In Depth Model Discussion, Analysis and Summaries

[johnholmes]

I'll assume that 'they' will read this as a request, a nice request, is from you Pit!

[chionomaniac]

I have to admit that I have been so busy in all areas of my life that I have not been able to put as much time and effort into my hobby as I would like. And when I have had time to look no clear pattern has emerged. Hopefully I will have some free time coming up soon to start some forecasts for May!

c

[Glacier Point]

Perhaps you could ask them nicely if they've got any ideas for the summer.

Some loose thoughts as follows:

The last 6 months has featured a solidly negative NAO. Feedback loops have helped to reinforce this -NAO signature in the North Atlantic, especially a warm anomaly in the tropical seas off Africa which strongly teleconnects to an upper weakness over the Azores (low pressure). These generally telconnect to unsettled and variable summer time conditions.

El Nino conditions are still persistent in the Pacific although declining. The tropics and extratropics are becoming nicely tied together so the waxing and waning associated with upper level tropical waves are varying the strength of El Nino global teleconnections. During March this signal was for east trade winds (working against El Nino) whilst this has switched in the last week and thunderstorm activity and cloudiness has increased east of the Dateline as the upper tropical wave moves eastwards. Hence why numerical model tools are gunning for a reintroduction of the blocking to the north. The periodicity of these waves is 6-8 weeks. Therefore, the weakening of this wave should begin in mid May and continue through to early to mid June allowing the NAO to become more neutral and probably quite a nice spell of weather. The period mid June through late July will likely favour a return of -NAO conditions. With a -AO more favoured during east QBO phase, this looks unsettled and cool during this time - but need to research this further. Late July through August looks slightly better with the NAO becoming more neutral based.

At this stage, I would be very against the development of a strong Azores ridge - the usual harbinger of summery weather for the UK. The SSTs in the Atlantic and residual Nino (not to mention the increasing amount of colder than normal waters across large parts of the north Pacific) do not favour this.

This leaves the options of a significant ridge over western Europe delivering warm and sunny weather or a mean ridge centred towards Iceland and the central northern Atlantic leaving NW Europe in a cool mean trough. The balance of probability favours the latter.

But, the key is likely to be the strength of blocking the north tied into the Arctic Oscillation. If it's weak, then more finer weather is likely. If strong, then it will be 4 poor summers on the bounce.

[cooling climate]

Refering to a post you made yesterday in the model thread the weather pattern

does seem to be enforcing the the sst tripole pattern in the Atlantic or vice versa.

If this pattern holds over the next 6 to 8 weeks then according to the Met office

this would favour another -NAO winter season 2010-2011.

[SteveB]

Thanks for the update.

[Glacier Point]

As we hit May the patterns start to get that little bit more interesting - can they tell us something about what is to follow in June and July ?

Total angular momentum is anomalously positive (net westerly wind anomaly) indicative of good interaction between tropical waves and the extratropical circulation. In this instance, westerly winds have been added through a strong burst of winds over the Himalayas:

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/gltotaam.sig.90day.gif

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/gltaum.90day.gif

A look at the global wind oscillation (GWO) shows us tilted towards the El Nino base state with an orbit in phase 5 and likely persistence in phases 5-6 for the next 10 days or so.

http://www.esrl.noaa.gov/psd/map/images/gcm/gwo_40d.gif

A measure of the coupling between tropics and extratropics is the MJO in similar phase space.

Composites for phase 6 of the GWO in early May depict a warming pattern with decreasing velocity allowing the jestream to become looped and downstream ridge building over Europe and the NAO to become neutral.

Long range model tools strongly support this idea with the CPC modified long-wave product and ECM ensemble mean anomalies both in tune:

http://www.cpc.ncep.noaa.gov/products/predictions/814day/814day.03.gif

http://www.meteociel.fr/modeles/ecmwf.php?ech=240&mode=1&map=1&archive=0

It should be noted however that the tropical waves will likely cause total angular momentum to fall back during the second half of May allowing a more -NAO pattern to take hold and I woulde be expecting the ridge to pull out into the mid Atlantic re-introducing a cooler NW flow to return. But for now, the prospect of some decent weather to start May.

[LomondSnowstorm]

Nice! It has been a decent April so far, at least for this part of the world, so long may it continue. I'm not sure if May's predominant conditions affect those later in the season and I suspect with most things in monthly forecasting that the link is fairly speculative at best but it would be nice to have a decent summer.

Not sure how this will affect the ash situation though - the high pressure has not helped at all in the last week but of course it depends a lot on the volcanic activity and also where the high will set up.

Great post as always GP!

[Glacier Point]

As we hit May the patterns start to get that little bit more interesting - can they tell us something about what is to follow in June and July ?

It should be noted however that the tropical waves will likely cause total angular momentum to fall back during the second half of May allowing a more -NAO pattern to take hold and I woulde be expecting the ridge to pull out into the mid Atlantic re-introducing a cooler NW flow to return. But for now, the prospect of some decent weather to start May.

Tonight's data runs are suggesting that the evolution above is taking place much more quickly than anticipated.

Cloudiness is increasing over the eastern Indian Ocean and Indonesia:

http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/winds/wavetrkP.html

This also indicates low level easterly winds increasing across the Pacific which is consistent with falling angular momentum. Angular momentum budgets are falling:

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/gltotaam.sig.90day.gif

and the GWO is responding with a rapid orbit into phase 0:

http://www.esrl.noaa.gov/psd/map/images/gcm/gwo_40d.gif

The composite anomaly for phase 0 indicates a well defined ridge in the Atlantic and trough over NW Europe:

It's quite possible that there will be a short bounce back towards phase 6 as mountain torques remove easterly winds (signal for ridge centred more towards the UK), but the overall trend seems well defined for falling momentum and mid Atlantic ridge.

[in the vale]

I am really trying hard to understand the tech model thread which, although generally quiet, is much more educational (in the bigger picture sense) than other model threads. Anyway, as we are mad campers, my question is this: GP's posts seem to suggest a possible earlier warmer spell followed by a more W flow which is pretty rubbish for camping (esp. in Wales), so is this a similar outlook to e.g. last year (my memories of two wet/windy weeks in Tenby in August are still to be erased) or is there still a chance of better weather later on? I apologise if this is the wrong place for this question, but it does seem to be a 'technical', long-range model issue rather than a 'next weekend' issue.  

[Glacier Point]

You might get some better weather late May although I wouldn't be holding out for June which looks unsettled IMO.

May should feature a solid mean ridge centred in the mid Atlantic so if it's close enough, west might fare best and as the jet flow will be over the top, precipitation should be below or close to average.

Edited April 26, 2010 by Glacier Point

[Eugene]

But for now, the prospect of some decent weather to start May.

Not so sure about that GP May looks like starting on a very cool and rather wet note but probably drying up after the first few days and as you say a solid mean ridge to our west keeping it rather cool but mainly dry.

Looks like we could be in for a cool first half to May, as you think i think the second half or last week will fare better but an unsettled June is on the cards, i'm personally going for a cool and wet June, warmer first half to July but still unsettled and improving mid to late July onwards.

[The PIT]

Seems to be falling into place at the moment.

[summer blizzard]

http://www.esrl.noaa.gov/psd/data/correlation/qbo.data

Good news in regards to July/August, April saw a surge in the easterly QBO value to -24, which indicates that the easterly QBO should peak in May or June assuming we do not see a record event, this tips the scales away from 70/30 cool, to 50/50 average imo and i would quite strongly expect an above average August now.

[in the vale]

Apparently rainfall was recorded near the North Pole in April. Does this have any implications for our weather? Maybe some of you 'in depth' folk would care to comment if it is relevant? Linky here: Story Here

Edited May 5, 2010 by in the vale

[Optimus Prime]

Apparently rainfall was recorded near the North Pole in April. Does this have any implications for our weather? Maybe some of you 'in depth' folk would care to comment if it is relevant? Linky here: Story Here

"A group of British scientists working off Ellef Ringnes Island, near the North Pole"

Enough said.

[in the vale]

"...50 to 60 years of historical weather data show no signs of rainfall ever occurring in April in the High Arctic." Despite it being near the Pole, rather than at it still makes this anomalous. Maybe it means nothing, but still.... 

[MR EXTREMES]

its alright it would have froze on impact adding to ice extent lol dont tell gray wolf lmao.

[Devon-Nelly]

http://www.esrl.noaa...lation/qbo.data

Good news in regards to July/August, April saw a surge in the easterly QBO value to -24, which indicates that the easterly QBO should peak in May or June assuming we do not see a record event, this tips the scales away from 70/30 cool, to 50/50 average imo and i would quite strongly expect an above average August now.

Would this be because the -QBO signal would weaken allowing the Jet stream moving further northand weaken the Northern blocking, making for warmer weather to be more likely?

And if so on that basis we could be looking at a fairly cool June, a more average July and Warm August? I personally think June will see fits and bursts of warm weather alternating with more prolonged cooler periods.

[summer blizzard]

Would this be because the -QBO signal would weaken allowing the Jet stream moving further northand weaken the Northern blocking, making for warmer weather to be more likely?

And if so on that basis we could be looking at a fairly cool June, a more average July and Warm August? I personally think June will see fits and bursts of warm weather alternating with more prolonged cooler periods.

Yes, a weakening -QBO would promote a more +AO towards the end of summer particulaly, so an improving picture as summer progresses would be my favoured senario.

[Glacier Point]

Total angular momentum is on the slide....

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/gltotaam.sig.90day.gif

This is very signifcant as it comes ahead of models showing an increasing shift towards La Nina in the latter part of the summer. It also suggests to me that the atmospheric circulation is wanting to embrace a negative tendency, far removed from an El Nino which would be the desired base state for a warm summer pattern.

Where do we go from here ? Well, models are starting to advertise a shift in pressure over the Atlantic, which is to be expected given the anticipated recovery in angular momentum as we approach June so a brief ridge developing over western Europe as the pattern and wavelengths alter looks plausible but models are likely to be too progressive in this change and it may take another 14 days to take effect.

However, I think the now very strong emerging signal is for an unsettled June pattern with low pressure anchored to the immediate west of the UK, probably on the cool side as well. As hinted at in previous posting, the mid Atlantic ridge will probably return into July.

[Eugene]

Thanks GP for your thoughts even though for summer fans the prospects sound dismal.

According to what you just said we might have to write off the whole of the summer with not much improvement even in August, could be worse than the past three summers that had some decent spells in August.

We could be in for a very cool July if you thoughts on the mid atlantic ridge returning are correct, that is the kind of pattern that brings cool 850's from the north and some very cool nights even with the chance of local ground frost in rural areas :lol:

Yes looks like we may see a brief weak ridge over western europe early next week but not lasting long enough or strong enough to halt atlantic LP systems rolling in.

[damianslaw]

Thanks GP for your thoughts even though for summer fans the prospects sound dismal.

According to what you just said we might have to write off the whole of the summer with not much improvement even in August, could be worse than the past three summers that had some decent spells in August.

We could be in for a very cool July if you thoughts on the mid atlantic ridge returning are correct, that is the kind of pattern that brings cool 850's from the north and some very cool nights even with the chance of local ground frost in rural areas :lol:

Yes looks like we may see a brief weak ridge over western europe early next week but not lasting long enough or strong enough to halt atlantic LP systems rolling in.

Yes a persistant mid atlantic ridge during the summer doesn't make for great reading for those wanting some sustained warmth, however, it wouldn't be too bad a pattern, it would prevent the steamtrain of low pressure system and consequently plenty of dry albeit probably cloudy weather, however yes it would deliver consistently cooler than average temps

Personally I'd much rather have the warmer stuff during June and July, others probably disagree and take the best weather in August with its associated school holidays, however, during August we quickly loose day light.

[Eugene]

Yes looks like we may see a brief weak ridge over western europe early next week but not lasting long enough or strong enough to halt atlantic LP systems rolling in.

Well that was wrong the azores high built in from the southwest, yes the mid atlantic ridge pattern that returns next week could actually give us a repeating dry ish cool pattern over the summer, it all depends on whether the jet picks up and tracks southwards, the jetstream really is dead atm and looks like remaining that way.

[Jezzer]

This may be a more relevant thread to post my query...

I was just wondering if someone could explain the movements of pressure systems. The latest runs obviously suggest that the High pressure that is building over the UK this weekend will retreat to the west allowing for the northerly air flow to kick in.

What is currently causing this westward movement of the highs, I always presumed that pressure systems usually drifted in an easterly direction towards and across the continent, if someone could explain to a novice I'd be grateful.

Many thanks

Jez

Edited May 20, 2010 by Jezzer

[summer blizzard]

This may be a more relevant thread to post my query...

I was just wondering if someone could explain the movements of pressure systems. The latest runs obviously suggest that the High pressure that is building over the UK this weekend will retreat to the west allowing for the northerly air flow to kick in.

What is currently causing this westward movement of the highs, I always presumed that pressure systems usually drifted in an easterly direction towards and across the continent, if someone could explain to a novice I'd be grateful.

Many thanks

Jez

Easiest way to explain this, is if you imagine a low over Iceland, and a high over the Azores, the high pressure is essentially being pushed eastward by the Jet Streak between them, whereas because we have a low further west, in the Central Atlantic, what we see is the low pressure acting as a vaccum to pull the high pressure system back, over the top so that the low can undercut it.