Summer 2016 update 4

Matt Hugo's view

What does summer 2016 have in store? The TWO summer forecast will be issued in late May but throughout April independent meteorologist Matthew Hugo, BSc, FRMetS, RMet will be giving us his view on what the current data is suggesting.

IMPORTANT: This may or may not correspond with the TWO view.

Overview

1) Latest seasonal models continue to produce rather contradictory data in terms of the likely pressure trends for the UK summer period, with still more in the way of higher pressure signaled now, especially during the early and middle part of the summer.

2) Within the last week or so changes in the SST (Sea Surface Temperature) anomaly pattern across the North Atlantic may now also support the possibility of higher pressure across the UK and N Europe in general.

3) Latest data suggests that whilst a rather active hurricane season is still likely, particularly compared with the last few years, the development and persistence of colder than average sea temperature anomalies off W Africa, within the region of tropical development, has lowered the overall risk of a more active season. As a result this reduces the risk of more unsettled conditions across the UK to an extent through late summer.

4) The updates and trends over the last 4 weeks have definitely shifted towards to a greater probability of a better summer across the UK than was expected at the end of March.

Seasonal model update

The late April period now provides a more comprehensive set of updated seasonal charts and information with the likes of the EC Seasonal, UKMO GloSea and CFSv2 models all being updated within the last week or so. The April update of the EC Seasonal model now has a slight bias towards higher pressure over and just to the west of the UK for the period that is June, July and August and this highlights a potentially drier than average summer. Temperature predictions from the EC Seasonal are more open to speculation with little bias towards above or below with generally near average temperatures preferred, which from the UK’s point of view would still support some warm or very warm weather at times. This change in evolution for the summer period creates a lack of model consistency after the March updates which favoured more unsettled conditions through the summer period.

The UKMO GloSea seasonal model (updated 11Apr16) has lost its signal for lower than average pressure to become influential as the summer progresses and now provides a rather vague output for the summer period. On balance there remains a signal for below average temperatures to be present through the summer period to the west of the UK, as exampled on the image on the right below. But overall there is now more in the way of a bias towards near or perhaps above normal temperatures for the summer period, particularly across central and eastern areas of the UK.

The signal regarding pressure through the summer is particularly vague in the latest update with a slight bias towards near-normal sea level pressure which would generally equate to reasonable summer conditions with the likelihood of the Azores high pressure building into the UK at times and leading to summery conditions, for example.

The latest update (22Apr16) of the CFSv2 seasonal model maintains a signal from the last week or two now for overall higher pressure to be more influential across the UK during the summer than low pressure, particularly during June and July. There remains a tentative trend within the month of August to perhaps see lower pressure develop. The images below highlight this with the orange colours indicating generally higher than average pressure, but note on the bottom image (August) the signal for low pressure to potentially develop to the northwest and perhaps lead to more of a westerly-oriented spell of weather during the month, which could lead to spells of unsettled conditions at times.

Atlantic sea surface temperature anomalies

The development of higher sea temperature anomalies across more southern and western areas of the N Atlantic, whilst lower sea temperatures continue further north and east, has also resulted in a general shift in the signal for the summer period. This pattern is known to promote generally higher pressure across the UK and N Europe through the summer period, but there is definitely a fine balance as below average sea temperatures within the N Atlantic can also lead to a more unsettled summer period, as was discussed in recent updates. The recent temperature anomalies across the N Atlantic are below and equally note the below average temperature anomalies that have developed off W Africa, which relate to the comments earlier about this year’s hurricane season.

Summary

Throughout the last four weeks there has been a subtle trend towards a greater possibility of higher pressure across the UK this summer than lower pressure. The initial thoughts during late March and early April were towards a progression towards far more unsettled conditions as July and August arrived, with some of the best conditions during early summer. However, May now looks to be quite a changeable and unsettled month which contradicts those initial thoughts to start with. As is often the case across the UK, the main indicators for the summer period are often far more contradictory and difficult to analyze than those prior to winter.

However, at this juncture given the information of the last four weeks, the overall outcome is now for the summer across the UK to be far more ‘average’, with a bias towards higher pressure over the three months than lower pressure. This would lead to some very pleasant summer weather at times with no doubt a number of individual spells of hot weather too, but there remains a tentative comment and bias towards more unsettled conditions potentially developing during August, with the best summer weather through June and July. At the moment the probability of an unsettled and cool summer period (for example, 2012) is low or perhaps very low and this summer may well be one of the better ones of recent years.

Matt will be providing weekly updates on the summer weather prospects throughout April.

Issued by: Matthew Hugo, BSc, FRMetS, RMet

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