NAO To Go Positive, Last Blast Of Summer For Western Europe?

Written by on August 11, 2013 in Rest of Europe, United Kingdom & Ireland with 1 Comment

It’s been a much more classic summer pattern across Western Europe over the last 10-15 days while the very air mass which baked us back in July sizzles central and eastern Europe. Much of the Med up into the Alpine region has seen blistering highs of 35-40C with an all-time high of 40.1C in Austria. The front passed through a couple of days ago, wiping out of the intense heat but as a result sparked tremendous and destructive storms. Moscow which is hosting the World Athletics Championships has backed in mid-30s.

For Ireland, the UK and France, Low Countries, Denmark and Scandinavia, the next 10+ days looks no different to the previous day, however, as the NAO is set to go back positive, so I speculate the potential for one last blast of true summer across Ireland, the UK and the western flank of mainland Western Europe.

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There is a much cooler flow of air set to cross the continent this week as the upper flow flattens and allows Atlantic air across the continent. The heat becomes focused to the south, over and surrounding the Med.

Nothing’s written in stone and between now and the 25th, it appears we see a deeper trough into the UK not this week but the following but beyond that and towards the end of this month, we may see a ridge try to make one last attempt at real summer and by that I mean highs into the 27-30C range in the South of England as well as northern France up through the Low Countries and into Denmark and southern Scandinavia.

Here’s the NAO ensemble.

nao_sprd2

As you can see, the NAO ensemble shows a sharp return to fairly strong positive which would suggest a deepening North Atlantic trough and ridging either side. Timing is difficult because while the GFS shows this return to strong positive as early as 5-7 days, the CFSv2 shows a deep trough, not ridge  building into the UK and western Europe mid to late August.

Both the ECMWF and CFSv2 show a zonal next 10 days with multiple systems riding the jet into Europe keeping things largely unsettled with small bubbles of high pressure in between lows which allow a day or two of warmth, sunshine and dry weather in between wet and at times windy conditions.

Below is the CFSv2 500mb height anomalies week 1 through 4.

Week 1 into 2 shows a predominantly zonal or west-east flow from North America to Europe but notice how week 2 sees a warm bubble slide across Canada and a cold pool drops out of the arctic towards the UK.

wk1_wk2_20130810_z500

Week 3 shows a Greenland block and a trough with potentially our first shot of chilly air into the UK, possibly extending into the western mainland.

Interestingly, week 4 (above) shows the complete reversal from high to low pressure over Greenland and so the potential is there for a reversal from trough to ridge over the UK and western Europe. The CFSv2 also sees the return of a positive NAO but later than the GFS and the CFSv2 has a deepening of the negative before the large scale turnaround in height field.

wk3_wk4_20130810_z500

How long any potential warm-up and return to summer lasts very much remains open to question. My bet is that it’s fairly short lived.

Will have my September thoughts tomorrow and hope to have my official autumn forecast for you this upcoming week.

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  1. Michael says:

    Interesting update. Perhaps one last and late blast of summer and then we head into autumn. It shall be interesting to see what the autumn and then later on the winter brings. Hopefully lots interesting weather to come the coming months. Looking forward to both your autumn and winter forecast.

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