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Summer 2018 weather

Forecast indicators


07/04/2018

What does summer 2018 have in store? The TWO summer forecast will be issued in late May but during the coming weeks we will be providing several updates which take a look at what the latest seasonal forecast data is suggesting. 

By by simonwakefield [CC BY 2.0], via Wikimedia Commons

Seasonal model overview

At this stage most of the seasonal forecasting models are not showing large temperature or precipitation anomalies for the meteorological summer (June, July and August) taken as a whole. However it is important to note that the skill level (accuracy) of seasonal forecast models is low for the UK.

Model Temperature Precipitation
GloSea (UKM) Above average Close to average
ECM Below average in the northwest Below average
Meteo France Below average in the south Below average in the north
CS3 No anomaly No anomaly
Jamstec Above average Slightly below average
CFSv2 Below average in the northeast No anomaly
Table 1: Forecast temperature and precipitation anomalies, June, July and August

 

The most notable thing at this stage is the suggestion of below average temperatures in at least parts of the UK by the Meteo France, ECM and CFSv2 models. It is particularly interesting that they don't suggest above average rainfall levels, because cooler and drier than average summers are unusual. 

The combination of below average rainfall and below average temperatures could possibly be explained in part by the lower than average sea surface temperatures around the UK at the current time. There is also some support from 500hPa height anomaly charts for periods of cooler northwesterly winds to occur more frequently than average during the June, July and August period.

Anomalies masking the outcome

Although the anomalies in the table above are not marked, it could also be the case that they are not telling the whole story. They are for the meteorological summer as a whole and there is a chance that extreme swings are cancelling each other out. For example hot and dry periods are matched by very cool and wet ones to give a close to average outcome overall.

Some of the recent CFSv2 charts for the summer months may offer support for that hypothesis. There is a suggestion of higher pressure to the north of the UK. That could easily lead to cool and wet periods as low pressure pushes up from the south. However it also offer a chance of hot and dry spells if high pressure cells extend southwards. At times during the long hot summer of 1995 high pressure was centred to the north of the UK.  

CFSv2 500hPa anomalyCFSv2 forecast 500hPa anomaly July

ENSO conditions

The state of the El Niño Southern Oscillation (ENSO) has significant impact on temperature and rainfall patterns around the globe. Nonetheless its link to the UK summer is considered to be weak. Latest forecasts suggest a transition from La Nina to ENSO neutral conditions will take place before the start of the summer and continue into the second half of the year. Therefore ENSO is not expected to favour a particular outcome.

Atlantic Hurricane season

According to the Colorado State University Tropical Meteorology Project the 2018 Hurricane Season is expected to be slightly more active than usual. In total 14 named storms (average 12), 7 hurricanes (average 6) and 3 major hurricanes (average 2) are forecast. This could increase the chance of more unsettled weather in the UK towards the end of the meteorological summer.

Summary

1) Seasonal models suggest weak temperature and precipitation anomalies over the June, July and August period taken as a whole.

2) ENSO conditions are expected to be neutral and not providing support for any one particular outcome.

3) The latest 2018 Atlantic Hurricane season forecast is for a slightly more active than usual season. This would possibly favour more unsettled weather towards the end of summer.

At this stage the likelihood of a cooler than average summer is considered higher than it has been during recent years. However the chance of major variations is considered higher than normal and there is a low possibility of prolonged spells of hot weather more than offsetting cooler and unsettled periods. If that happened the summer could make it into the warm or even very warm category.

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