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Jet Stream Jet Stream Forecast Long Range Forecast Update (June)

The transition to El Nino type conditions has happened a lot quicker than expected and this has lead us to revising our June forecast to average or just above average temperatures for southern parts of the UK and average for northern half of the UK. Rainfall amounts will continue to be below average in June for the south, closer to normal further north and showery, unsettled conditions are likely to persist.

At this stage, there are still indications for sustained periods of dry, sunny and warmer weather in July and August so we are keeping our broad summer forecast for above average temperatures and below average rainfall.

Original Summer Forecast 2011
"Shades of 1976"

Of the post war summers, 1976 was perhaps the epitome of the long, hot and dry season, perhaps most remembered for the drought which most parts of the UK endured. This coming summer there is a number of parallels with 1976 and, following a drier than average winter and very much drier than average spring, we are very concerned that a sustained spell of warm and dry weather will only compound pressure on water resources.

Overall our forecast for summer 2011 (June, July and August) calls for much above average temperatures with sustained periods of dry or hot, warm and settled weather, particularly during June and July which would contrast markedly with the summers over the last three years which have been characterised by long unbroken periods of cool and unsettled weather. Rainfall is likely to be much below average with little in the way of organised weather fronts crossing the UK with any rainfall likely to come from thundery showers, particularly during August which we consider as being the least settled of the summer.

Long range weather forecasts are about probabilities and this year we see a high probability for a warm and sunny summer due to a number of factors all pointing to a particular pattern of high and low pressure systems with the normal path of low pressure systems bringing rain and wind held well out to the west and north of the UK.

This year, those factors are:
  • A transition away from La Nina in the Tropical Pacific.
  • The location of tropical thunderstorm activity in the Pacific.
  • The profile of sea surface temperatures in the Atlantic.
In the Tropical Pacific Ocean, surface sea temperatures associated with climatic phenomenon known as the El Nino Southern Oscillation have weakened rapidly from the La Nina (cold water) phase. The atmosphere has generally lagged behind these changes (and this has been a key driver of the dry and exceptionally mild spring) although significantly in the last few weeks there have been growing signs that a full blown transition will take place during the summer.

For the summers where such a transition in this key driver of global weather patterns took place the weather patterns were characterised by high pressure over North-west Europe and low pressure in the Atlantic. This type of pattern draws up humid, often hot air from Spain as the normal flow of weather is blocked by high pressure to our east.

Remarkably, the location of tropical thunderstorm activity many thousands of miles away in the Tropics can have a significant influence on global weather patterns. This year we anticipate there to be an increase in levels of activity tied into an increase in solar activity and favourable conditions for storm development in the western and central Tropical Pacific Ocean tied into the weakening of La Nina. Where this activity becomes organised, a powerful influence on global weather patterns can arise. In this location, this is likely to reinforce the pattern of high pressure to the east and south of the UK and low pressure stalled in the Atlantic bringing southerly winds and hot often sultry weather interspersed by isolated thunderstorms.

The mean position of the jet stream (a band of high altitude winds steering weather systems across the globe) can be deflected by relatively warmer or cooler sea surface temperatures over a sustained period of weeks or even months. The current profile of surface temperatures in the Atlantic is characterised by a much colder than normal pool of water in the mid Atlantic and around the Azores. Temperatures around the coastal waters off Europe are well above normal.

Figure 1: Profile of sea temperatures in the Equatorial Pacific since January
Sea Temperature Profile
Sea Temperatures
Click to enlarge
Figure 2: Profile of sea surface temperatures in the Atlantic (difference from normal)
Sea Temperature Anomaly
Sea Temperature Anomaly
Click to enlarge
Where colder than normal waters persist, the expected impact on the jet stream is to draw it southward. Warmer waters tend to shunt the ‘jet’ northward and take the attendant wind and rain further north. The ‘coupling’ of the colder than normal waters in the mid Atlantic and warmer waters around North-west Europe will result in the jet stream being ‘amplified’ with a trough or dip in the Atlantic and ‘ridge’ of high pressure and warmer weather held over much of Europe.

So here we have three major influences on global weather all suggesting a similar result. This indicates high probability for the weather over much of north and west Europe to be warmer and drier than normal. Crucially, just as in 1976, the normal high pressure belts extending out of the Azores which traditionally gives the UK its fine weather will be strikingly absent but replaced with high pressure close to or just east of the UK drawing up southerly winds. This is a very durable and repetitive weather pattern which can become difficult t o dislodge, often reinforced by attempts for low pressure to approach from the west but stalling and fizzling out as high pressure reasserts from the east.

The main feature of this summer’s weather is likely to be the sustained warmth with long runs of hot days, high humidity making it feel sticky and lack of appreciable rainfall. What rainfall there is will be showery in nature which will do nothing to alleviate pressure on water resources as showers are hit and miss and not everyone is guaranteed to catch them and during hot weather what rainfall there is tends to be evaporated away quickly.

Within the summer, we expect the core of the hot and settled weather to occur during June and July, particularly June which we think will be very pleasant with high pressure in charge across much of western Europe and the UK.

During July, the centre of the high pressure delivering settled weather is likely to shift more towards Scandinavia allowing pressure to fall over southern Europe and thunderstorms to develop on the continent, possibly affecting the south of the UK later on. Temperatures are however likely to be well above average and rainfall well below average.

Into August and the weather patterns tied into a weakening La Nina hint at the high pressure retreating further west into the Atlantic allowing more in the way of showers to develop. Temperatures may fall back a touch closer to average or just above and finally we may see some alleviation for parched soils in the way of average rainfall for the month although this is the most uncertain aspect of the forecast.

Figure 3: Forecast pressure and rainfall pattern for Summer 2011
Pressure and Rainfall
Pressure and Rainfall
Click to enlarge
Forecaster: Stewart Rampling for

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