What does summer 2017 have in store? The TWO summer forecast will be issued in late May but in the run up to it we'll be providing updates on the latest forecast data.
Blyskawica" by Ziemor at pl.wikipedia. Licensed under CC BY 2.5 via Wikimedia Commons
At this stage seasonal forecasting models on balance suggest a warmer than average meteorological summer (June, July and August). Since the last summer weather update the likelihood of below average rainfalls levels has increased. 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 update (issued 11/05) continues to favour above average temperatures over the June, July and August period taken as a whole. 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 latest GloSea update hints at below normal precipitation levels. The April update suggested a bias towards wetter than average conditions, so this is a significant change. It is now close to what some of the other models such as Jamstec and the International Research Institute (IRI) have suggested.
Pressure forecasts (or heights of pressure levels) from the seasonal models provide another way of looking at things. There is good agreement for positive height anomalies over the UK (higher than average pressure over the three month period).
Recent runs of the Climate Forecast System (CFS) v2 show positive 500hPa height anomalies over the UK in July. The signal for June and August is weaker. The UKMO GloSea model strongly favours positive 500hPa height anomalies this summer.
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 and El Nino conditions are nearly equally favoured during the summer and autumn. Since the last update this tentatively suggests drier than average conditions could continue through the summer.
1) Seasonal models suggest above average temperatures.
2) The latest signal for rainfall is more strongly supportive of a drier than average season.
3) A switch to El Niño still looks possible but it is not certain and ENSO neutral conditions are considered as likely to continue. El Niño could increase the risk of more unsettled late in summer relative to ENSO neutral.
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 chance of drier than average conditions over the season as a whole has increased.
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