In Depth Model Discussion, Analysis and Summaries

[damianslaw]

Not liking the prospects of the polar vortex weakening later in April as this will not doubt herald the return of generally cooler conditions after what is looking a very mild period for the time of year. I'd much rather have the milder conditions later in April and into May than now - early spring warmth at this time of year is a waste - by May you begin to feel the benefits much more. A cool unsettled April or a cool unsettled May I'd much rather have a cool unsettled April. I worry all the dry mild conditions of late will be quickly used up so to speak leaving us with another poor wet summer.

[sundog]

I worry all the dry mild conditions of late will be quickly used up so to speak leaving us with another poor wet summer.

[Alex]

A weak el nino and +qbo would be ideal for summer!

[summer blizzard]

A weak el nino and +qbo would be ideal for summer!

Indeed, although the chances of getting the combination are minimal.

[cooling climate]

El Nino + positive QBO = good summer, I am positive the weather is far

more complicated than that. What phase the AO is in this summer will be

far more important.

[summer blizzard]

El Nino + positive QBO = good summer, I am positive the weather is far

more complicated than that. What phase the AO is in this summer will be

far more important.

The ENSO and QBO state are two of the biggest factors which influence the state of the AO. Having conducted reasearch and posted it in the Spring/summer thread, neutral/+QBO along with a El Nino were present in over 80% of months which achieved a CET greater than 18.0C since 1950.

[Supercell 89]

I love reading this discussion and how all everything clinks together. I wanted to ask a question about Angular Momentum, firstly what is it? i have a hunch its the winds that circulate the globe but i'm not brilliantly sure. Secondly how does this effect the rest of the climates around the world including ours?

Also in regards to summer, i know its along way off but its something i've been pondering on alot lately. My thoughts are that La Nina isn't really going to go anywhere fast and that we're in the middle of a mutli La Nina sequence, and that the current La Nina will only wane to a weak stage during the summer only to strengthen again during the winter. Also this combined with a QBO which i think will flip negative in the next month or two will aide with the weak La Nina to bring an increasingly cool and wet pattern.

Im probably not making sense and reading the signals wrong but still trying to get to grips with how everything clogs together.

[snowlover2009]

Im thinking based on some things I know, that if La nina weakens through out summer, we should get a better late july/august? If we can keep a weakening la nina, maybe we will get another very harsh november/december period once again,maybe if we are lucky, we could see a weak el nino start to develop? Just a thought? Odds are in favour I would imagine, considering how long we will have had la nina, chances are it will weaken and chances are that an el nino should develop, but probably later in the year and it should be weak to start with. But that is just based on a bit of my knowledge, I could be wrong, it is just a rough guess, but I do think, summer could be very wet at stages and the end of the year atm will have some moments.

[Glacier Point]

Some similarity with the weather pattern experienced in early to mid February so there is some backing for an unsettled spell, but I have to question just how unsettled (especially for southern UK) and how long this pattern will last ?

Comparison of relative angular momentum plots identifies an increase in westerlies in the mid to high latitudes in the Northern Hemisphere. This is excatly the same as the period experienced during the first half of February following an increase in tropical convection.

Reanalysis of that period does reinforce long range modelling, showing below normal heights in the Atlantic although look at the anomalous ridge still persistent over Europe which might just hint at a weakness in the modelling which generally shows a jet angled NW-SE into the Alps.

Additionally, the Global Wind Oscillation is currently in phase 0 having gone through a distinctive phase 4-0 orbit typical of anomalous mid latitude ridges following a rise in angular momentum. As commented last posting, we should expect to see the lagged impacts of the last MJO wave-induced bout of tropical convection throughout the next three weeks or so. This should maintain angular momentum in phase 0-4 type orbits, possibly some entering of low amplitude phases 1-2-3 although latest MJO forecasts are beginning to hint at a second increase in angular momentum over the tropics late month which would prevent angular momentum falling too low. The current modelling suggesting a very unsettled phase is more akin to phase 1-2-3 orbits of the GWO, so some disagreement there.

Compare plots for GWO phases 0 and 4..

....with phases 1-2-3

Temperatures in the polar stratosphere have risen sharply and zonal winds have started to reverse. The lagged impacts are going to be tricky to decipher although it should be noted there is a lot of residual westerlyness in the atmosphere. I suspect we will begin to see an emergence of high pressure around the North Atlantic (Iceland and Scotland being the centre) as the net result of this. How long it lasts will be interesting to monitor to gauge our prospects for the summer.

So in terms of probabilities, I would favour the south of the UK to hold onto the more settled weather and the north to be more windy and unsettled. In the longer timeframe, there are suggestions that settled weather may well extend further north agaun althoug temperatures are not likely to be as warm as they currently are, although average or slightly above is favoured.

[Glacier Point]

The longer range modelling is starting to look a lot more realistic from my perspective with a general trend towards troughing in the Atlantic and ridge developing over Europe / Scandinavia, which is consistent with where the GWO signal is right now.

A couple of observations for the dynamical ocean-atmospheric coupling for the coming months:

1) If we take a look at trade winds and anomalies since January, we can see that the easterly trades are at their weakest generally, second only to a brief period in January associated with a strong MJO event. In particular, take a look at the right hand side panel which shows the weakening of easterly zonal wind anomalies around the Dateline (180W). This is consistent with the atmosphere and sea surface during a decaying La Nina event (although the Southern Oscillation continues to tank positive). This increases the prospect of subsequent MJO waves to gain strength and induce further westerly wind bursts into the global system in the future. Also note how the latest westerly wind burst is the strongest this year around 140E.

Why does this matter ? It matters because westerly winds are what drive ridge development in the low and middle latitudes. More westerlyness increases the chances of high pressure for Europe into the summer.

2) Analysis of the profile of SSTAs in the North Atlantic suggests that over the past month, large areas of the NW Atlantic extending up the eastern US and Canadian Coast and into the central-western sub-tropical Atlantic have cooled by 0.25C to 1C. The surface waters around the eastern Atlantic including the south-west approaches have correspondingly warmed. This is an increasing signal for troughing or cut off lows in this area as a baroclinic response with an increased probability of ridge development in the NE Atlantic and Western Europe.

Both issues raise a big question in my book. Where is summer rainfall coming from ?

Edited April 11, 2011 by Glacier Point

[Gavin P]

Looks like we're heading for a summer 1975/1976 type post La Nina pattern then GP?

Edited April 11, 2011 by Gavin P

[snowlover2009]

Looks like we're heading for a summer 1975/1976 type post La Nina pattern then GP?

June to me, was always looking the most unsettled and to some people, but I wonder if we may be lucky to see a reasonable June? Then we also have July and August which are looking decent, well from my perspective, people's opinion and going on what Lrfs are showing.

Going to be a hell of a lot of water shortage

Edited April 11, 2011 by snowlover2009

[Gavin P]

Going to be a hell of a lot of water shortage

I think most places should be OK for this summer. Yes, they'll be hosepipe bans and obviously it'll have a disasterous effect on agriculture, but as far as people keeping their water supply goes, we should be OK this year.

Now, if we were to repeat the 75/76 pattern and had a dry autumn/winter 11/12 followed by another intense heatwave and drought through summer 2012, then next year would be looking far more serious. That would take quite some doing, though.

Edited April 11, 2011 by Gavin P

[snowlover2009]

I think most places should be OK for this summer. Yes, they'll be hosepipe bans and obviously it'll have a disasterous effect on agriculture, but as far as people keeping their water supply goes, we should be OK this year.

Now, if we were to repeat the 75/76 pattern and had a dry autumn/winter 11/12 followed by another intense heatwave and drought through summer 2012, then next year would be looking far more serious. That would take quite some doing, though.

You know your stuff I suppose as an established member

I have no evidence or sources on why, but I think we will have a wet autumn all round from October to November. I have no idea on winter, but if it is like the last 3, then it will be possibly another dry one, I am speculating another cold winter once again, with the cold lasting longer than the winter just gone. If we see El nino developing at the end of the year, then I guess you could call that one piece of the jigsaw. I am getting carried away now...

[Cheshire Freeze]

I don't think it's as cut and dried as that though. If it was, long range forecasting would have been cracked a long time ago. GP is excellent with his long range forecasting though so it is a concern as a dry summer would not be much fun from my perspective.

[snowlover2009]

I don't think it's as cut and dried as that though. If it was, long range forecasting would have been cracked a long time ago. GP is excellent with his long range forecasting though so it is a concern as a dry summer would not be much fun from my perspective.

I suppose the prediction of a dry summer, would take torrential downpours into account? If the prediction was, a warm summer with above average rainfaill, one would imagine, the reason for it being predicted to be wetter than average, would be the intensity of the rain and not the frequency of rain? I suppose it could be the other way round though.

Basically does a predicted dry summer take into account, the intensity of downpours or just the freuqency of rain?

Edited April 11, 2011 by snowlover2009

[Cheshire Freeze]

I suppose the prediction of a dry summer, would take torrential downpours into account? If the prediction was, a warm summer with above average rainfaill, one would imagine, the reason for it being predicted to be wetter than average, would be the intensity of the rain and not the frequency of rain?

A hot summer with interludes of torrential downpours would still be classed as dry as 1. the rainfall would be localised and 2. short, intense downpours do little to infiltrate a couple of centimeters below soil surface.

I'm envisaging a mixed summer.....1976 out of the equation for me sorry. I do agree there will be some fine summer spells and I do think we will break into the 30's on a couple of occasions.....but I don't see 1976 and I don't see any drought problems; except for local exceptions towards the SE.

Edited April 11, 2011 by CreweCold

[snowlover2009]

A hot summer with interludes of torrential downpours would still be classed as dry as 1. the rainfall would be localised and 2. short, intense downpours do little to infiltrate a couple of centimeters below soil surface.

I'm envisaging a mixed summer.....1976 out of the equation for me sorry. I do agree there will be some fine summer spells and I do think we will break into the 30's on a couple of occassions.....but I don't see 1976 and I don't see any drought problems; except for local exceptions towards the SE.

I have to agree, 76 sounds too good to be true, I think the summer will be better than 2010 and tbh that was the best since 2006, even if we did have a poor july and august, they were nothing compared to previous ones.

I think we will see quite a wet may, with at least a week of reasonable weather or noteworthy, start of June will follow from the wet May, June I think could be similar to 2009, maybe slightly wetter. July and August, possibly some amazing summer weather.

[Cheshire Freeze]

I have to agree, 76 sounds too good to be true, I think the summer will be better than 2010 and tbh that was the best since 2006, even if we did have a poor july and august, they were nothing compared to previous ones.

I think we will see quite a wet may, with at least a week of reasonable weather or noteworthy, start of June will follow from the wet May, June I think could be similar to 2009, maybe slightly wetter. July and August, possibly some amazing summer weather.

You have pretty much summed up my gut feelings on proceedings at the minute. A wet early season followed by some warmth in July and August. May and June look wet to me.

[Alex]

Well your last comments crewe cold and snowlover are going against what GP thinks, he hasn't said summer like 76 but he has said I'n the medium term expect may and June to look dry. Let's hope all 3 months are dry and Sunny!!!

[Rob]

Well your last comments crewe cold and snowlover are going against what GP thinks, he hasn't said summer like 76 but he has said I'n the medium term expect may and June to look dry. Let's hope all 3 months are dry and Sunny!!!

Fingers crossed!

[feb1991blizzard]

A hot summer with interludes of torrential downpours would still be classed as dry as 1. the rainfall would be localised and 2. short, intense downpours do little to infiltrate a couple of centimeters below soil surface.

Forgive me if i am talking rubbish but surely how wet a summer is defined by the same criteria as winter. How wet 2cm below the soil surface is irrelavent as other variables can affect this such as sunshine hours. A wet summer is defined by rainfall above the 30 year average. If 1mm above the monthly average falls in one thunderstorm and nothing else for 30 days in June, July and August i was under the impression that it is still statistically a wetter than average summer.

Edited April 11, 2011 by feb1991blizzard

[Cheshire Freeze]

Forgive me if i am talking rubbish but surely how wet a summer is defined by the same criteria as winter. How wet 2cm below the soil surface is irrelavent as other variables can affect this such sunshine hours. A wet summer is defined by rainfall above the 30 year average. If 1mm above the monthly average falls in one thunderstorm and nothing else for 30 days in June, July and August i was under the impression that it is still statistically a wetter than average summer.

Sorry, I should have clarified.....

During the summer months, evaporation potential is much greater than during the winter months due to the angle of the sun and it's concentration of surface heating of the land. Therefore it comes down to perception and practicality- a short intense downpour may produce a localised accumulation of rainfall but it is more likely to run of over the land and into drains......'overland flow'. This is in contrast to a more persistent bout of rainfall which would be more likely to infiltrate the ground and be transported across the water table via 'throughflow'. Short, intense downpours do little to fill aquifers and reservoirs whereas steadier bouts of rainfall tend to help a little bit more. Therefore although localised places may see signifficant short term accumulations through thunderstorms, the perception would be that it had not been that wet as it wasn't persistent and the practicality of that rainfall would be limited due to most of the water being wasted as run off.

p.s to quote Disraeli, 'There's lies, damn lies and statistics' In this case if 1mm over the monthly average fell in one short, sharp downpour and nothing thereafter, the fact that most of the water would flow into the drains and what was filtrated into the ground would most probably be evaporated very quickly, the perception would be that it was a dry month.

Edited April 11, 2011 by CreweCold

[feb1991blizzard]

Sorry, I should have clarified.....

During the summer months, evaporation potential is much greater than during the winter months due to the angle of the sun and it's concentration of surface heating of the land. Therefore it comes down to perception and practicality- a short intense downpour may produce a localised accumulation of rainfall but it is more likely to run of over the land and into drains......'overland flow'. This is in contrast to a more persistent bout of rainfall which would be more likely to infiltrate the ground and be transported across the water table via 'throughflow'. Short, intense downpours do little to fill aquifers and reservoirs whereas steadier bouts of rainfall tend to help a little bit more. Therefore although localised places may see signifficant short term accumulations through thunderstorms, the perception would be that it had not been that wet as it wasn't persistent and the practicality of that rainfall would be limited due to most of the water being wasted as run off.

Yes i see your point. I suppose its the same as having a winter where you have 6 weeks of the winter with massively below ave temps and blizzards interspersed with 6 weeks of extreme mild. Satistically it would be only an average winter but perception would be that it would be a very severe winter.

[The PIT]

Looking like GP's idea of a cooler end too April is dead in the water and the warm to very warm dry weather to continue.