UK Winter forecast: another notably colder winter favored, though milder at times.
Temperature: overall temperature is expected to be slightly colder than average across the British Isles, with no clear-cut signal for one single month being especially colder than the rest. A winter similar to last winter is possible, in terms of slightly colder than average temperatures, however we suspect the cold will be more consistent this winter, with various cold snaps throughout, rather than the “odd severe spell”, like we saw with last year’s beast from the east.
Snowfall: snowfall may well materialize as close to average, however a snowier than average season can’t be ruled out, especially for northern hills. There is little to suggest a snowless or season with less snowfall than average at present.
Precipitation: Between cold snaps, some more active jet stream periods will occur. This will increase the chance of heavy rainfalls, and windy weather, however these periods will be fairly short lived in nature.
In the short term, I released a post recently discussing the potential for significantly colder weather in around 10 days time. You can click here to view that post. In the short term it does look like we’re a gloomy, rather cold spell of weather, but not much in the way of snow. An area of high pressure will situate itself close to the British Isles by next weekend and into the following week allowing for cool, dry and quite gloomy weather. Temperatures could fall slightly below average next week (19th-25th) of November.
Long range forecasting isn’t an exact science – it is prone to something we call “uncertainty”, which is what weather forecasters use to describe the decrease in confidence of forecasts after about 10 days time. However, we can use some signals, or “factors” to try and predict UK winter weather. Ever since the “beast from the east”, which I’m sure you all remember, the atmosphere has been in a different state. There has been an increased amount of “blocking” in the atmosphere. Blocking is a feature in atmosphere circulations that increases the chance of cold weather in winter. It stops off the normal jet stream flow, from west to east, allows cold air to drain off the pole into the British Isles.
Essentially the chances of cold or snow are dictated by blocking, this drives the cold air from the pole and into the mid latitudes, like the UK. It’s these large areas of high pressure that promote cold weather during the winter. However, they need to be in the correct position to deliver cold. A large high pressure system sat to the south of the country, or directly to the east, can promote milder south westerly winds, such as seen below during December 2016.
In this example, the blocking signal is strong, so the chance of cold is there, but the jet stream is simply to strong. Because of this strong jet stream, the high pressure “blocking” is prevented from moving into the correct position to promote blocking. So what needs to happen to allow cold weather to occur is a) a weak jet stream to allow blocking to move into the correct position to promote cold and b) a strong signal for blocking to form. Now there are various factors that influence jet stream strength and blocking formation. One of those is solar activity. The number of sunspots on the face of the sun – yes – does have a significant impact on UK winter weather. Large declines or periods of solar “inactivity” can lead to jet stream abnormalities, which can promote cold weather. One stark example are the winters from 2008 to 2010 which all coincided with solar minimum during the last solar cycle. We’re currently in the next solar minimum. Solar minimum correlates with an increased risk of stratospheric warmings, which can promote colder winter weather.
The uncertainty is this forecast for an “increased chance of cold” forecast, stems from the El Nino, warm sea surface temperature in the northern Pacific and generally “unfavorable” global sea surface temperature pattern. Also worth mentioning is that the Arctic at the moment is significantly warmer than average, this means we need more intense synoptic patterns to deliver the same degree of cold as say we may have needed a few decades ago.
Other factors, also favor increased levels of blocking. However we are also fought with a fairly active jet stream this winter. This could produce a west/east split across the British Isles. Significantly wetter conditions are likely in the west, with drier conditions in the east. The blocking signal will be stronger to the east of the UK, at times allowing for mild southerly winds to affect the British Isles. This “battleground scenario” could allow for some “stuck” weather fronts in the west, leading to perhaps an increased risk of flooding events. Given the increased signal for blocking, during periods of “jet stream weakening”, the blocking would have a fairly easy time “expanding” leading to potentially some cold or very cold periods, one of those *could* occur during December, with a more significant signal into January and February, thanks to declining solar activity.
Although I wont go into the details regarding the other “factors” used for this forecast, I will mention them. 1) QBO – equatorial wind oscillation, easterly and westerly phases. The easterly phase promotes colder weather. A neutral phase will be dominant for winter 2018/2019. 2) El Nino – A weak El Nino is progged for winter 2018/19, this increases the risk of northern blocking around central and northern Europe, whilst also allowing for a fairly active jet stream. 3) North Pacific warm “Blob” – increasing jet stream strength. This is more significant in our U.S and Canadian winter forecast. 4) Hurricane season – a fairly active hurricane season during 2018 may allow for more cold outbreaks due to stronger northern blocking.