Summer Forecast 2019

Forecast Overview

Summary

This has proved to be a difficult summer to forecast because many of the signals are weak and conflicting, in contrast to the summer of 2018, for which there was a fairly strong signal for relatively warm, settled weather for June and July before breaking down into August.

It looks probable that after an unsettled start to June, towards mid-June the weather will turn quieter with a reduced westerly flow over the British Isles relative to normal, and a relatively high frequency of easterly and southerly winds, suggesting mostly above average temperatures and potentially a high frequency of thunder for some parts of the country.

For July and August the signals are somewhat mixed, but with strong suggestions that conditions will not be far from the long-term normal overall, with relatively anticyclonic (but not especially warm) weather suggested for August, perhaps not dissimilar to what we had in the Augusts of 1993 and 2007.

It’s not looking likely to be a repeat of summer 2018, so if the projections come off, 2018 will probably go down as a one-off dry sunny summer rather than as the “first of two” as per 1975/76 and 1989/90.

Read below for more details on the forecast, and the factors that were considered when putting it together.

ENSO Signal

ENSO has been running in a weakly positive state during the first four months of 2019, mostly close to or just below +1. The NMME (above), Met Office and ECMWF forecasts are agreed that this will probably continue during the summer and beyond, although the level of uncertainty in the forecasts increases substantially by August.

The closest analogues for this situation are 1953, 1969, 1977, 1991 and 1993.

The overall signal from these analogues is very weak for the summer quarter as a whole. For June there is a slight tendency for below average pressure over central Europe and a reduced mean westerly flow relative to normal over the British Isles. For July there is no substantial signal, while for August the mean sea level pressure is about 2mb above average over much of the UK, suggesting a weak tendency for relatively anticyclonic Augusts. Above average pressure is suggested over the Arctic which may be detrimental to the Arctic sea ice extent come September 2019.

Stratospheric conditions

The QBO has moved into a westerly phase during early 2019, but the QBO and the stratosphere temperature are not generally known to play a large role in the weather over the British Isles in summer. The westerly QBO phase, if it continues, could increase the chances of a mild late autumn and winter in 2019/20.

MJO

The Madden-Julian Oscillation is moving from phase 1 into phase 2 as of the beginning of June 2019, but is then forecast to move to a very low amplitude, suggesting that the MJO will probably not have much of an impact on Britain’s weather during the middle part of June. The MJO generally does not play as major a role in our weather in summer as in winter, but those looking for fine anticyclonic weather are advised to watch out for phases 3/4 in June and July.

Arctic Sea Ice

Arctic sea ice is anomalously low, about the same as in 2018, with a similar anomaly pattern to 2018, with relatively near normal sea ice extent on the Atlantic side of the Arctic, but much below average extent over the Beaufort and Chukchi Seas. Only 2018, 2016, 2015 and 2011 had comparably low sea ice extent at this time of the year. Note that 2012, currently the year with the lowest September sea ice extent, sea ice extent was actually much higher as of late May.

This is a very mixed bag of summers, and unsurprisingly the seasonal composites show little correlation with sea level pressure over the UK, but with an anomalous tendency for high pressure over Greenland, which is strongest for the months of July and August, which also on average have a slight anomalous cyclonic tendency over the UK. For June there is not much of a correlation. The possible implication is that we may see a relatively settled June followed by more changeable conditions in July and August with relatively high pressure over Greenland and the jet stream deflected to the south of its normal position, but it is worth pointing out that this did not happen in 2018. Even the relatively unsettled August of 2018 still had below average pressure in the Arctic, with an enhanced westerly flow relative to normal over the British Isles (rather than a southerly tracking jet stream).

North Atlantic Oscillation and Arctic Oscillation

The signals for the North Atlantic Oscillation and the Arctic Oscillation are somewhat weak for the upcoming summer, but with the low Arctic sea ice extent perhaps pointing to an increased chance of a negative Arctic Oscillation (though summer 2018 had a strongly positive Arctic Oscillation, which casts doubt on the reliability of this association, especially as it relies on just a few years of data).

Temperature and rainfall forecasts

The Met Office ensemble mean gives conflicting signals, with mean temperatures projected to be just slightly above the long-term normal (0.0 to 0.5C) with rainfall above normal, but with mean sea level pressure also a little above normal with an anomaly centre in the eastern North Atlantic, perhaps suggesting a mean westerly flow over the UK that is veered from normal, giving an anomalously high frequency of northerly and/or north-westerly winds. However, the Met Office probability maps suggest that the signal for above average rainfall is only weak, with a rather stronger signal for above average mean sea level pressure.

The Netweather long-range model is suggesting a slightly more anticyclonic than average June (mean sea level pressure ~1mb above the long-term normal) with close to average sea level pressure for July and August. There is a signal for drier than average conditions for central and southern areas in June but wetter conditions for many areas in July and August, particularly over large areas of Scotland.

The CFS model gives conflicting signals for June precipitation but with agreement that in July rainfall will be near or below normal, and that in August it will be near or above normal. Temperatures are forecast to be near or above normal in June, above normal in July and near normal in August.

The long range forecast guidance from ECMWF suggests that near to slightly above average temperatures are most likely for the UK this summer, with the British Isles lying on the eastern flank of the persistent cool anomaly over the North Atlantic Ocean. The signals for mean sea level pressure and precipitation are weak, but point towards above average rainfall over northern Scotland and a slightly stronger than average mean westerly flow over the British Isles. Mean sea level pressure is forecast to be near normal.

The CanSIPS forecast model has not been performing well recently, but is suggesting a relatively cyclonic/westerly June, a less westerly than normal July with near average sea level pressure overall, and then an anticyclonic but fairly “northerly” August.

The JAMSTEC model outlook appears to strongly agree with the ECMWF model, with slightly above average temperatures for most of the UK but near to slightly below for Northern Ireland, which lies closest to the cool anomaly in the North Atlantic, and with above average rainfall for northern Scotland and near average elsewhere.

Temperatures

Temperatures are most likely to be about 0.5C above the 1981-2010 long-term average over most of the country during this summer, perhaps more than that in the south-east of Britain and in northern Scotland, but very close to the long-term normal over Northern Ireland, which will lie on the eastern flank of a cold anomaly in the eastern North Atlantic Ocean.

June is expected to be warmer than average due to an above average incidence of anticyclonic, easterly and southerly regimes, by between 1 and 2C in most parts of the country. For July and August close to average temperatures look most probable, with slightly below average temperatures likely for Northern Ireland.

Rainfall

Rainfall is most likely to be near to slightly above normal but with above normal rainfall about 70% likely for the north of Scotland. Given that June will probably be less “westerly” than normal, with the prevailing westerlies probably not really establishing until the very end of June or into early July, the implication is that this excess rainfall for northern Scotland will come mostly from July and August.

Due to an unsettled start to June and then a likelihood of some thundery rainfall at times associated with easterly and southerly winds, June’s rainfall will probably come out near or rather above normal, but with considerable local and regional variations, and it will probably be drier than average in the north and east of Scotland.

Confidence for July and August rainfall is much lower, but it will probably be wetter than normal in the north of Scotland, with close to average rainfall in most other parts of the UK. July is more likely to be on the wet side, August on the dry side, but neither month is expected to be particularly warm.