What does summer 2017 have in store? The TWO summer forecast will be issued in late May but throughout we'll be taking a look at what the latest forecast data is suggesting.
By by simonwakefield [CC BY 2.0], via Wikimedia Commons
At this stage most seasonal forecasting models suggest a warmer than average meteorological summer (June, July and August). The signal for rainfall is more mixed. However it is worth remembering:
1) Even during warmer than average seasons cool or cold periods usually occur
2) The skill level (accuracy) of seasonal forecast models is low for the UK
The UK Met Office (UKMO) GloSea seasonal model shows a strong signal for temperatures to be above the average over the three month period. The probability of above normal, near normal and below normal temperatures is shown on the right hand image below. The key thing to note is the yellow and orange shading over the UK and Europe on the above normal 2m temperature plot. This indicates the most probable outcome.
The GloSea also hints at above normal precipitation levels. This is different to some of the other models such as Jamstec and the International Research Institute (IRI) which indicate a drier than average season.
Pressure forecasts (or heights of pressure levels) from the seasonal models provide another way of looking at things. There is reasonable agreement for positive height anomalies over the UK (higher than average pressure over the three month period).
The Climate Forecast System (CFS) v2 chart below shows positive 500hPa height anomalies over the UK in July. The signal during June and August is weaker on the latest updates. The UKMO GloSea model favours positive 500hPa height anomalies.
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 ENSO neutral conditions will give way to an El Niño pattern by late summer. This may increase the risk of wetter weather late in the season.
1) Seasonal models suggest above average temperatures.
2) The signal for rainfall is more nuanced. The UK Met Office GloSea5 is weakly pointing towards above average rainfall, but a number of other seasonal models favour dry conditions.
3) A switch to El Niño conditions late on could increase the risk of more unsettled weather occurring towards the end of summer.
4) The latest 2017 Atlantic Hurricane season forecast is for a near average number of storms. This would possibly favour average or more settled than average weather continuing in the UK during August and September.
On balance a warmer than average summer is considered likely at the present time. The signal for rainfall is more mixed, but close to average is considered the most likely outcome. The wettest part of the summer may occur towards the end but the hurricane season forecast possibly goes against this.
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