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WARNING - LONG RANGE FORECASTING IS EXPERIMENTAL AND CAN BE VERY, VERY WRONG! I am not a meteorologist, just an interested amateur. The views below are my own, but are not intended to be relied upon!
Every year the tabloid newspapers publish scare stories about winter. Worst winter for 20/50/100 years. 100 days of snow. Coldest winter on record. Etc etc. This year is no different as can be seen from this article published on 29th September WEATHER WARNING: UK set for HEAVIEST SNOWFALL in years as forecast warns Winter BIG FREEZE | Weather | News | Daily Express
We are likely to see another burst of tabloid hysteria as the long range models are all to some degree forecasting cold weather in November, some of it very cold and snowy. I thought people might appreciate something approaching a layman's explanation. References to a warm or cold winter are for the UK - obviously this will be different in other parts of the world.
There are several things that the weather models and forecasters use when considering the winter forecast. These include sea surface temperatures, the rate of expansion of arctic ice and siberian snow cover and the general prevailing pattern of pressure across the north pole. In addition there are predictable cycles which include the sunspot cycle (about 11 years) and something called the QBO which has about an 18 month cycle.
Last winter was very warm and very wet, and this was for the most part predictable. The sea surface temperature pattern (strong El Nino, warm north pacific, cold north atlantic) all served to enhance the jet stream, blasting straight across the atlantic bringing warm, wet and windy weather from the west. The air in the stratosphere, high over the arctic was very, very cold, which causes the winds to circulate strongly around the pole, holding in the cold air and preventing it from escaping south. This is known as a strong polar vortex. The QBO was in its westerly phase, enhancing the strong jet stream even further. The sun spot cycle was dropping from maximum in 2014 towards minimum (expected around 2018-19) - high sunspot numbers seem to favour a milder winter. Only the snow cover seemed favourable to a cold winter, and against everything else, that was not enough.
This year things look very different. El Nino is no more and we are now approaching a weak La Nina. The north pacific is still warm, but the north atlantic is a lot less cold. The jet stream is weak and meandering north and south with long, deep loops. The stratosphere is doing something nobody has seen before at this time of year, with the polar vortex splitting into 2 parts and an area of warmth developing over the pole (this normally happens later in the year or early in the new year if at all, and often causes cold weather a couple of weeks or so later). The QBO, which is normally as reliable as clockwork, has done an about turn, having begun its easterly phase, and is now westerly again. This has never happened before and nobody knows what it means. Solar activity has dropped since last winter, and siberian snow cover is high with an increase forecast soon. The arctic ice is freezing very very slowly and this is said to favour a cold winter (this is due to the position of high pressure which has warm air beneath and therefore inhibits the ice formation).
(Probably as a result of all this) we have a large area of high pressure situated over the arctic - exactly the opposite to last year. This situation is known as "northern blocking" and gets the meteorologists very excited because it is one of the ingredients of a cold winter, and for some reason they all seem to really love snow. The blocking has to be orientated correctly for us to get snow, and the classic scenario for this is high pressure over Scandinavia, which then migrates west (retrogression) towards Greenland, dragging the winds round to the north as it does so. This is what the long range models are predicting to happen in November.
IF what the models are predicting actually happens we could find ourselves in quite a cold spell. The area of high pressure is huge and isn't going anywhere fast, and IF the block gets stuck over Greenland long term we could be in for a very cold and snowy winter indeed.
There is huge uncertainty in the predictions, but this situation has been slowly developing over the last month or so. What happens will depend crucially on the positions of the high and low pressure systems over the arctic, Greenland and the continent and how long the northern blocking lasts. The current thinking, linked to the splitting of the polar vortex, is that retrogression will occur and we are likely to be cold by mid November. What happens next is pretty much anyone's guess.
In the shorter term the next few days look mostly dry and mild for late October, although there is a risk of fog which could be quite thick and keep the temperatures low where it persists. This is because we have low pressure to the west and high to the east, which drags up warm air from the south - exactly the opposite from the situation described above. I think one of the reasons the models are predicting the cold is because the current situation leads to warm air being sucked up into the arctic. I remember this happening before the exceptionally cold late November and December in 2010, but that doesn't necessarily mean that things will develop that way again.
My advice is, if you are planning on growing crops that don't like cold over winter, for example in your greenhouse, make sure that you have some insulation to hand, or somewhere you can put them which won't freeze if it gets really cold. The risk is definitely there.
Every year the tabloid newspapers publish scare stories about winter. Worst winter for 20/50/100 years. 100 days of snow. Coldest winter on record. Etc etc. This year is no different as can be seen from this article published on 29th September WEATHER WARNING: UK set for HEAVIEST SNOWFALL in years as forecast warns Winter BIG FREEZE | Weather | News | Daily Express
We are likely to see another burst of tabloid hysteria as the long range models are all to some degree forecasting cold weather in November, some of it very cold and snowy. I thought people might appreciate something approaching a layman's explanation. References to a warm or cold winter are for the UK - obviously this will be different in other parts of the world.
There are several things that the weather models and forecasters use when considering the winter forecast. These include sea surface temperatures, the rate of expansion of arctic ice and siberian snow cover and the general prevailing pattern of pressure across the north pole. In addition there are predictable cycles which include the sunspot cycle (about 11 years) and something called the QBO which has about an 18 month cycle.
Last winter was very warm and very wet, and this was for the most part predictable. The sea surface temperature pattern (strong El Nino, warm north pacific, cold north atlantic) all served to enhance the jet stream, blasting straight across the atlantic bringing warm, wet and windy weather from the west. The air in the stratosphere, high over the arctic was very, very cold, which causes the winds to circulate strongly around the pole, holding in the cold air and preventing it from escaping south. This is known as a strong polar vortex. The QBO was in its westerly phase, enhancing the strong jet stream even further. The sun spot cycle was dropping from maximum in 2014 towards minimum (expected around 2018-19) - high sunspot numbers seem to favour a milder winter. Only the snow cover seemed favourable to a cold winter, and against everything else, that was not enough.
This year things look very different. El Nino is no more and we are now approaching a weak La Nina. The north pacific is still warm, but the north atlantic is a lot less cold. The jet stream is weak and meandering north and south with long, deep loops. The stratosphere is doing something nobody has seen before at this time of year, with the polar vortex splitting into 2 parts and an area of warmth developing over the pole (this normally happens later in the year or early in the new year if at all, and often causes cold weather a couple of weeks or so later). The QBO, which is normally as reliable as clockwork, has done an about turn, having begun its easterly phase, and is now westerly again. This has never happened before and nobody knows what it means. Solar activity has dropped since last winter, and siberian snow cover is high with an increase forecast soon. The arctic ice is freezing very very slowly and this is said to favour a cold winter (this is due to the position of high pressure which has warm air beneath and therefore inhibits the ice formation).
(Probably as a result of all this) we have a large area of high pressure situated over the arctic - exactly the opposite to last year. This situation is known as "northern blocking" and gets the meteorologists very excited because it is one of the ingredients of a cold winter, and for some reason they all seem to really love snow. The blocking has to be orientated correctly for us to get snow, and the classic scenario for this is high pressure over Scandinavia, which then migrates west (retrogression) towards Greenland, dragging the winds round to the north as it does so. This is what the long range models are predicting to happen in November.
IF what the models are predicting actually happens we could find ourselves in quite a cold spell. The area of high pressure is huge and isn't going anywhere fast, and IF the block gets stuck over Greenland long term we could be in for a very cold and snowy winter indeed.
There is huge uncertainty in the predictions, but this situation has been slowly developing over the last month or so. What happens will depend crucially on the positions of the high and low pressure systems over the arctic, Greenland and the continent and how long the northern blocking lasts. The current thinking, linked to the splitting of the polar vortex, is that retrogression will occur and we are likely to be cold by mid November. What happens next is pretty much anyone's guess.
In the shorter term the next few days look mostly dry and mild for late October, although there is a risk of fog which could be quite thick and keep the temperatures low where it persists. This is because we have low pressure to the west and high to the east, which drags up warm air from the south - exactly the opposite from the situation described above. I think one of the reasons the models are predicting the cold is because the current situation leads to warm air being sucked up into the arctic. I remember this happening before the exceptionally cold late November and December in 2010, but that doesn't necessarily mean that things will develop that way again.
My advice is, if you are planning on growing crops that don't like cold over winter, for example in your greenhouse, make sure that you have some insulation to hand, or somewhere you can put them which won't freeze if it gets really cold. The risk is definitely there.
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