Summer Forecast 2018

Summer 2018 Forecast Overview

There is general agreement that this summer will be warmer than average over the UK but not exceptionally so, although the ECMWF long-range forecast goes for close to average temperatures. The majority of the models are also suggesting a slightly drier than average summer, particularly for the south.

Breaking it down by month there is a recurring theme of a relatively anticyclonic June, possibly with a reduced mean westerly flow relative to normal, and with easterly winds frequent in the first half but the traditional "June return of the westerlies" seeing the jet stream power up to the north of Britain late in the month. For July the signals are rather more mixed but with the majority suggesting that there will be a reduced mean westerly flow relative to normal, with above-average pressure to the north and below-average pressure over central and southern Europe. For August, the Netweather long-range model is suggesting close to average pressure but the majority of other models, and the signals from ENSO and the Arctic sea ice, are pointing towards a cyclonic August.

Thus, it looks like being an "early summer", with plenty of warm and dry and probably sunny weather likely during June, possibly extending into July as well, but with somewhat more unsettled weather establishing by August. In June, it is likely that northern and western areas will see the warmest and sunniest weather relative to normal during the first half of the month, with southern areas more favoured late in the month when there is also potential for it to turn hot at times.

Forecast Factors

Summer 2017

The summer of 2017 started with a rather unusual June, which was very wet for some parts near both ends but had the most significant June hot spell since 1976. It was a very warm, dry and fairly sunny month overall for the south-east but most other parts of the country were notably dull. During July and August the weather turned generally unsettled and August was on the cool side for most.

ENSO Signal

Following the moderate La Nina event of winter 2017/18, ENSO is forecast to move into a neutral to slightly positive state during the summer of 2018. The EUROSIP multi-model forecast has the majority of ensembles suggesting a near-neutral to slightly positive ENSO state for the summer. The NMME, NOAA and Met Office forecasts all go for a slightly positive ENSO state as the most likely outcome. The Met Office model in particular points towards El Nino territory for the winter of 2018/19 but the EUROSIP ensemble indicates that there is a fair amount of uncertainty over this.

Analogues for past years with this kind of ENSO progression throw up 1957, 1963, 1996, 2001 and 2006 as the nearest matches.

For June-August as a whole, the mean sea level pressure composites show a very weak tendency for above-average pressure extending from Iceland to Scandinavia and below-average pressure over central Europe, pointing to a reduced mean westerly flow over the British Isles. Breaking it down by month, those Junes on average had an anticyclonic tendency over and around the British Isles. The Julys had a tendency towards high pressure just to the north of the British Isles and a reduced mean westerly flow, but the Augusts in contrast were on average substantially more cyclonic than normal over the British Isles.

The ENSO signal therefore points weakly towards June and July having more anticyclonic and/or easterly types than usual and towards August being dominated mainly by low pressure.

Stratospheric conditions

The QBO (Quasi-Biennial Oscillation) remains in an easterly phase and it is possible that this may have contributed to the recent preponderance of easterly winds, particularly during March when we had two exceptional, if relatively short lived, cold spells. However, the stratosphere tends not to play a major role in the weather patterns of north-west Europe during the summer.

MJO

The Madden-Julian Oscillation is currently heading into phase 4 which points towards a relatively anticyclonic start to June, but is forecast to head into phase 5 towards midmonth which will promote relatively cyclonic conditions. A high amplitude MJO would also tend to promote cyclonic conditions for most of the rest of June. Progression into phases 7 and 8 could lead to relatively anticyclonic weather setting in into early July. It must be stressed though that the relationship between the MJO and atmospheric circulation is generally weaker in the summer than it is in the winter half-year.

Arctic Sea Ice

Arctic sea ice is anomalously low again, and is currently tracking second lowest on record (after 2016) according to NSIDC. Sea ice extent is record low around the Bering Sea, which has had remarkably low sea ice extent since last summer, and relatively unexceptional in the Atlantic and Barents/Kara region.

Going by NSIDC, the other years with unusually low Arctic sea ice extent at this time of year were 2015 and 2016, and to a lesser extent 2004, 2006, 2010 and 2011.

These years tended to have low pressure more dominant than usual over the British Isles with a strong anomalous high pressure centre over Greenland. Breaking it down by month, June tended to be relatively anticyclonic, July mildly cyclonic/south-westerly and August markedly cyclonic. Thus the Arctic sea ice signal joins ENSO in pointing towards an August dominated mainly by low pressure.

North Atlantic Oscillation and Arctic Oscillation

Most of the above analogues are suggesting that we may have a negative Arctic Oscillation during this summer. While this is not great news for the survival of Arctic sea ice, it is worth noting that the exceptional Arctic summer sea ice melt years of 2007 and 2012 are not in the analogues, and that both years had exceptionally wet summers over the UK.

Temperatures and rainfall

The Met Office probability and ensemble mean maps are suggesting a slightly more anticyclonic than average summer with the greatest positive anomaly centre over Scandinavia. Note the below-average pressure over southern and south-eastern Europe, which is also a feature of the ENSO analogues. The ensemble mean also suggests a drier and warmer than average summer, albeit not exceptionally so.

The Netweather long-range model is suggesting that both June and July will be more anticyclonic than average (though not exceptionally so), with a reduced mean westerly flow in June and a tendency for below-average pressure over central and southern Europe. For August pressure is forecast to be close to average. The long-range CFS is suggesting a predominantly anticyclonic June with frequent easterly winds in the first half, followed by the more traditional westerlies in the last third of the month but with the jet stream often deflected a fair way to the north.

The CFS model is broadly in agreement, pointing towards a positive NAO and AO in December and a mix of anticyclonic, westerly and south-westerly types, and then more blocking to the north in January and February promoting some easterly outbreaks, but also some milder interludes in between. July is currently forecast to be a mixed bag of a month, with August predominantly cyclonic.

The CanSIPS forecast model is suggesting an anticyclonic/westerly June, a weaker than average westerly flow in July and a marked anomalous easterly flow in August, but I have noted that the CanSIPS model has not performed well in recent months. Meanwhile the JAMSTEC model is going for a warmer and slightly drier than average summer.

The ECMWF model is suggesting above-average sea level pressure as the most likely outcome for the summer over Britain, but with relatively low confidence (it lies in the 50-60% range). A slightly drier than average summer is suggested, particularly for the south, with temperatures, perhaps surprisingly, close to the long-term normal, with a cold anomaly in the North Atlantic and warm anomalies over southern Europe.

Temperatures

Temperatures are most likely to be moderately above normal during the summer as a whole with positive anomalies (relative to the 1981-2010 reference period) between 0.5 and 1C most likely, but about a 30% chance of the anomaly being rather greater than 1C.

Temperatures will probably be significantly above average during June, with positive anomalies (relative to the 1981-2010 reference period) most likely between 1 and 2C and possibly above 2C in some regions. The warmest/hottest weather is likely to come during the last third of the month when there is potential for ridges from the Azores High to pull tropical air up from a fair way south at times. With a reduced mean westerly flow likely relative to normal, July is also (about 70% chance) likely to be rather warmer than normal, but with somewhat more uncertainty over the extent of the warm anomalies. The reduced westerly flow implies that it will also be cooler relative to normal near North Sea coasts. However, suggestions of a mostly cyclonic August point to temperatures during August coming out slightly below normal for most regions.

Rainfall

The rainfall signal for this summer is quite weak but with the most favoured outcome (about 60% chance) being below-average rainfall across much of England and Wales, particularly the south. For Scotland and Northern Ireland near-average rainfall looks most probable.

A drier than average June looks highly probable (roughly 75% chance, although rainfall totals may be swollen by thundery downpours in some regions). July will probably also be drier than normal for most, but with less confidence (about 60% chance). Given a likelihood of cyclonic weather, August will probably be wetter than normal for most (about 70% chance) but, given the overall signal for a drier than average summer towards the south, southern and particularly south-eastern Britain may see August's rainfall come out relatively close to the long-term average.