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Atmospheric circulation this summer

PostPosted: 2012 Jul 21, 14:24
by Peter Wright
Before the summer I made a tentative forcast, but I did not publish it. I was impressed by the buildup of warmth over much of Europe, and I visualised that this would continue and provide an anchor, with us getting fringe benefits as the warmth extended, at least periodically, westward. How wrong I would have been!

On June 1, the jet stream got into a pattern with an anticlockwise flow located in the SE North Atlantic, which favoured the development of low pressure on its left flank, namely over the British Isles, often extending to Scandinavia. The result, in the surface charts, was that every small or medium low pressure in the North Atlantic made a beeline for the UK, and when it arrived, it sat over us without losing its intensity for many days. This amazing and grossly disappointing pattern happened time and time again! This pattern got set in its ways, and prevailed throughout June, with the hot area present over SE Europe just as persistent on the S side of the same jet stream.

On July 3, with a forecast for the next 2 weeks of 'same again', I departed for Croatia, where I came into the hot zone. The weather was just as constant as in Britain. But the opposite. Britain had day after day of cloud and rain, with maxima 15-20C. Croatia had day after day of hot, virtually cloudless skies, no rain, wind varying from calm to fresh, and temperatures which every day reached around 36C, and the nights felt like the upper 20s or higher. It was certainly 'better' than in Britain, but the afternoons were not times for doing much -- a bit of averaging-out would have pleased both us and you! Only on our last day was the weather a much more pleasant 30C by day and a cool breeze (sometimes strong) getting down to perhaps the lower 20s at night.

In Britain the first changes seemed to be starting also, leading up to the current high pressure spell, while Croatia is forecasting unsettled and much cooler weather with thunderstorms. So is that persistent pattern breaking down? On current forecasts, sadly no, this is just an interlude. We seem to be in for a return to attracting every depression that approaches us from whatever direction.

I can only conclude that some pattern elsewhere in the hemisphere is persistent and has a teleconnection which favours low pressure over Britain. That pattern might have started in April, which exhibited a remarkably instant shift from a 2-year drought to a very wet pattern, and April indeed had depressions close to us many times. When will this end? Well, the drought period (when pressure was repeatedly high over southern Britain and weakened almost every rainband that came our way) lasted over 2 years!

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 21, 15:11
by greg_gruner
On current forecasts, sadly no, this is just an interlude. We seem to be in for a return to attracting every depression that approaches us from whatever direction.

A good assessment of the summer so far, but I was wondering what these "current forecasts" are? GFS charts? ECMWF? Or some forecaster's opinion? The ECMWF charts show dominant high pressure near the UK until July 31st. The longe range GFS do show some lows returning, but must be taken with a pinch of salt. Jet stream forecasts that I have seen (http://virga.sfsu.edu/scripts/jetstream ... _fcst.html) indicate no return to the previous pattern.

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 21, 16:49
by Peter Wright
I look at the GFS charts in www.wetterzentrale.de . They start having a low close to the British Isles by Thu Jul 26. This is within the first week ahead and there has been some consistency over recent model runs. But I'm pleased to see that the ECMWF has a more optimistic outlook, as indeed the GFS had until about 3 days ago. I agree the 2nd week ahead cannot be relied on.

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 21, 20:25
by greg_gruner
There seems to be some divergence between GFS and ECMWF for July 26-31, the ECMWF has a low brushing past us in the North Sea, while the GFS has a low over the UK for some days. However, beyond Aug 2nd (the inaccurate period, I know) it has high pressure returning.
I hope the ECMWF is correct! I think the Olympics will start in warm sunshine, then a spell of cooler weather with some rain and showers, followed by sunshine again. We shall see!

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 21, 20:26
by Martin Rowley
Peter Wright wrote:I look at the GFS charts in http://www.wetterzentrale.de . They start having a low close to the British Isles by Thu Jul 26. This is within the first week ahead and there has been some consistency over recent model runs. But I'm pleased to see that the ECMWF has a more optimistic outlook, as indeed the GFS had until about 3 days ago. I agree the 2nd week ahead cannot be relied on.


... I'm not sure how widely known-about these output are, but buried on the Meteogroup site here:-

http://www.weathercast.co.uk/services/e ... ecast.html

you can see a limited selection of ensemble 'plumes' from both the GFES & ECEPS plotted on the same chart. Unfortunately, only London is shown for our part of the world, but at the range we're considering, it's enough to see (in this particular case) the 'step-change' that occurs around 27th/28th - reflected in both models and has been for a few days. They do differ, interestingly (where they overlap) in amounts of rainfall consequent upon the change - presumably a consequence of exactly where the trough & its disrupted upper low ends up in relation to London.

Martin.

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 22, 13:07
by greg_gruner
Thanks, Martin, I hadn't seen that on Meteo Group before.
The latest ECMWF and GFS seem to show a shallow low developing over France on Friday, moving north east towards Scandinavia and pulling in a cooler northerly. The GFS (but not ECMWF) also has a seconday low crossing the UK in the northerly flow. But afer Aug 2nd high pressure seems to regain control.

My thinking is that it should just about stay dry for the Olympic opening ceremony, but there is the chance of some big storms in the SE as the cooler air moves in. Then several days of cooler showery weather before increasingly sunny again. If the ECMWF is right, I can't see much rain falling in the SW, as we always do well in a N wind and the showers are further east.

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 23, 11:56
by Peter Wright
Current (1200Z on 23rd) GFS charts show a gradual return to the June/July pattern of low pressures approaching or settling over the British Isles, and the jet stream back where it was then.

(Edited at 1700Z:) Looking appreciably better at the latest run -- we are still forecast to be in a cool area of theflow, but not the centre of action of low pressures.

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 23, 19:54
by greg_gruner
I'm looking for evidence that we still have a summer to come, and the latest GFS for Aug 6th + shows high pressure - but that's speculative. ECMWF and GFS have diverged a bit over the last few days, as well as changing from day to day. This I think is why the Met Office outlook has some uncertainty: "Locations and timings of rain are currently uncertain. " A forecasting headache...and just when the Olympics start.
I have to smile when I think of how many people will be arriving in our country this week for the Olympics, no doubt with umbrellas and raincoats, having seen reports of heavy rain and floods, only to be greeted at Heathrow by sunny skies and a temp near 30C!! I am sure they will get the other side of the British summer before they leave, though...

Re: Atmospheric circulation this summer

PostPosted: 2012 Jul 24, 09:00
by greg_gruner
Aha! We have some consistency in today's charts. Looks like the weekend low will stay to the NE and introduce a cool NW flow (should not be a great deal of rain in the S and W). Then a low moves south and crosses the UK at the end of July/beginning of August. Then, according to GFS, high pressure builds again from Aug 3rd onwards with the jet stream moving north again. I have high hopes for some more summer weather in August!