely as the fundamentals will still be against this- the Westerlies are likely to shift north of Britain across the still relatively-warm Norwegian Sea so this will preclude frigid Arctic or Siberian air reaching these shores. The best likelihood of seeing any snow will be in the mountains across northern Britain with the squally colder Westerlies following the passage of depressions.

So to conclude,  a mild rainy, windy and unsettled Winter 2020-2021 with snow in northernmost mountains at times, with the best chance of brighter colder weather in February, looks to be on the cards.                    

 

Thanks Ian.

Actually, December is the month I most disagree on, I think there is a good chance of blocked patterns until the strat and trop vortexes connect this year, which could deliver some cold spells, agree that January will be wet mild and windy.  February will depend on SSW or not, so much so, that we can’t hazard a guess at that month yet.  We will see...

  • Like 2
Link to post
Share on other sites

This forecast is completely the opposite to other Winter forecasts for the UK - 2020 /21, I have, seen. 

It looks like December will begin cold by day, frosty by night and dry - certainly not beginning with your prediction of a month "dominated by persistent south-westerlies, with plenty of wind and rain."

Time will tell though. 

 

 

 

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...